Chapman Piloting & Seamanship

How a Compass Works • How It Should Be Selected & Installed How It Should Be Used • Variation & Deviation • Compass Compensation

The marine compass is a remarkable instrument, and the single most important navigational tool on any boat. It requires no power source; it guides you on all oceans and waterways, in fair weather and foul.

Your boat’s safety may well depend on her compass, and on your ability to use it properly. On long runs out of sight of land or aids to navigation, your compass enables you to steer accurately to your destination. In poor visibility you may have no other means of keeping on your course. Knowing the limitations of your magnetic compass is essential, but within these, you should trust it to guide you safely.

Magnetism is a phenomenon that can only be known from its effects. It is a basic natural force and one that is essential to a vast number of devices, from incredibly complex electronic equipment to your boat’s simple magnetic compass.

Magnetism appears as a physical force between two objects of metal, usually iron or an alloy of iron and other metals, at least one of which has been previously magnetized and has become a MAGNET. The area around a magnet in which its effect can be detected is called a MAGNETIC FIELD. This is commonly pictured as innumerable LINES OF FORCE, but these are purely a convention for illustration, and actual lines, as such, do not exist; see Figure 13-01, left, which is a typical illustration of a BAR MAGNET with some of the lines of force shown. (An electric current flowing in a wire also creates a magnetic field; more on this later.)

Figure 13-01 The external effects of a bar magnet (left) exist within its field, represented by imaginary “lines of force.” The magnetic properties of the earth can be visualized as emanating from a powerful bar magnet at its center (right). Because the field is not quite aligned with the earth’s axis of rotation, the magnetic poles are offset from the geographic poles.

Basic Law of Magnetism
Each magnet, regardless of size, has two POLES where the magnetic action appears to be concentrated. These poles have opposite characteristics and are termed NORTH and SOUTH. The basic law of magnetism is simple: Opposites attract; likes repel. Thus an N pole is attracted to an S pole but repels another N.

The earth as a whole has magnetic properties and can be thought of as having a powerful bar magnet near its center; refer to Figure 13-01, right. The magnetic properties of the earth are not uniformly distributed, and as a consequence its magnetic poles are not at the locations of geographic poles. The magnetic pole in the northern hemisphere is named the NORTH MAGNETIC POLE because of its general location; actually, it is more than 400 nautical miles from the North geographic pole, the pole of the earth’s rotation, and moves over time (more on this later). The SOUTH MAGNETIC POLE is more than 1,500 nautical miles from the South geographic pole.

The basic principle of a magnetic compass is simple—a small bar magnet freely suspended in the earth’s magnetic field will align itself parallel to the lines of force of that field, and thus establish a DIRECTION. The end of a magnet that points generally north is termed its NORTH POLE, although it is actually the “north-seeking” pole; the other end is its SOUTH POLE. Normally, the compass will not point to geographic north nor even provide the correct direction to the North magnetic pole. It gives, however, a reliable and consistent direction, and for the purposes of navigation this can be considered as being relatively constant over a period of several years.

The navigator usually has at hand information and rapid means for converting the reading of his compass to true geographic directions. He knows that if any local influences from magnetic geologic formations, iron or steel objects, and electrical wires can be neutralized (or measured and recorded), his compass will be a dependable magnetic direction finder, needing neither mechanical nor electrical power.

The construction of a typical compass is shown in Figure 13-02; it has more than one bar magnet from which to derive its force to align with the earth’s magnetic field. A circular disk of light, nonmagnetic material is mounted on a wire frame, also of nonmagnetic material. This is the CARD of the compass; it is marked around its circumference with graduations from which the direction can be read. Two or more magnets are attached to the frame, aligned with the N-S markings on the card.

Figure 13-02 This cross-section of a typical marine compass shows parallel permanent magnets mounted beneath the card. The magnets and card are attached to a light frame, which is supported on a pivot. The space inside the compass is filled with a liquid to dampen oscillation, and a diaphragm to allow for expansion and contraction with temperature changes. Other features, not labeled here, include a protective hood over the compass dome and a night light, usually red.

Centrally placed under the frame is a BEARING that rides on a hard, sharp point called the PIVOT; this in turn is supported from the outer case, which is the BOWL of the compass. A cover of transparent material, the DOME, is rigidly fastened to the top of the bowl with a leakproof seal; this may be either flat or hemispherical. Through a pluggable opening, the bowl is filled with a special nonfreezing liquid. An EXPANSION DIAPHRAGM in the lower part of the bowl allows the fluid to expand and contract with temperature changes without bubbles being formed.

A FLOAT may be attached under the frame to partially suspend the card and magnets, and thus reduce friction and wear on the bearing and pivot. The compass liquid also helps to dampen any rapid oscillations or overswings of the card when direction is changed or rough weather is encountered. GIMBALS, when provided, either internally or externally, are intended to help the compass card remain essentially level despite any rolling or pitching of the vessel. The best arrangement is two sets of rings, one pivoted on a fore-and-aft axis and the other on an athwartship axis.

Compass Cards
The card of a modern compass is divided into DEGREES, 360 to a full circle; the degrees increase clockwise around the card. North is 0° and the scale continues on around through 90° (East), 180° (South), and 270° (West) back to North, which is 360° or 0°. The spacing between graduations on the card will vary with the size of the compass, but 5-degree intervals are typical for most small-craft compasses (see Figure 13-03, left) with some larger compasses having 2-degree cards. Ships will normally have compasses with cards having 1-degree divisions; see Figure 13-03, right. Note that this card is also marked with the “points” of the compass (see Table 13.1). Numbers may be shown every 10 degrees from 10° to 350°, every 20 degrees from 20° to 340°, or every 30 degrees from 30° to 330°.

Figure 13-03 Compass cards for boats (left) are commonly graduated at 5-degree intervals, with the 10-degree marks heavier for easier reading. Numbers are usually at 30-degree intervals, and cardinal headings—North, East, South, West—may be shown as letters rather than numbers. Compasses for larger vessels (right) may have finer graduations: two degrees or even one. Intercardinal and combination headings are often shown; on the largest cards, “bypoints” and even individual points, half-points, and quarter-points may be shown.

Over or just outside the card is the index mark against which the graduations are read. This is the main or forward LUBBER’S LINE; it may be a mark on the inside of the bowl or it may be a part of the internal gimbal system; see Figure 13-04, and refer to Figure 13-02. There may also be additional marks at 45° and/or 90° from the main mark.

The lubber’s line should be close enough to the card so that there is minimal PARALLAX ERROR, the change in the apparent value of a reading when viewing from one side of the compass as compared to reading the card from directly to the rear.

On some compass cards, the width of the major degree marks and the lubber’s line have significance. If the lubber’s line is one degree in width, for example, that fact can be used to estimate values that are intermediate between markings on the compass card; refer to Figure 13-05.

Figure 13-05 On some compasses, the width of the lubber’s line equals one degree on the card. The center drawing above shows how this feature can be used to read one degree either side of a main graduation. As shown at right, a reading slightly more or less than halfway between marks would indicate two or three degrees more or less than the nearest graduation.

Some smaller and medium-size compasses are designed for mounting so as to be read from the front (i.e., the side nearest the viewer) rather than from above. All bulkhead-mounted compasses are front-reading. The lubber’s line is aft, facing the helmsman, rather than on the far side of the card. This results in the increasing number sequence being to the left of the lubber’s line rather than to the right; refer to Figure 13-04. This can be confusing to a skipper accustomed to a more conventional “top-reading” compass. Some front-reading compasses can also be read from above using the lubber’s line above the front of the card—great care must be taken to understand the use of this style of compass; see Figure 13-04.

Figure 13-04 When a compass is designed for front viewing, the lubber’s line is on the side nearest the helmsman. The readings increase to the left, and a person accustomed to a conventional card must take care to avoid confusion when reading values between marks. These compasses are sometimes found on smaller craft. The scale at the bottom displays the boat’s inclination (angle of heel) to port or starboard.

In olden days, compass cards were subdivided into POINTS—32 points to a complete circle, one point equal to 11¼ degrees (or 11°15’); refer also to Figure 1-37. These were named, not numbered, and a seaman early learned to “box the compass” by naming all the points. There were also half- and quarter-points when a smaller subdivision was needed.

The CARDINAL points—North, East, South, and West—and the INTERCARDINAL points, Northeast, Southeast, Southwest, and Northwest—are still in common use as rough directions and as descriptions of wind directions. The COMBINATION points, such as NNE, ENE, and so on, are sometimes used for general directions, but the BY-POINTS, N by E (North by East), NE by N, NE by E, and the like, are rarely used.

The combination point system is now effectively obsolete. For those who do not wish to forget the past or want to learn points for ease in steering sailboats, however, a table of conversions between the point and degree systems is provided as Table 13-1.

Some modern compass cards label the intercardinal points as well as the cardinal headings, and a few mark all points and even quarter-points; refer to Figure 13-03, right. The examples and problems that follow, however, will all be in terms of degrees. Note, however, that some statements of direction will be in degrees and minutes, there being 60 minutes in one degree. Minutes will have to be converted to decimal fractions for calculations.

When the top over a compass is hemispherical, the dome serves to magnify and make the card appear much larger than it actually is. This aids in reading it closely, making it possible to steer a better course or take a better bearing. On some compasses with a hemispherical cover, the card itself is concave or dish-shaped rather than flat. This feature, combined with the shape of the dome, makes it possible to read the compass from a lower angle and at considerably greater distances; it is not necessary to stand more or less directly over the compass to read it. A compass with such a card of 5-inch (12.7 cm) “apparent diameter” (actually somewhat smaller in diameter) can be read from as far away as 10 feet (3m) or more.

If the compass as a whole is spherical, bowl as well as dome, there is a considerable gain in stability of the card. The effect of a whole sphere is to permit the fluid inside to remain relatively undisturbed by roll, pitch, or yaw; the result is superior performance in rough seas.

Some compasses have special features for use on sailboats. These include more extensive gimbaling so as to permit free movement of the card at considerable angles of heel, and additional lubber’s lines at 45 degrees and 90 degrees to either side of the one aligned with the craft’s keel as an aid in determining when to tack. These additional lubber’s lines are also useful when the helmsman sits to one side, as when sailing with a tiller. They can also be used to sight distant objects to determine distance off.

Table 13-1 The point system was in common use before World War II. Few boaters now will learn to “box the compass” through all its 32 points, but the major directions are still familiar and very useful. In descending order, the points are known as cardinal (N, E, S, and W), intercardinal (such as NE and SE), combination points (such as NNE and ENE), and by-points (such as N by E, NE by N, etc.).

Compasses are sometimes mounted directly on or through a horizontal surface. More often, however, the compass is mounted in an outer supporting case called a BINNACLE. This can be a simple case, sometimes with a light-shielding hood, which is then fastened down; refer to Figure 13-06. Or it can be a complete pedestal stand as might be used on larger sailboats. On smaller sailboats, a compass is sometimes mounted on a vertical or near-vertical bulkhead forward of the helm position.

The outside of the binnacle or compass case, or any mounting bracket, should be black, preferably with a dull finish. There should be no shiny chromed parts that could reflect sunlight into the eyes of the helmsman.

Figure 13-06 Compasses offer a variety of mounting possibilities. A light-shielding hood cuts off excessive sunlight to aid in reading the compass. Most modern compasses have two sets of internal compensating magnets, usually at the bottom of the binnacle.

When compasses are lighted for night use, the lighting system is often a part of the compass case, cover, or binnacle. The light should be red in color and its intensity at a suitable level. If the light is not red, or is too bright, there is a risk of loss of the helmsman’s night vision—this will make it more difficult, or even impossible, for him to see aids to navigation, vessels, or other objects. On the other hand, a too dimly lit compass can be a source of eyestrain and a detriment to good steering. In some installations, a means is provided for varying the intensity of the light; this is a desirable feature.

As will be detailed later in this chapter, it usually is not possible to avoid all magnetic effects from iron and steel objects on the boat. The compass must then be COMPENSATED (ADJUSTED) with small additional bar magnets located and polarized so they counteract these unwanted magnetic influences. Such COMPENSATORS may be external to the compass and binnacle, but are more likely to be internal, usually a part of the binnacle unit; refer to Figures 13-02 and 13-06. Refer to Check for Magentic Influences for more on local magnetic influences and compensation.

Although a compass is relatively simple in its construction and operation, it is probably the most important navigational tool on board any boat. Considerable thought should be given to its selection. This is not the place for a hasty decision or for saving money by buying a small, cheap unit. In fog or night, or any other form of reduced visibility, your compass is the basic instrument that shows direction. Your safety will depend upon it; it has to be right!

Quality
Almost any new compass will look fine in a store or on board a boat in the quiet motion of a marina slip or at a mooring. But its behavior underway, when the sea makes up and the craft pitches, rolls, and yaws, is of supreme importance. Will the card stick at some angle of heel? Will its motion be jerky, or smooth and easy? Are the card graduations legible and are different headings easily distinguished? Is the compass protected against large temperature changes? What if a bubble appears under its glass that might distract the helmsman? The answers to these questions will depend upon the quality of the compass. Select a compass fully adequate for your expected needs, and if a choice must be made, choose the better quality.

Examine a number of compasses before buying one. Pick them up; tilt and turn them, simulating motions to which they would be subjected in actual use afloat. The card should have a smooth, stable reaction, coming to rest without oscillating about the lubber’s line. Reasonable tilting, comparable to the rolling and pitching of your boat, should not appreciably affect the reading. In fairness to the compass, however, if it has internal correctors, they must have been zeroed-in before you make these motion tests; see later section on zeroing-in for more.

All too often, a small compass will be selected and installed on a small boat for the simple reason that the boat is just that: small. This is not the correct approach. Because the boat is small, it will be subject to greater movement, both in normal operation and in rough waters. A small, cheap compass will have less stability and may even be completely unusable in severe conditions, perhaps just when it is most needed.

A compass for a boat should be as large as the physical limitations of mounting space sensibly permit—and the quality should be just as high as the owner’s budget can possibly allow.

In selecting a compass for your boat, pay particular attention to the design of the card. Its graduations should be suited to the intended use. That a large craft may be held on course more easily and precisely than a small one is readily understandable. Thus the larger the vessel, the greater the number of divisions normally found around the outer edge of the card. Skippers of large oceangoing craft seem to prefer cards that can be read directly to two degrees, or even to single degrees. Studies have shown, however, that subdivisions smaller than five degrees may not be desirable. Cards with finer subdivisions may produce more eyestrain, require a higher intensity of illumination, and may result in greater steering action although no better course is made good. The experiences and preferences of the individual must govern here.

The helmsman of a sailboat is usually in an unprotected cockpit exposed to the weather. Spray and rain on the glass cover of the binnacle may obscure or distort the markings on the compass card. The smaller and more uniform the markings on the card, the easier it is for the helmsman to confuse one marking for another, lose his place on the card, and wander off course. This is especially true at night or offshore when he has no landmarks to warn him that he has been steering, for example, 055° instead of his intended course of 065°. And when he discovers his error, he may not know for how long he has been off course.

If the pattern on the card shows a large and prominent patch of contrasting color at the cardinal points and an even larger one at “North,” even an inexperienced helmsman can steer with surprising accuracy by relying on the general position of this distinctive pattern as his guide, vague and blurry though it may be.

Although not always reliable, price is generally a fair indicator of quality. There are, however, simple tests that you can perform yourself to aid in judging between different units. When making any of these tests, make sure that any other compasses are at least four feet (1.2m) distant.

Test for minimum pivot friction (it should be zero) by turning the compass as a whole until a card graduation mark is exactly aligned with the lubber’s line, then bring a small magnet or an iron/steel object just close enough to the compass to cause the card to deflect 2 to 5 degrees to either side. Next, take it well away from the compass quickly, watching the card as you do so. The card should return to its former position exactly. This test is completed by drawing the card off to the other side—by moving the magnet or iron/steel object to the other side of the compass. Again the card should return to its initial position when the influence of the external magnet or steel object is removed. (Make sure that the object is moved far enough away each time for a fair test; turn a small magnet end-for-end—the card should not be affected.) Sticky pivots are rare, but they do occur, and the test is too simple and easy to make to omit doing it. Do not purchase a compass that does not pass this test.

Repeat the above procedure, but this time draw the card off to each side about 20 to 30 degrees. Release the external influence abruptly and observe the amount of overswing as the card returns back to, and past, its original setting. A compass with proper dampening will have a minimum overswing and will return to its proper position with a minimum of oscillation about that position.

These tests are relative; there are no absolute values to use as a standard. Thus these tests should be made on comparative basis between several different compasses of the same model and between different models. Be fair, be careful—try to see that comparable tests are made on each compass.

SUMMARY OF SELECTION CRITERIA

When shopping for a compass, choose a quality instrument. Consider the points below, and remember that your compass is the most important navigation tool on board your boat.

• The compass (and binnacle if applicable) should be of a design that can be mounted onboard your boat in a location that allows accurate, comfortable viewing.

• The card should be easily read and graduated to your preference.

• The card should remain level and not stick through reasonable angles of pitch or roll.

• The card should move, but slightly and uniformly, during any course change (simulated by slowly and smoothly rotating the compass through 90 degrees or more). Some small lag due to the inertia of the card is permissible, but the card should be deadbeat, swinging but once to a steady position, not oscillating about the lubber’s line.

• There should not be any significant parallax error when viewed from the side as compared from the rear.

• The compass or its binnacle should have built-in compensating magnets.

• The compass or binnacle should include provision for night lighting. (A means of adjusting the intensity of the light is desirable.)

• The dome should be hemispherically shaped rather than flat.

• The compass should have a fully internally gimbaled card and lubber’s line.

• The compass should be of a design that can be mounted on your boat in the most suitable position for it, with easy access to the adjusting screws for the internal compensators.

• The compass should have a metal or rubber expansion bellows or diaphragm in the assembly to prevent bubbles forming in the liquid due to temperature changes.

• The compass or binnacle should be waterproof and have some form of light-shielding hood.

• If the compass is second-hand, it should be very carefully checked.

• Buy a quality instrument and pay the price. This is no place to cut corners; you get what you pay for.

The installation of a compass, from initial site selection through to the actual mounting, is of critical importance to its performance. If any step is omitted or is not done carefully, it may never be possible to get the compass properly adjusted for precise navigation. Follow each step in detail and don’t rush the work.

Locating the Compass
The first step is selecting the location in which to mount the compass. There are a number of factors to be considered, and some limitations that must be faced. Actually, you should consider where to place the unit before you buy the compass.

Ideally, the compass should be directly in front of the helmsman, placed so that he can read it without physical stress whether he sits or stands. Give some thought to comfort in rough weather and in conditions of poor visibility, day or night. His position is fairly well determined by the wheel or tiller. The compass has to be brought into what might be called his zone of comfort. Too far away, he bends forward to watch it; too close, he bends backward for better vision. Much of the time that person may be not only the helmsman but the forward and after lookout as well. So put the compass where he can bring his eyes to it for reading with a minimum of body movement. For the average person, a distance of 22 to 30 inches (56–76 cm) from his eyes with the head tilted forward not more than 20 degrees is about right. Additional details will be found in ABYC Standard T-17.

Locating a compass on a small boat may be more of a problem. It must, of course, be located where it can be easily seen by the helmsman, preferably directly before him; see Figure 13-07. Such a location may not be physically possible, or it may be subject to undesirable magnetic influences that will cause DEVIATION in the compass readings. It may be necessary to locate the compass to one side of the helm position. If so, check carefully for parallax error—the change in apparent reading when the compass is viewed from one side as compared with from directly behind. A slight difference in reading, perhaps up to 3 degrees, may be accepted, but more than that, which can easily occur, will result in inaccurate and perhaps dangerous navigation. If you have this mounting problem, check several models of compasses, as some designs will have less parallax error than others.

Figure 13-07 Boats smaller than this one nearly always present a major problem in locating a compass due to limited space and a poor magnetic environment. Finding the best location takes ingenuity and patience.

A compass is often located off the centerline of the boat, and special care must be taken to ensure that it is properly aligned. A line running through the center of the card and the lubber’s line must be parallel with the centerline of the hull; see Figure 13-08.

On some boats that are either wide or are steered from one side or the other—such as catamarans, racing sailboats, or even workboats—additional compasses can be mounted at either side to make viewing more direct and accurate.

Figure 13-08 If the compass is not mounted directly over the keel line, take care to make sure that a line through the center of the card and the lubber’s line is exactly parallel to the centerline of the boat.

Zeroing-In
A modern compass with internal compensators should be ZEROED-IN before being mounted on a boat. Zeroing-in is nothing more than adjusting the internal compensating magnets so that they have no effect on the compass. A compass that is subject only to the earth’s field has no need for compensation: there is no deviation to be removed. Thus before being installed, any deviation caused by the improper position of the compensators themselves must be removed so that the unit can go on board ready for whatever magnetic influences the boat may have. With these magnets properly set, any deviation will come from external sources. A quality compass will have been shipped with its internal compensating magnets set to zero, but check before installation to be sure that the settings have not been changed. The slots of the screws for the N-S and E-W compensators should be horizontal, but aligning the screw slots by eye, or with marks on the case, may not be sufficient for accurate navigation. Zeroing-in by trial and error is simple and effective. It does not require any knowledge of the direction of magnetic north.

Zeroing-in must be done off the boat and in an area well away from any magnetic influence such as iron or steel pipes and girders, radios, electric motors, etc. Don’t work wearing a steel belt buckle or wristwatch strap; keep all tools well away from the work area.

Select a board with two exactly parallel edges. Temporarily mount the compass on this board using nonmagnetic screws or tacks. Line up the lubber’s line with the parallel edges as closely as possible, but precise alignment is not necessary. On a level, flat surface—a plank will do fine, but not a table that may have screws or braces of magnetic material—place a reference edge, such as a book or straightedge, and the compass on its small board; see Figure 13-09. Move the board with the compass into close contact with the reference edge. Rotate the board and reference edge as a unit until the compass reads exactly North. Hold the reference edge steady in this position; it provides a fixed direction reference. Remove the board and compass, turn it end for end, then bring the other parallel side of the board snugly against the reference edge. The lubber’s line has now been exactly reversed. If the compass now reads South exactly, the N-S internal compensator is already zeroed-in and requires no adjustment.

Figure 13-09 A board with parallel edges and a large book are all that is needed to accomplish the zeroing-in process. A nonmagnetic screwdriver or a thin coin must be used to turn the adjusting screws for the compensating magnets.

If, however, the reading is not exactly South, an adjustment is needed; read the difference from 180° and make a note of it. Adjustments must be made with a nonmagnetic tool—a “screwdriver” made from a piece of brass rod with one end ground or filed down to a flat blade is often used; bend the other end over by 90° to form a handle that will facilitate small turns. A thin coin, such as a dime, can be used with some compasses. Slowly turn the N-S screw until half this difference is removed. (If the difference increases when you first turn the screw, turn it in the opposite direction until the desired result is obtained.) Now slightly realign the reference edge until the compass reads South exactly. Hold the reference edge firmly and again reverse the board.

The compass should now read N exactly. If it does not, the difference should be much smaller than the initial difference at South. If there is a difference, halve it by turning the N-S adjusting screw. Next, slightly realign the straightedge and board until the compass reads exactly North, always keeping the board snug against the reference edge. Once again, reverse the board, and check to see if the compass reads South. If it does not read South exactly, again halve the difference. Continue this procedure until the reversal yields zero difference, or as nearly zero as possible. When this process has been completed, the North-South axis of the compass coincides with the magnetic meridian.

Next adjust the East-West compensators in the same way. Line up on either an East or West heading, reverse the board and compass, and if the reading is not the exact opposite, halve the difference by turning the E-W screw in the correct direction as determined by trial and error. Continue working on the East and West headings just as on North and South until the desired results are obtained.

If exact reversals are not attainable, some unseen magnetic influence may be present. Move the zeroing-in project to a different location, and try again as above. It is also possible, although unlikely, that the compass is defective.

If the compass manufacturer provided instructions for zeroing-in, they should be followed.

Do not discard the temporary mounting board. It may be useful in testing for a possible site on the boat.

Check for Magnetic Influences
With the compass properly zeroed-in, and the best location selected from the viewpoint of visibility and use, now check for undesirable magnetic influences. The site should be at least 2 feet (0.6 m) from engine instruments, radios, bilge vapor indicators, electric gauges and instruments, and any iron or steel. (Stainless steel is nearly always nonmagnetic, but there are many different alloys, and each item of such metal should be checked.) When one or more of these magnetic influences is too close, either it or the compass must be relocated. Fortunately, magnetic influence is subject to the “inverse square law”—double the distance from a disturbing force to the compass and you have reduced the effect to one-fourth; triple the distance and the effect is down to a relatively insignificant one-ninth.

Test for magnetic materials or influences that may be concealed from ready view. Make your test with the compass itself. Move the compass slowly and smoothly all around the proposed location without changing its orientation with the boat’s centerline. Watch the card; one thing only will make it turn: a magnetic influence—find this with the compass. If the influence cannot be moved away or replaced with nonmagnetic material, test to determine whether it is merely magnetic metal, a random piece of iron or steel, or a magnetized object. There is a difference between the effect of an object that is magnetized and one that is merely made of magnetic material. Successively bring the North and South poles of the compass near it. Both poles will be attracted if it is not magnetized. If it attracts one pole and repels the other, the object is magnetized. Demagnetization may be attempted (more on this later in the chapter).

Metal objects that move in normal operation should be moved. Turn the steering wheel fully in both directions; work the throttle, move the gearshift lever; open and close the windshield if it is near the compass. Try to duplicate all the changes that can occur in normal operation of the boat.

Check for Electrical Influences
Electrical currents flowing in wires near the proposed location can also exert undesired influences on the compass. Hold or tape down the compass at the proposed location, then test every circuit that might affect the compass.

Switch on and off, one at a time, all the electrical loads: radio, bilge pump, depth sounder, lights, windshield wipers, etc. Don’t overlook anything controlled by a switch on the panel or located near the proposed compass site. Start the engine so that its electrical instruments will be operating. If there is an auxiliary generating plant, start it. Make full sets of checks with the boat on two cardinal headings 90 degrees apart.

Twisting the two wires that carry current to the compass light (or any other electrical load near the compass) around one another will create a “twisted pair,” canceling the magnetic field created by the flow of current through the wires. Twisted-pair wiring should be used to connect the compass light, instruments, and switches near the compass.

In summary, check everything, one item at a time, that might influence the compass at the intended location, carefully watching the card as items are switched on and off, moved back and forth, started and stopped. When the card moves during any of these tests, the compass has been affected. Ideally, the compass should be relocated or the cause of the influence removed or demagnetized. In practice, however, you may have to settle for two separate states of magnetic environment; for example, with and without windshield wipers working.

Vibration
Vibration can make a compass completely unusable at an otherwise desirable location. It can actually make a card slowly but continuously spin. Even if vibration does not cause the card to spin, it could result in excessive pivot wear. Mount the compass temporarily but firmly in the planned location and watch for vibration at all ranges of engine rpm and boat speed; vibration is unpredictable and may occur only at certain engine speeds.

To avoid compass vibration, mount it on as solid a part of the boat as possible; a beam or deck is better than a wood or fiberglass panel. If vibration remains a problem, it may be eliminated or reduced by placing a block of foam rubber between the bottom of the compass and the boat’s structure. Craft with slow-turning diesel engines may require a special vibration-dampened binnacle.

Mounting
Once the zero setting of the compass has been verified and the prospective location checked for undesirable magnetic influences, you can proceed to the actual compass mounting. Certain basic requirements must be met.

• A line from the center of the card through the lubber’s line must be exactly parallel to the keel, the centerline of the boat; refer to Figure 13-08. (Ideally, the compass would be directly over the centerline. This often is not practical, but it must be mounted exactly parallel to the centerline.)

Establishing this line parallel to your boat’s centerline is quite simple, but it almost always takes a little more patience and time than you anticipate. Find the center of the transom using a measuring tape, and accurately mark that center on a piece of masking tape (so as not to permanently mark the boat). This is usually easy. Next you need a second center point at some convenient location forward of the compass position—this might be the track of the mainsheet traveler of a sailboat. Alternatively, you can use the bow rail and a second location aft of the intended position of the compass.

With help from some additional pairs of hands, stretch a string tightly between the two center points. Accurately measure out (athwartship) to the location you selected for your compass, and after marking off this distance at your transom and forward location, move the string to this new pair of points. If the string is higher than the area where the compass base will be, use a plumb bob to project the line. Of course, the boat must be on an even keel if a plumb bob is used.

Once you have checked to make sure you have accurately established your compass mounting line parallel to the boat’s centerline, you might just want to carefully scratch or mark a short permanent line for future reference. You must install your compass so the lubber’s line is exactly lined up with it. Many compasses have small alignment marks (forward and aft) where the housing touches the mounting surface, to aid in positioning on the scratch marks.

This alignment is extremely important, because an improperly aligned compass can never be properly adjusted for deviation and will have a constant error (in addition to deviation) no matter what your boat’s direction.

• The compass should be level; this is often at least partially accomplished by mounting brackets; if the compass has a cylindrical case, the bottom may need to be shimmed so that the unit is level. Bulkhead-mount compasses must be of the type intended for such installation; the mounting should be no more than a few degrees from true vertical.

• Magnetic material in the vicinity of the compass should be minimal. Wires near it, carrying direct current, must be twisted.

Drill one mounting hole only. If during compensation later the compass must be slightly turned to correct any misalignment of the lubber’s line, more than one hole could present problems. When compensation has been accomplished, the remaining hole(s) can be drilled and the fastening job completed; a check of the compensation should be repeated after this is done. Use only nonmagnetic screws, and if possible, nonmagnetic tools.

A compass bracket that has only two holes for mounting requires precise alignment and leaves very little room for further adjustment. Often circular compass mounts have slots built in for fastening, and fasteners are tightened when alignment is completed. If such slots are not present, try using either masking tape or duct tape to hold the compass temporarily in place.

If Your Boat Already Has a Compass
If you have a boat and it already has a compass installed, or if you are buying a boat that comes equipped with a compass, you can still benefit from the above information, as all too many factory- and owner-installed compasses have lubber’s lines not truly parallel to the centerline.

Figure 13-10 Electronic fluxgate compasses react to the same magnetic field of the earth as do traditional compasses, but they have no moving parts. The output is a digital signal that can be displayed for steering, used as the input to an autopilot, or as an input to electronic navigation systems.

A simple magnetic compass is a basic navigation tool that should be on every boat. However, more and more small craft are additionally fitted with electronic compasses; this is nearly always a FLUXGATE COMPASS; see Figure 13-10. These are more expensive than the previously described ordinary magnetic compasses, but not greatly so. A fluxgate compass has an electronic output that can be used in several ways. Applications can include digital displays, with as many remote readouts as you wish, or signals to an autopilot, or input data to electronic navigation systems. Various designs are used, but the basic principle is the sensing of the earth’s magnetic field by coils of wire and the amplification of these very weak signals by solid-state electronics. There are no moving parts, thus avoiding such problems of an ordinary magnetic compass as friction, lag, etc.

Electricity is required for the operation of a fluxgate compass, but not a significant amount; in the event of a loss of electrical power, an electronic compass, of course, becomes inoperative, and you are back to your regular old-fashioned mariner’s compass. Fluxgate compasses are subject to the same disturbances as ordinary compasses, but the sensing unit can be remotely located in a position where these influences can be mitigated or minimized. (A common location is under a midship berth.) If needed, compensation can be applied, either with magnets or electronically. Variation can be electronically inserted so that the instrument gives true headings and directions. Careful alignment of the sensor unit with the vessel’s centerline is just as essential as for an ordinary mariner’s compass, but the remote readout units can be positioned anywhere that is convenient, and these have no need for precise alignment.

A more sophisticated, high-tech (and more expensive) version of a fluxgate compass includes three-axis gyro stabilization for improved accuracy and precision. Its electronic output can be used with a variety of navigation devices, such as autopilots, chartplotters, and radar displays, as well as a direct display of heading. The normal output is a magnetic heading, but if a GPS receiver is connected, or variation is entered manually, a true heading is available.

USING AN UNCOMPENSATED COMPASS

A compass that has not been compensated—had deviation removed or else had a deviation table prepared—cannot be used to navigate with a chart, but there are simple and practical uses for it.

An uncompensated compass may not measure directions correctly relative to magnetic north, but it does indicate directions in a stable, repeatable way if local conditions on the boat are unchanged. You can proceed from one point to another—for example, from one channel marker to the next, read the compass when you are right on course, and record that value. If you make the same run on another day, the reading will be the same, provided that you haven’t changed any magnetic influences near the compass. Thus if the day comes when it is foggy, or night catches you still out on the water, you can follow with confidence the compass heading that you recorded in good visibility.

It’s a good idea to record compass headings on all the runs you regularly make, particularly a route you might follow during period of reduced visibility. Run the route in clear weather and record the compass headings. One word of caution, however: run the course in both directions for each heading. You cannot add or subtract 180 degrees from the forward run to get a reading for the return; residual deviation may give an erroneous reading.

A compass is read by observing the graduation of the card that is opposite the lubber’s line. The card of a typical boat compass has major graduations every ten degrees, with lighter marks at intermediate five-degree intervals; refer to Figure 13-03, left. On some models the width of the lubber’s line is just about one degree. Thus it is possible to use this width to estimate fairly accurately a reading of one degree more or less than each card marking. A reading of slightly less or more than midway between the card markings gives the individual degree values; refer to Figure 13-05.

Always remember when a boat turns, it is the boat, and the lubber’s line, that turns—the compass card maintains the same orientation; see Figure 13-11.

Like all other piloting instruments and techniques, your compass should be used frequently. Use it in good weather and sea conditions, even though it might not be needed. This will make you more familiar with how it is used and increase your confidence in it when it becomes a vital piloting tool at night or reduced visibility. Line up with a channel whose magnetic heading you know. Does your compass read that value (with any correction for deviation that is necessary)? Make note of your readings on frequently run tracks. Keep your compass course records handy and check the various headings frequently. If there is a change, look for the reason and eliminate it.

Using a compass for steering a boat and taking bearings is covered in later chapters.

Figure 13-11 When a boat changes heading, it is the boat itself and the lubber’s line of the compass that turns—the compass card maintains the same orientation as before the turn.

The worst enemy of a compass is direct sunlight. When a boat sits day in and day out in a marina slip or on a mooring, something can and should be done about excess sunlight.

When a compass is not in use, it should be thoroughly shielded from the sun to prevent discoloration of the liquid and perhaps of the card. Sometimes a binnacle will have a light-shielding hood; if so, keep it closed whenever the compass is not in use. If such a shield is not a part of your compass, improvise one of lightproof material; if you use cloth, make sure it is a tight weave that will not “leak” direct sunlight.

Before a compass can be used for precise navigation, you must understand COMPASS ERRORS—the differences between compass readings and true directions measured from geographic North. Initially, when using charts, you probably will plot courses and bearings with respect to true North. With more experience, you may find it easier to use magnetic directions.

Figure 13-12 Variation is the angle between the magnetic and the true meridians. Variation depends on geographic location but is the same for all vessels in any vicinity. In North America, right, variation is generally easterly on the Pacific Coast and westerly on the Atlantic Coast and on the Great Lakes. There is a line of zero variation between these areas.

Variation
The first and most basic compass error is VARIATION (V or VAR). For any given location on land or sea, this is the angle between the magnetic meridian and the geographic meridian; see Figure 13-12. It is the angle between true North and magnetic North as indicated by a compass that is free from any nearby influences. Variation is designated as EAST or WEST in accordance with the way the compass needle is deflected. Any statement of variation, except zero, must have one of these labels: E or W.

Variation is a compass error about which a navigator can do nothing but recognize its existence and make allowances for. Variation is the compass error that would exist on a boat entirely free from any onboard magnetic materials or other internal magnetic influences.

The earth’s magnetic field is not uniform, and, because the magnetic poles are not at the geographic poles (refer to Figure 13-01,), variation changes with location; refer to Figure 13-12. The result of failure to apply different variation as a vessel moves from one location to another can be seen in Figure 13-13.

At any given place, the amount of variation is essentially constant. There is usually a small annual change, but if you are using a chart not more than two or three years old, this quite small amount of change can be ignored. The amount of variation, and its annual rate of change, is found within each COMPASS ROSE on the chart; see Figure 13-14. On nearly all types of charts published by the National Ocean Service, part of NOAA (National Oceanic and Atomospheric Administration), there will be found several compass roses suitably placed for convenient plotting. These are circular figures several inches (perhaps 5 to 10 cm) in diameter with three concentric circular scales. The outer scale is graduated in degrees and it is oriented so that its zero point, indicated by a star symbol, points to true (geographic) North. The inner pair of scales is graduated in degrees and in the point system down to quarter-points. The zero point of these scales, identified by an arrow symbol, shows the direction of magnetic North at that point. On some charts, the innermost scale in points is omitted.

Figure 13-13 Do not overlook the change in variation with change of location. In this example, the true and magnetic courses from X to Y are initially very nearly the same, but the magnetic course soon changes, while the true remains constant. By staying on the same magnetic heading, an aviator would soon be at Z, 150 miles west of his destination—the skipper of a boat would have run aground on the Rhode Island coast!

The angle at the center of a compass rose between the star and arrow symbols is the variation in that vicinity for the year stated. This information is printed at the center of the rose to the nearest 15’; for example, in Figure 13-14, “Var 17° 45’W(2003).” The annual rate of change is noted to the nearest 1’, and whether the variation is increasing or decreasing; for example, “Annual Increase 2’ ” or “Annual Decrease 6’ ” or “No Annual Change.” On older charts, apply the accumulated annual change to the variation value at the center of the rose. In all cases, use variation to the nearest whole degree—fractions are not practical for small craft.

Figure 13-14 Most charts have several compass roses that show the variations both graphically and descriptively. They are located so as to be convenient for plotting. On any chart, especially small-scale charts covering large areas, always use the compass rose nearest your position or the reference point you are using.

As noted above, magnetic variation changes with location. Thus, except for charts of a limited area, such as a single harbor and its approaches, variation will most likely be different in various areas of a typical chart. For this reason, the variations graphically indicated by the several compass roses on any particular chart may be different. Always refer to the compass rose that is nearest your position on the chart to read variation or to plot from its scales. You can obtain up-to-date variation data on the web. See www.ngdc.noaa.gov/geomag/.

In addition to the general overall magnetic variation, the navigator may also encounter LOCAL ATTRACTION. (Here “local” means a limited geographic vicinity, as opposed to magnetic influences on board the boat.) In a few localities, the compass is subject to irregular magnetic disturbances in the earth’s field over relatively small areas. A striking example is found at Kingston, Ontario, Canada, where variation may change as much as 45 degrees in a distance of a mile and a half (2.4 km).

Charts of areas subject to such local attraction will bear warnings to this effect. Other means of navigation are needed when within range of such disturbances.

Deviation
A boat’s compass rarely exists in an environment that is completely free of nearby magnetic materials or influences. Normally it is subject to magnetic forces in addition to those of the earth’s field. Material already magnetized, or even capable of being magnetized by the magnets of the compass, will cause the compass needle to deviate from its proper alignment with the magnetic meridian. Currents flowing in improperly installed electrical wiring can have the same effect.

Figure 13-15 Deviation is the difference between North as indicated by the compass and magnetic North. It can be easterly or westerly, and depends on the magnetic conditions on the vessel. It changes with the boat’s heading but is not noticeably affected by changes in position within a geographic area.

The deflection of the compass from its proper orientation is called DEVIATION (D or DEV). It is the angle between the magnetic meridian and a line from the pivot through the North point of the compass card; it is the angle between the direction the compass would point if there were no deviating influences and the direction in which it actually does point. Theoretically, it can range from 0° to 180°, but in practice large values cannot be tolerated. Just as for variation, deviation can be EAST or WEST depending on whether compass North lies to the east or west of magnetic North. Deviation must carry one of these labels, E or W, unless it is zero; see Figure 13-15.

While variation changes with geographic location, deviation changes with the craft’s heading and does not change noticeably in any given geographic area. (Causes for this effect will be explained later.) To cope with these changes, a vessel needs a DEVIATION TABLE—a compilation of deviations, usually for each 15 degrees of heading by the compass; see Table 13-2. For added convenience, a deviation table can also be prepared in terms of magnetic headings; see Table 13-3. (Note: In these examples, the values of deviation are large for purposes of illustration. They are typical of an uncompensated compass in a poor magnetic environment.)

Table 13-2 The first deviation table prepared will be in terms of compass heading. This table is useful for correcting compass bearings so that they can be plotted. (Note that the table’s figures are much larger than would be tolerated in actual piloting. If deviations are this large, the compass needs compensating to reduce deviation to zero or nearly so.)

For ordinary navigation, it is normally sufficient to use the value of deviation from the line of the table that is nearest to the craft’s actual heading, either compass or magnetic. This is particularly true for compensated compasses, on which the deviations will rarely exceed a few degrees.

For more precise navigation, such as in contests and perhaps some races, it will be necessary to INTERPOLATE between tabular entries to get a more precise value of deviation.

Note that any deviation table is valid only on the boat for which it was prepared, and only for the magnetic conditions prevailing at the time the table was prepared. If magnetic materials within the radius of effect—some 3 to 5 feet (0.9–1.5m)—are added, taken away, or relocated, a new deviation table must be prepared—or at a minimum, sufficient checks must be made of the old table to ensure that its values are still correct. If any electronic equipment is installed, removed, or relocated, check to see if a new table is needed.

Table 13-3 Deviation in terms of magnetic headings, rather than compass headings, is needed to determine the compass course to be steered for a plotted magnetic direction. Here the values have been calculated and tabulated for each 15° magnetic heading.

A boat’s course is the direction in which it is intended to be steered, and its heading is the direction it is pointing at any given moment. Three lines of reference have been established for identifying a course or heading: the direction of true North or the true meridian, the direction of the magnetic meridian, and the direction of the North point of the compass. Thus there are three ways to name a course or heading: TRUE, MAGNETIC, or COMPASS. The three reference lines and the three measurements of direction for one heading are illustrated in Figure 13-16. The above remarks also apply to the measurement of directions for bearings.

Figure 13-16 A course, heading, or bearing, can be named in any one of three systems based on the reference direction used.

Applying Variation & Deviation
To be able to use your compass fully, you must develop an ability to CONVERT directions of one type to the other two quickly and accurately. These directions include headings, courses, and bearings. You use compass courses in steering your boat; you get compass bearings when you take a reading across your compass to a distant aid to navigation or landmark. The process of calculating true directions for plotting on your chart and making entries in the log involves an intermediate step, magnetic directions. These can be used directly if properly identified. Hence, there is need for continual interconversion between the three systems of naming direction.

Variation and deviation can be in the same direction (have the same label), or they may be opposite, and thus partially canceling; see Figure 13-17. (The amount of variation and deviation in this figure is exaggerated for greater clarity.)

Figure 13-17 Variation and deviation are sometimes combined algebraically into a single value, “compass error (CE).” These intentionally exaggerated drawings show the four possible combinations of easterly and westerly variation and deviation.

Although most compass calculations involve a double conversion from true to magnetic to compass or vice versa, it is best to consider single-step computations first.

The terms “correct” and “correcting” are used in easily remembered phrases to help determine whether to add or subtract deviation and variation. This is done by considering true directions as being more “correct” than magnetic, and magnetic as being more “correct” than compass directions. This can be rationalized and easily remembered by observing that true has no errors, magnetic has one error (variation), and compass includes two errors (variation and deviation). Going from compass to magnetic—or magnetic to true—can thus be termed “correcting.” Conversely, the term “uncorrecting” is used when going from true to magnetic or from magnetic to compass.

The basic rule is: When correcting, add easterly errors, which can be shortened to “Correcting add east” and memorized as “C-A-E”; see Example 1.

EXAMPLE 1
Given:The magnetic course is 061°; from the chart, the variation for the area in which we are navigating is 11°E; there is no deviation.

Required: The true course, TC.

Calculation:The conversion is one of correcting because we are going from magnetic to true. The variation is east, so the basic rule is used, Correcting add east. 061° + 11° = 072°
Answer: TC 072°

When “correcting” a magnetic course with easterly variation to a true course, add the variation.

The basic rule is easily altered for application to the three other operations of conversion. Remember to always change two, but only two words in the phrase for any application—never only one, never all three; see Examples 2 and 3.

EXAMPLE 2
Given: The magnetic course is 068°; the variation is 14° W; there is no deviation.

Required:The true course, TC.

Calculation:The conversion is correcting and the variation is westerly. In the basic rule, EAST has been changed to WEST; one word has been changed, another must be. Since this calculation is correcting, “add” must become “subtract.” The rule to be applied here is thus: Correcting subtract west. 068° - 14° = 054°
Answer:TC 054°

When “correcting” a magnetic course with westerly variation to a true course, subtract the variation.

EXAMPLE 3
Given:The course of the boat by its compass is 212°; the deviation for that heading is 5°E.

Required:The magnetic course, MC.

Calculation:The conversion is correcting and the deviation is easterly; the application rule is Correcting add east.

212° + 5° = 217°
Answer: MC 217°

Now consider the reverse process, uncorrecting. We again change two, but only two words to the basic rule and we get: Uncorrecting add west and Uncorrecting subtract east. (Don’t worry about remembering all four rules. It’s easier to remember the basic rule of C-A-E and how to change it to fit the other situations); see Examples 4 and 5.

EXAMPLE 4
Given:The true course is 351° and the variation is 12°W; there is no deviation.

Required:The magnetic course, MC.

Calculation:The conversion is uncorrecting and the variation is westerly. The applicable rule is Uncorrecting add west.

351° + 12° = 363° = 003°
Answer: MC 003°

EXAMPLE 5
Given: The magnetic course is 172° and the deviation for that heading is 7°E.

Required: The compass course, CC.

Calculation: The conversion is uncorrecting and the deviation is easterly. The application rule is Uncorrecting subtract east.

172° - 7° = 165°
Answer: CC 165°

In this example, a true course is “uncorrected” to a compass course, with the error of variation and deviation both westerly.

The same rules apply when two-step conversions are made from true to compass, or from compass to true. It is important to remember that the proper rule must be used for each step separately. It will always be correcting or uncorrecting for both steps, but the addition or subtraction may not be the same, as this is determined individually by the east or west nature of the variation and deviation; see Examples 6 and 7.

EXAMPLE 6
Given: The true course is 088°, the variation is 18°W, and the deviation is 12°W.

Required: The compass course, CC.

Calculation: Both conversions are uncorrecting and both errors are westerly. The rule for both calculations is Uncorrecting add west.

088° + 18° + 12° = 118°
Answer: CC 118°

EXAMPLE 7
Given: The compass bearing is 107°; the variation is 6°E, and the deviation for the heading that the boat is on is 2°W.

Required: The true bearing, TB.

Calculation: Both conversions are correcting; the variation is easterly but the deviation is westerly, so two rules are required: Correcting add east, subtract west.

107° + 6° - 2° = 111°
Answer: TB 111°

The basic rule of Correcting add east and its variations is only one set of memory aids for compass calculations; there are many more. You need to know only one; study several and pick the one that is easiest for you to remember and apply.

Another that is easily memorized and used is Compass least, error east; Compass best, error west. This can be used for conversions such as those shown in the examples above, but it is of particular value when the difference between compass and magnetic is known numerically and it must be decided whether the deviation is east or west. For example: if the magnetic course is known to be 192° but the compass reads only 190°, what is the deviation? The numerical difference is 2° and the compass is “least”—190° is less than 192°—so the error is east; deviation is 2°E. If on the same heading, the compass had read 195°, the compass would then have been “best”—195° being more (better) than 192°—and the deviation would be 3°W.

When considering the variation as the difference between true and magnetic, apply the same phrase but with magnetic substituted for compass—the rhyme is not as good, but the principle is the same. For example: if a true bearing is 310°, and the variation is 4°W, then the magnetic bearing would be “best” (more than the value of the true bearing), 314°. If the variation had been easterly, then the magnetic bearing would have been “least” (or less than the true bearing), 306°.

Many boaters find a pictorial device a better memory aid than a phrase or sentence. Letters are arranged vertically, representing the three ways of naming a direction—true, magnetic, and compass—with the respective differences—variation and deviation—properly placed between them. The arrangement can be memorized in several ways, such as True Virtue Makes Dull Company or in the reverse direction as Can Dead Men Vote Twice.

Figure 13-18 A pictorial “TVMDC” diagram can be an aid in remembering the rules for converting between true, magnetic, and compass directions. Use the up (right) arrow for correcting and the down (left) arrow for uncorrecting.

On each side of the letters, an arrow is drawn to indicate the direction of conversion. Also on each side, the appropriate arithmetic process is indicated for use with easterly or westerly errors; see Figure 13-18. The right side indicates correcting; the left side illustrates uncorrecting. If one memorizes D-A-W for Down add west, then the correct signs can always be added to the letters and arrows. Or use the mnemonics True Virtue Makes Dull Company, Add Whiskey and Can Dead Men Vote Twice At Elections as reminders to "add west" when uncorrecting and to “add east” when correcting.

EXAMPLE 8A
Given: From the chart, the true course is 107° and the variation in the locality is 5°E.

Required: The magnetic course, MC.

Calculation: The conversion is uncorrecting, so use the left side and read down the diagram. The variation is easterly so it is to be subtracted.

107° - 5° = 102°
Answer: MC 102°.

Down	T	107°
Subtract	V	(-) 5°E
East	M	102°

EXAMPLE 8B
Given: For this magnetic heading, the deviation aboard this boat is 4°W.

Required: The compass course, CC.

Calculation: The conversion is still uncorrecting, so read downward on the diagram; continue to use the left side. The deviation is westerly so it must be added.

102° + 4° = 106°
Answer: CC 106°.

Down	M	102°
Add	D	(+) 4°W
West	C	106°

EXAMPLE 9
Conversions such as Examples 8A and 8B could be done in a single operation.

Given: A boat is on compass course (CC) 193°. Its deviation on this heading is 6°E; the variation for the vicinity is 9°W.

Required: The true course, TC.

Calculation: The conversion is correcting, upward on the diagram; use the right side. Easterly deviation is to be added; westerly variation is to be subtracted.

T	190°	Read up
V	(-) 9°W	Add
M	199°	East
D	(+) 6°E	Subtract
C	193°	WEST
Answer: TC 190°

Always Trust Your Compass
If you must depend on your compass for navigation, make a quick check for any objects near it that could cause additional, unmeasured deviations—typical objects that get placed too close to a compass include knives, beer and soft drink cans (if not aluminum), radios, some cameras and microphones, your flashlight, and tools of all sorts. If you have non-boater guests on board, be especially careful that one of them doesn’t unknowingly bring a “forbidden” object into the vicinity of the compass.

If your magnetic situation has been checked and is all clear, then trust your compass to direct you to your destination—it is better than your instincts!

Keep in mind the results of not running an accurate course. If your compass or compass calculations should be in error by only 5 degrees, you will be a full mile off course for every 111/2 miles run (1 km for every 11.4 km). This could be hazardous when making a landfall or a coastal run at night or in poor visibility. Also, running a narrow channel in poor visibility with an inaccurate compass would be dangerous. Note the increasing seriousness of even small errors as shown in Table 13-4.

This table shows the effect of various steering errors—the distance off course after traveling one mile, and the distance traveled before being one mile off course.

Deviation is a property of each individual boat, and its effect must be determined for your own craft. It is the angle between the compass-card axis and the magnetic meridian; refer to Figure 13-15. On board any boat, the direction of compass North is easily learned by a quick glance at the compass card. Not so the direction of the magnetic meridian—magnetic North. Thus it is not possible to visually compare the angle between these two reference directions to determine the deviation on that heading. But the difference between an observed compass bearing and the known correct magnetic direction for that bearing yields that value.

Deviation should always be checked after purchase of a boat, new or used, and before it is taken on its first cruise. It is also desirable that deviation be checked annually, usually at the beginning of a boating season, and after the installation of any new electronic or electrical equipment near the compass. If a compass is replaced, a new deviation check is required even if the new unit appears identical to the old one.

The bearing is sighted over the compass with the boat headed directly toward the object or range. The magnetic direction is taken from the chart. The distant target can be a range of two objects, or it can be a single object if the location of the boat can be fixed precisely, such as close aboard an aid to navigation with the object far enough away that any slight offset of the boat is immaterial.

The range technique is the more accurate and is done as follows. Select two accurately charted, clearly visible, and easily identified objects from the chart. Measure and record the magnetic direction from the nearer to the farther mark. (The best ranges are those in the Light List; their true direction is given precisely, just apply variation to get magnetic.) See Figure 13-19; A and B are two charted objects, with B visible behind A. The magnetic direction of this range is 075°. The boat is run straight toward A on the range, keeping A and B visually in line. While steady on this heading, the compass is read and recorded. In this example it reads 060°; the amount of the difference is the deviation, 15 degrees. Reference to any of the conversion rules makes it clear that the deviation is east. You can set up other ranges and find the deviation on such headings by repeating the bow bearing procedure as above. Use the side of a long object (pier or large building) as a range if its direction can be determined from the chart. Use a buoy and a fixed object, or even two buoys, only if a range of fixed objects is unavailable. Remember that the charted position of a buoy is that of its anchor, and wind or current, or both, may swing it around its charted position. Large ships and tugs sometimes collide with buoys and move them off-station inadvertently in thick weather. Buoys thus give less accurate deviation table results; replace the data as soon as you can with a table made under better conditions.

Figure 13-19 The simplest way to determine deviation on any heading is to locate two objects to use as a range whose magnetic direction can be established from the chart. Head your boat toward this range and read the compass. The difference is the deviation on that heading; if there is no difference, the deviation on this heading is zero.

Steering a visual course from one aid to navigation to another and noting the compass heading will, if the observation is made at the start of each leg, give reasonably good values of deviation when compared with the magnetic direction from the chart; see Figure 13-20. Sighting down the center of a long straight channel defined by aids to navigation or banks close by on either side is another excellent means of getting a compass heading to compare with the chart.

Figure 13-20 Carefully run courses between two charted positions can be used to determine deviation on a number of headings. Deviation will not be the same for reciprocal courses, and each direction must be run individually.

A precise technique for developing a deviation table is to SWING SHIP about a compass by running different headings across a range, recording the compass bearing of the range of each crossing. For example, two prominent charted objects are on a range with a magnetic bearing measuring 095°. On a calm day, the boat Water Witch sails across this range on compass headings successively 15 degrees apart. Using sight vanes and an azimuth circle mounted on the compass her skipper takes a bearing of the range for each crossing, and records and tabulates the results; see Table 13-5A.

Table 13-5A When determining deviation values, the first step is to construct a table as shown above. You already know that you will be reading range bearings at every 15° of compass heading, so list the compass headings in the first column of the table.

The skipper averages the various compass bearings and gets 94.91°; see Table 13-5B. He notes that this is very close to the magnetic bearing of the range from the chart and decides he has a good set of data.

Table 13-5B For the second step, use a pelorus (see “Pelorus” later in this chapter) to obtain range bearings for every compass course heading, and record them in column two. Then find the average of your range bearings by adding all the values and dividing by the number of values; round off to the nearest whole degree. This is what the range bearing would have been if the compass had no deviation. (These are illustrative values; they will be different for your boat.)

The first and last columns of this table now constitute a deviation table for Water Witch provided no changes are made in the magnetic environment of the compass; see Table 13-5C. Note also that the table’s deviations are in terms of compass headings. These are not, of course, the same as magnetic headings; when on a heading of 090° magnetic the boat’s bow points in a direction quite different from when she heads 090° by compass. This leads to a problem. The skipper determines from the chart his magnetic course; he then wants to know what deviation to apply to get the compass course he should steer. Use of Table 13-5C for that purpose requires time-consuming trial-and-error steps. To avoid this, he makes a second table listing the deviations in terms of magnetic headings; refer to Table 13-3. This can be done by trial and error. Deviation can also be determined by using a GPS receiver.

Table 13-5C The third step is to determine the difference between each range bearing (column 2) and the calculated magnetic bearing (column 3). This difference, entered in column 4, is the deviation. If the compass bearing is larger than the magnetic bearing, the deviation is west; if it is smaller, the deviation is east. From this table, another table in terms of magnetic headings can be calculated; refer to Table 13-3.

It can be assumed that a boat’s compass will have deviation errors. The area available for a control station on most craft, especially motorboats, is so limited that it is impossible to have magnetically undesirable objects at a safe distance from the compass. It is possible to live with known values of deviation provided they are not too large—deviations much over about 6 degrees can cause trouble in rough waters. It is, however, much better if the compass is COMPENSATED (also referred to as ADJUSTED) so that the deviation is reduced to zero, or nearly so, on as many headings as possible. This procedure is within the capabilities of the average skipper.

Two basic techniques are used to adjust compasses; both use one or two pairs of small COMPENSATING MAGNETS. In one instance the magnets are internal to the compass case or binnacle; in the other they are external and are mounted near the compass location. The objective of both methods is the same—to provide weak magnetic fields that cancel out the disturbing influences.

Compensation consists of adjusting the effect of each of these magnets to achieve the counterbalances of their fields with the fields of the disturbing influences.

Using Internal Compensators
Most modern boat compasses now have internal compensating magnets. The two magnets, or pairs of magnets, are installed at right angles to each other. Usually there are two slotted screw heads at the edge of the compass; one marked E-W, and the other N-S; refer to Figure 13-02.

As discussed previously, the internal compensators should be at zero effect before the compensation procedures are begun. Check also that the line from the center of the compass through the lubber’s line is exactly parallel to your boat’s centerline. Select an object at least one-half mile (0.9 km) distant. With your boat held absolutely motionless, sight on that object both down the centerline and then over the compass; the sight lines should be the same.

Have a nonmagnetic screwdriver available to adjust the screws of the compensating magnets. (You can also use a dime or a piece of heavy sheet copper or brass, or a brass machine screw filed down to a flat point.)

Store each magnetic article of the boat’s gear in its regular place, put the windshield wiper blades in their normal “off” position, and keep no magnetic material near the compass. Steel eyeglass frames, steel partial dentures, or even the steel grommet in a yachting cap, if brought within a few inches of the compass, might cause deviation; someone may be working that close to the compass during adjustment.

Disposable Buoys Several disposable buoys will be needed. Excellent ones can be made of cardboard milk cartons or plastic bleach bottles ballasted with some sand or dirt. Newspapers wadded into a ball about the size of a basketball and tied with string will work if they are weighted with a small heavy object such as a large bolt or old spark plug at the end of three or four feet (0.9–1.2 m) of string; wire or rope should not be used for such a buoy; see Figure 13-21.

Figure 13-21 A disposable buoy for use in maneuvering to determine compass deviation can be made from either a ballasted plastic container or a ball of wadded newspapers attached to a small weight. All such buoys should be picked up and properly disposed of after the operation is completed.

The essential element of compensating a compass by running reciprocal courses is achieving accurate reversal, putting and holding the boat on a course over the bottom that is exactly the reciprocal of the original unknown magnetic heading. This is not as difficult as might be expected.

Establishing the Courses Between any two visible marks, or on a range, reversing the magnetic heading is obviously easy. In the procedures of compensation, however, there is need to run at least the four cardinal magnetic directions: East and West, North and South. Seldom will you find in your local waters natural marks or aids to navigation on these exact magnetic headings. If you are fortunate, you may find special ranges on these headings that have been established for compass adjustment purposes. More likely, though, you must make your own temporary basic courses.

The basic technique is quite simple—departing from a fixed mark (a buoy will do, but a day-beacon or light is better), run on a steady compass heading until ready to reverse your course. Drop a disposable buoy creating the desired range. (With many high-speed craft, the path through the water may be distinctive enough that a marker will not be required.) Next, execute a “buttonhook” turn (known technically as a Williamson turn; Figure 12-07,)—line up the disposable buoy with the object marking the original departure point, steady on this range visually, and ignoring the compass, head for the initial point. Run down the disposable buoy, and continue back toward the departure point; see Figure 13-22. Make the turn as tightly as possible, keeping the buoy in sight, so as to complete the turn before the buoy has had time to drift from the spot at which it was dropped. Choose a right or left turn, whichever can be made tighter if there is any difference; swing enough to the opposite side before making the main turn so that when the main turn is completed, you can pick up the desired reciprocal course smoothly without overshooting and having to apply opposite rudder. This maneuver is not difficult to learn; all that is required is a few trials and then some practice. Under some circumstances, as discussed in Chapter 12, it is also the most accurate procedure for returning to the spot where a person or object has been lost overboard.

This maneuver will put the boat accurately on a reciprocal course, providing that the buoy remained where it was dropped and that no wind or current set the boat to either side of her heading on the outward run from the starting point.

Compensation will be inaccurate if the boat has moved crabwise, heading one way but actually going another. The various courses of compass compensation procedures must therefore be run when wind and current conditions will produce no leeway.

The skipper who hesitates to run directly over the disposable buoy should take it very close alongside, touching it if possible. On a range of ½ nautical mile, when the buoy is 10 feet (3m) to one side of the boat’s centerline, the course error will be only 11’, less than ¹/₅ of a degree.

Figure 13-22 With a little practice a skipper can learn to make a buttonhook, or Williamson, turn of 180° and come back directly over the spot where the turn was started. Swing out far enough initially on the opposite side of the turn so that the reciprocal course can be picked up without overshooting it. (Sketch not to scale.)

As noted above, compensation must be accomplished when wind and current will not set the boat to either side of the intended course; the winds and seas must also be such that accurate steering is possible. Adjustments are made first on the East-West cardinal headings.

First steer 090° (or 270°) by compass from the chosen departure mark, holding rigorously to this course by the compass for at least a half mile; a mile would be better, but you must be able to see the starting mark. It makes no difference whether the initial run is toward the east or west; this can be determined by the available water area and object used for a departure mark. Have someone drop a disposable buoy over the stern, execute the turn described above, and run the reciprocal course; refer to Figure 13-22. Run this course not by the compass, but by heading for the original departure point. Read the compass and record the figures.

If the compass reading is now 270° (or 090°), there is no deviation on east or west headings and compensation is not needed; the E-W internal compensator should not be touched.

If the compass reading for a return to the starting point is not 270° (or 090°), compensation is required. Use your nonmagnetic screwdriver to remove half the difference between 270° (090°) and the observed reading. Halving the difference is based on the assumption that the deviation of reciprocal courses is equal and opposite. This may not be exactly so, but it serves as a good initial approach.

Assume, for example, that the compass reading was 290°; then one-half of the 20-degree difference, or 10 degrees, is to be removed while the reciprocal course is carefully run. Turn the E-W adjusting screw, and watch the compass closely. If the compass reading increases rather than decreases, the screw has been turned in the wrong direction (there is no way to tell in advance which is the correct direction); turn the screw in the opposite direction. Move the screw slowly until the compass reading is 280° while the boat is exactly on the reciprocal course.

Return to the starting mark and make another east-west run. Head out again on a compass course of 090° (or 270°), the track through the water will not be the same as for the first run; drop a new disposable buoy, turn, and line up for the reciprocal course back to the point of departure. If the compass reading is not now 270° (or 090°)—it should be close—again remove half the difference by turning the screw of the compensating magnets.

Repeat these east-west runs until the deviation is eliminated or reduced to the smallest possible amount on these headings. Do not touch the E-W adjusting screw again after it is finally set.

Follow a similar procedure on north-south headings. Run from a starting point—it can be the same as for east-west runs, or it can be a different one as required by the available water area—on a compass heading of 000° (or 180°) by compass, drop a disposable buoy, make the turn, and head back visually toward the departure point. Remove half of the error by adjusting the screw marked N-S.

When the north-south compensation has been completed, make a quick check for any east-west deviation; it should not have changed, but it’s worth checking. The deviation on all cardinal headings should now be zero, or as near zero as is attainable in this boat.

You can determine any deviations remaining on the intercardinal headings—NE, SE, SW, and NW—by any of the methods discussed earlier in this chapter, or you can estimate them by making reciprocal runs on these headings. In this method, the deviation will be half the difference between the outward course steered by compass and the reading of the compass when the turn has been made and the craft pointed back to the starting point. Be sure to note the direction of the deviation, east or west. Do not touch the N-S or E-W adjustment screws.

Intercardinal heading errors can be especially troublesome on craft with a steel hull, or if there is a larger mass of iron near the compass. In such situations, the services of a professional compass adjuster should be obtained.

Failure to Achieve Compensation
When compensation cannot be achieved satisfactorily, you undoubtedly have an undetected magnetic field on your boat. Prime suspects include the steering mechanism and the engine throttles and shift levers. Test them with a thin machinist’s steel feeler gage a few thousandths of an inch thick (0.004 inch, or 0.1 mm, is ideal). Make sure the metal of the gauge is not magnetized.

Touch one end of the steel feeler gauge to the part being tested for magnetism and gently pull it away. If it tends to stick to the part, the latter is magnetized. Test all around thoroughly. Open the wheel housing or look under the instrument panel; test all metal parts found.

If a magnet is located, demagnetization will be required. You may need professional advice and assistance, but try your own hand at the job first.

The Final Deviation Table
When the compass has been adjusted to the greatest possible extent, make a complete deviation check and prepare a table for any deviations found; use any of the methods described earlier.

Make this check under all conditions in which the compass might be used; check with the navigation lights on and off (they may be distant from the compass, but wires running to their switch will be nearby), with the windshield wipers operating and not operating, and with the depth sounder, radio, radar, and chartplotter each on and off, one at a time. Look also for other electrical equipment whose operation could affect the compass. Remember to check on two cardinal headings 90 degrees apart. Most of this equipment will not disturb the compass, but some could, and you must be aware of it. If any checked items have to be on when the compass is in use, you may need to have more than one deviation table. You may also sometimes have to turn on certain equipment, whether needed or not, to re-create the magnetic environment that existed when the deviation table was prepared. A sailboat may need different deviation tables for port and starboard tacks because of different heeling errors. If two or more deviation tables are necessary, it is a good idea to use different color paper for each.

Before preparing the final deviation table, plot a simple graph of the values to be used. Lay out a horizontal baseline for compass headings from 0° to 360°; plot easterly deviation vertically above the baseline and westerly deviations below that line at a suitable scale; see Figure 13-23. When all values of deviation have been plotted, draw a smooth curve through the points, but don’t expect all values to be exactly on that line, since the deviations have been measured only to the nearest whole degree. You should be able to get a smooth curve through or near all of the points; if any value lies significantly off the curve, it is probably in error and should be rechecked. If you can draw a smooth curve, but its center axis does not coincide with the horizontal base line for 0° deviation, the lubber’s line may be out of alignment; recheck carefully.

Figure 13-23 When you have all your data, and before making a final deviation table, make a simple plot of the values to spot any that are not consistent with the others. (Data deviations have been exaggerated for emphasis.)

The final deviation table can be alternatively prepared as a direct-reading table of critical values; Table 13-6. Whichever style of deviation table you choose to prepare, it can be used with confidence if care is taken to see that the magnetic environment is not altered. Check it each year, even though you think no changes have been made that would upset the deviation table.

Table 13-6 A deviation table—shown here in two formats—permits the conversion of directions from magnetic to compass, or from compass to magnetic. A conventional table, upper, requires interpolation to get values between the lines of data. For example: for a magnetic heading of 10°, the deviation lies between 7°E for 000° and 1°E for 015°; interpolation gives 3°E. With the critical values table, lower, you can go directly to the value needed; no interpolation is required. (Deviation values have been exaggerated for clarity; on an actual vessel, they would have been reduced by compensation.)

In addition to protecting the instrument from direct sunlight when not in use, compass maintenance has two aspects. The first is the preservation of the magnetic environment that surrounds it. Except for occasional testing, no piece of iron or steel should be brought or installed near it, lest it cause unknown deviations. The second aspect of maintenance consists mainly of getting to know your compass. Watch how it swings. Check that its readings are consistent on frequently run courses. Note if it appears to become sluggish and above all if it becomes erratic—these conditions warn you of undetected disturbances or a damaged pivot bearing.

Test for a damaged bearing or undue pivot friction by deflecting the card two or three degrees with a small magnet or piece of iron. If the card does not return to its former position, the compass should be removed from the boat and taken to a reputable shop for tests and, probably, repair.

As mentioned, a bubble can be removed by adding some liquid, but the liquid must be the same as that with which the compass is filled. This is best left to a compass repair shop.

Lightning strikes on board or nearby may change the craft’s magnetic field; electric welding can have the same effect. After exposure to either of these, check deviations.

In northern waters, care should be taken that metal objects on the boat have not acquired a magnetic field over the winter storage period; a newly magnetized piece of metal could, of course, confuse the compass and require readjustment of the compensators or the making of a new deviation table.

Do read and follow the manufacturer’s instructions for winter storage if the boat is taken out of regular service. Doing so may add years to the compass’s useful life.

The following sections on various topics on magnetism and compasses are generally beyond the “need-to-know” of the average skipper. Some, however, will be of value to those with special problems, and many others will be of interest to those skippers who want to have a bit more than the minimum knowledge.

Magnetism
Contrary to some popular ideas, the location of the earth’s MAGNETIC POLES is of little interest to the navigator. (Should he ever get to their vicinity, he would then have to rely for directional information on some instrument other than his magnetic compass.)

Variation is not the angle between the direction to the true and magnetic poles. This concept, a fictitious visualization useful in helping students to realize that true and magnetic north are usually different directions, has been accepted by many as truth. This must be recognized as merely a learning aid, not a factual representation. The magnetic pole does not control the compass. The controlling force is the earth’s magnetic field. The compass magnet does not necessarily point in the precise direction of the magnetic pole.

Actually, two specific points on the earth’s surface that are the magnetic poles do not exist. There are north and south MAGNETIC POLAR AREAS containing many apparent magnetic poles, places where a dip needle would stand vertically. If this seems strange, consider the problem of pinpointing the precise point in the end of a bar magnet that is the pole.

For scientific purposes, approximate positions of the theoretical North and South Magnetic Poles have been computed from a large number of continuing observations made over the world for many years. They are not diametrically opposite each other as are the geographic poles. The magnetic poles do not stay in the same place. The north magnetic pole, for example, is moving approximately north-northwest at 30 nautical miles (55 km) per year, and in 2017 it was at 86.5°N, 172.6°W. The south magnetic pole was at 64.2°S, 136.2°W. A magnetic compass at 87°N, 172.6°W would show north in a direction truly southward of the compass.

Magnetic force lines dip downward toward a point far below the surface except near the equator, creating a DIP ERROR that increases with latitude. Compasses sold for use north of the equator are balanced to offset this error. Compasses balanced for use in southern latitudes will provide improved performance in these waters.

On smaller scale charts covering larger areas, variation may be shown by ISOGONIC LINES; every point on such a line has the same variation. Each line is labeled with the amount and direction of variation and the date; see Figure 13-24. The line joining all points having zero variation is called the AGONIC LINE.

Figure 13-24 Offshore charts of large areas often show magnetic variation by a series of broken magenta lines for each degree of variation. Each such isogonic line is labeled with its variation and the year for which it applies.

Masses of iron or related metals that do not show any magnetic properties under usual conditions will acquire INDUCED MAGNETISM when brought near a magnet and into its field. The polarity of an induced pole is opposite to that of the nearest pole of the magnet that caused it. When the inducing field is removed, any magnetism that remains is termed RESIDUAL. If the object retains magnetism for a long period of time without appreciable reduction in strength, this is PERMANENT MAGNETISM.

Although the copper of electrical wires is nonmagnetic material, currents flowing in them produce magnetic fields. It is this effect that makes possible electrical motors, generators, relays, electromagnets, etc. A direct current in a wire results in a field that can affect a vessel’s compass. (Alternating-current electricity also produces magnetic fields, but not of the type to disturb a compass.) The polarity of a field around a wire carrying DC is related to the direction of the current, a fact that can be used to advantage. The two wires carrying power to any load have currents in the opposite directions, thus producing opposing fields. If these wires are twisted together, the field around one wire cancels out the field produced by the other, and the net effect on the compass will be zero, or nearly so.

Always use two wires, never use a common ground for return current, and always twist pairs of wires that pass near a compass. Don’t fail, however, also to make an actual check by observing the compass closely as the electrical circuit is switched on and off; make this test on two cardinal headings 90 degrees apart.

If tests with a thin strip of steel have shown that some metal object at the control station is magnetized, it may be possible to DEMAGNETIZE it with a device used by electronic repair shops, such as a degaussing coil for TV sets or a magnetic-tape bulk eraser. If the magnetized object cannot be removed from the boat for undertaking this effort, it is mandatory that the compass be taken off lest the effectiveness of its magnets be destroyed.

The demagnetizing device is operated from 120-volt shore power; it uses alternating currents to produce magnetic fields that reverse at the 60 Hz frequency. Hold the device about a foot (0.3 m) from the target object and turn on the power; do not turn off the device until the full process has been carried out. Advance the demagnetizer slowly toward the object until it touches; move it around slowly over all the accessible surfaces of the object. Remove the device slowly away until it is at least the starting distance away and then turn it off. (If power to the demagnetizing device is interrupted at any time during the process, start over again and repeat all steps.) Take care not to demagnetize anything that is properly magnetic, such as electrical instruments or radio speakers.

Now test the disturbing object again with the thin strip of steel. If it has been possible to reach all of the object, it should be demagnetized. Often, however, complete access is not possible, and the object may have to be disassembled and/or removed from the boat for complete demagnetization.

After the demagnetization process is completed, remount the compass, and, in all cases, make an entirely new deviation table.

Aboard a boat, the compass is subject to two magnetic forces: that of the earth and that of objects on the craft. If the boat were absolutely free of magnetism—no permanent magnets, no material subject to acquiring induced magnetism, and no fields from electrical currents—there would be no deviation on any heading; see Figure 13-25. The earth’s force depends upon geographic locations; the compasses of all vessels in the same harbor are subject to the same variation. The deviations aboard these ships and boats will be dissimilar because of the differences in the magnetic characteristics of each vessel. Thus if all of them put in identical magnetic headings, their compasses would differ, except in the remote circumstance that each compass was completely compensated.

DEVIATION, it will be remembered, varies with the craft’s heading because of magnetic material on board. A few examples may make this clearer. Refer again to Figure 13-25, the nonmagnetic boat. No heading of this craft affects its compass card; its North point always lies on the magnetic meridian; the compass has no deviation. As the boat changes heading, the lubber’s line, turning with the bow, indicates the magnetic heading on the card.

Now put aboard the craft, in the vicinity of the compass, items such as radios, electronic depth sounders, electrical gauges, and other instruments. Also on the boat, but usually farther from the compass, are metallic masses such as anchors, chain, fuel and water tanks, and other magnetic items, including the engine (the engine is by far the largest mass of magnetizable material, but it is relatively distant from the compass). The boat has now acquired a magnetic character of its own; in addition to any permanent magnetism of some objects, there are now masses of unmagnetized material that can affect the compass by acquiring induced magnetism.

The effects on the compass of permanent magnetism and induced magnetism are different and must be considered separately.

Figure 13-25 If there are no magnetic disturbances on the boat to disturb its compass—which is rarely the case—there is no deviation on any heading.

On typical boats of wood or fiberglass, the effect of permanent magnetism is by far the greater. To visualize this, let us assume that the same net effect would be exerted by a single permanent magnet located aft of the compass and slightly askew of the boat’s centerline; see Figure 13-26. This representation by an imaginary magnet is quite applicable; it simulates conditions found on most boats other than those with a steel hull. (The exact position of the imaginary magnet, its polarity, strength, and angle of skew are not important in this illustration of principles.)

Two forces, that of the field of the earth and that of the field of the imaginary magnet, now affect the compass; its poles are attracted or repelled in accordance with the basic law of magnetism. Without the influence of the imaginary magnet, the compass magnets and card will line up with the magnet meridian. The 0° mark of the card is an N (north-seeking) pole, and the 180° mark is at an S pole. Now, in addition to the effect of the earth’s magnet poles on them, the N and S poles of the compass magnets will be attracted or repelled by the S pole of the imaginary magnet (the pole nearer the compass in these examples). The result is easterly deviation on some headings, westerly on others, and zero on those where the sign of the deviation changes from east to west, or from west to east.

Consider a typical boat, Morning Star, headed north (000°) magnetic; see Figure 13-26A. The S pole of the compass, 180° on the card, is repelled by the S pole of the imaginary magnet. The result is that the card is rotated counterclockwise; the compass reading is increased, 006° in this instance, and there is westerly deviation.

On a south magnetic heading, as in Figure 13-26C, the N pole of the compass magnet, 000° on the card, is nearer the S pole of the imaginary magnet and is attracted by it. The card is deflected clockwise and the compass reading decreases, to 174° and deviation is easterly.

If the magnetic heading of Morning Star is to the east, as in Figure 13-26B, the N pole of the compass magnet is the nearer pole to the S pole of the imaginary magnet and it is attracted. This results in a counterclockwise rotation of the compass card, giving westerly deviation. On a west magnetic heading, the deflection of the card is clockwise, for easterly deviation. (It is a good exercise to diagram this situation for yourself.)

In the above examples, the deviation is westerly on an east magnetic heading, but easterly on a south magnetic heading. Thus there must be some intermediate heading where the deviation passes through zero as the sign changes from west to east. This situation is approximated in Figure 13-26D. There is still attraction of the N pole of the compass by the S pole of the imaginary magnet, but on this heading the compass card has its N pole as close to the S pole of the magnet as it can get. Thus there is no deflection of the card in either direction, no deviation.

As has been illustrated above, deviation resulting from permanent magnetism will normally swing through one cycle of west to east and back to west in 360°, with two headings having zero deviation.

Do not assume that the location, polarity, and angle of skew of the imaginary magnet would be the same on any other boat as in the above examples, which are for the sole purpose of illustrating principles. A change of one or more of these factors would result in an entirely different set of deviations.

Figure 13-26 On a north magnetic heading, A, the S pole of the compass is repelled by the S pole of the imaginary magnet representing the net effect of all magnetic influences on the boat. The card is deflected counterclockwise; there is westerly deviation. On an east heading, B, the N pole of the compass is nearer the S pole of the imaginary magnet, and is attracted toward it. The deflection is still counterclockwise; the deviation is still westerly. On a south magnetic heading, C, the N pole of the compass is nearest the S pole of the imaginary magnet and is attracted to it. The card is deflected clockwise; the deviation is easterly. On an intermediate heading, D, the N pole of the compass is attracted to the S pole of the imaginary magnet, but is already as close to it as it can get. There is no deflection, and thus no deviation on this heading.

The effect of induced magnetism on a compass can be visualized in a series of examples much like the above with an imaginary mass of magnetizable material. In this case, the disturbing pole would be the one induced in that mass, and its sign would change as determined by the changing sign of the nearer pole of the inducing magnet. The sign of the deviation would change four times in 360 degrees, with four headings having zero deviation.

If the effects of permanent and induced magnetism are combined, a complex situation results; fortunately, this is unlikely for most small craft.

Deviation can change with changes in MAGNETIC LATITUDE of a vessel. The strengths of most magnetic influences on a boat are unchanged at different geographic positions, but the horizontal component of the earth’s field lessens in higher latitudes. Hence the interaction of these two forces, the cause of deviation, changes with significant change in magnetic latitude.

Compass Calculations
COMPASS ERROR (CE) is sometimes used in compass calculations as a specific term. It is the algebraic sum of the variation and deviation. Because variation depends on geographic location, and the deviation upon the craft’s heading, there are various possible combinations of these two quantities; refer to Figure 13-17. Similarly to variation and deviation, it is expressed in degrees and direction, for example 5°E or 7°W; in boating, it is given to the nearest whole degree.

This is a term used for convenience. The compass is not actually in error; it is operating according to the forces that control its behavior. Compass error is merely the angle between true north and compass north.

The process of determining intermediate values between tabular entries is called INTERPOLATION. It is most easily explained by several examples. Consider the following extract from a deviation table:

Assume that we need the deviation for a compass heading of 100°. The interval between tabular entries is 15 degrees; the difference between deviation values is 3 degrees. Since 100° is ¹⁰/₁₅ of the way between 090°and 105°, we multiply the value difference by that fraction: 3°x ¹⁰/₁₅ = 2°. Since the deviation is decreasing, this is subtracted: 7°E - 2°= 5°E, the deviation for 100°.

If the values are not so even as above, an answer with a fraction will result. If we had desired the deviation for 098°, the fraction would have been ⁸/₁₅, and the result 3°x ⁸/₁₅ = 1.6°; then 7°- 1.6 = 5.4°E; but for boat use deviation is not used more precisely than the nearest whole degree and thus the deviation for 098°would be recorded and used as 5°E. If the fractional value is 0.5°, use the nearest even, not odd, degree.

Caution must be used when the two values of deviation are of opposite name. The value difference between 3°W and 1°E is four degrees, not two.

Critical Values Tables
A skipper will need deviation values in terms of both compass headings (for correcting calculations) and magnetic headings (for uncorrecting). A simple way to combine both of these, and have an easier-to-use table as well, is to prepare a direct-reading table of CRITICAL VALUES. Such a table is prepared in basic terms of deviations rather than headings. There is a line in the table for each whole degree of deviation; opposite this is listed on one side the range of compass headings for which that value of deviation is applicable; on the other side is the range of magnetic headings that have that value of deviation. See the below partial example and Table 13-6, where different numbers are used.

Special Compensation Situations
The procedures of compensation described earlier in this chapter will fit most, but not all, recreational boats and their compasses. Special situations are discussed next.

If the boat’s compass (or binnacle) does not have internal compensating magnets, it still can be adjusted to near zero deviation if there is space to place external magnets where they are needed.

Two compensating magnets will be needed. Each is an encased permanent magnet having two holes in the case for screw fastening. The ends of the magnets will either be marked N and S or be colored red on the north end and blue on the south. Masking tape is good for temporary fastening; be sure that the screws used for final fastening are nonmagnetic. Carefully mark the longitudinal center of each magnet. In the areas where the external magnets will be placed, carefully mark out in chalk two lines through the compass center, one for fore-and-aft and one athwartships; see Figure 13-27.

Figure 13-27 Before installing external compensating magnets, chalk in two lines, shown in red above, through the center of the compass mounting location. The magnets must always be placed so that their centers are on one of these lines. Magnets are placed forward or aft of the compass and on one side, above right. When final placement has been determined, they are fastened down with nonmagnetic tacks or screws.

General Procedures The compensation process consists of making outward runs on cardinal headings by compass, dropping a disposable buoy, turning, and heading back toward the point of departure. All this is done in the same manner as was done for internal compensators. Keep all external magnets far from the compass until each one is to be used.

Adjustment on E-W Headings If compensation is shown to be needed—for example, the compass reads 290° on the reciprocal of a 090° outward run—the correction is made by placing an external compensating magnet in a fore-and-aft position centered on an athwartship chalked line. This can be placed on either side of the compass in position A or A’ of Figure 13-27, right. Should the compass reading now become more than 290°, the compensation is increasing the deviation; move the magnet to the other side of the compass or turn it end-for-end (but don’t do both). The card will then move in the desired direction, toward 270°.

Should the card stop moving before 280° (the reading for removal of half of the error), the compensator, in a position such as at D, is too far from the compass. On the other hand, if the card moves past 280°, the compensator is too close to the compass, as at C, and must be moved farther away. Move the external magnet closer to or farther from the compass until the desired reading is attained. Now tape the compensator down—no screws yet. Do not let it get off the centerline as at E. As with internal compensators, a second E-W run is then made, or perhaps more, until zero deviation is achieved.

Adjustments on N-S Headings After the turn back on a north or south run, one-half of any deviation is adjusted out by placing a compensator in an athwartship position centered on the fore-and-aft line, such as at B in Figure 13-27, right. If there is no room for the magnet in front of the compass, place it aft of the instrument in the vicinity of B’. Move it closer to the compass or farther away—and reverse ends if needed—in the same manner as on east-west runs.

When, after sufficient runs, the deviation is zero, or reduced to a minimum, screw down the magnets (remember, nonmagnetic screws) with care that their position is not shifted. It is wise to make one more E-W and then N-S run after the magnets have been secured in place as a final test of your work.

Compensation by Shadow Pin
A method of sailing reciprocal courses without setting up temporary ranges with disposable buoys uses a vertical shadow pin in the center of a horizontal disc graduated similar to a compass card; the disc should be gimbal mounted.

As the craft heads east (or west) by the compass, the disc is rotated by hand so that the shadow of the pin caused by the sun falls on the 090°(270°) mark. The turn is made and the boat is steadied on such a heading that the shadow falls on the opposite side of the disc, exactly on the 270°(090°) mark. If the compass now reads 270°(090°), there is no deviation on E-W headings. If not 270°(090°), the previously described procedures are followed for adjustment. A similar technique is used for N-S headings.

This procedure has advantages. The boat need not run far on any heading. No departure mark or disposable buoy is required. Neither the wind nor current will affect the result. But there is one complicating factor—the sun and its shadow do not stand still. Regardless of how steady the boat is being held on a heading, the shadow is moving. Time becomes a factor in the execution of the procedure; compensators must be adjusted before the shadow moves enough to upset the process, taking the boat off a true reciprocal heading.

Compensation on Sailboats
Sailing craft, heeling out of a horizontal trim, frequently require HEELING MAGNETS below the compass to reduce their deviation. Small-craft skippers can make a rough compensation, but an exact adjustment is a job for a professional compass adjuster.

Place your sailboat on a N-S heading and heel her slightly, maintaining a constant heading. Place a correction magnet vertically under the compass so as to eliminate only the deviation induced by heeling; reverse the ends of the magnet if necessary. Even if you are unable to make this adjustment, try heeling your sailboat to see the effect on your compass.

Compensation on Steel Boats
Steel hulls may present problems whose solution may require installation of soft iron QUADRANTAL SPHERES; see Figure 13-28. Another solution might be the use of FLINDERS BARS, or possibly heeling magnets. All this is for the professional adjuster or for the skipper who has achieved professional competence.

Iron and steel vessels are subject to changes in deviation upon large changes in latitude. This must be considered when cruising in such craft.

Figure 13-28 Vessels, including boats, with steel hulls present special cases for compass compensation. It will normally be necessary to mount two quadrantal spheres of soft iron on the binnacle and then adjust them very carefully—this is work for a professional compass adjuster.

Several other direction instruments will be of general interest to boatmen, although few skippers will have more than one or two of them.

Hand-Bearing Compass
A small compass (some equipped with a handle) designed to be easily held in front of one’s face is termed a HAND-BEARING COMPASS; see Chapter 16. This instrument is normally used for taking bearings, especially those that cannot be conveniently sighted across the steering compass.

A more sophisticated (and more expensive) hand-bearing compass combines a 5 x 30 monocular, a digital compass, an electronic rangefinder, and a chronometer in a single lightweight, handheld instrument. It can show the distance to an object as well as its compass bearing and the time of the observation; up to nine bearings can be recorded with the push of a button.

Tell-Tale Compass
A TELL-TALE COMPASS is one located belowdecks, usually overhead above the skipper’s bunk. These go back to the days when the captain used one to keep an eye on the course steered when he was not on deck. Today, they can serve the same purpose on long passages, but the more likely use is to watch out for wind and current shifts when riding at anchor for the night.

A tell-tale compass is an ordinary magnetic compass built upside-down so that the bottom of the card can be read. Some models have markings on both sides of their cards so that they can be used equally well in normal fashion or inverted as a tell-tale mounted overhead.

Pelorus
A PELORUS is an instrument having sight vanes and a compass-like card, either of which can be clamped in a fixed position. Sometimes referred to as a DUMB COMPASS, it is used to take bearings when it is not possible to do so across the steering compass. The pelorus is placed in a location suitable for observation; its zero point is adjusted to either compass North or the boat’s current heading, depending upon whether compass or relative bearings are desired.

Gyrocompass
Ships and the largest of yachts will normally be fitted with a GYROCOMPASS, a complex device that senses changes in the direction of the vessel. One type of gyrocompass holds a preset orientation, but gradually drifts off; it is independent of the earth’s magnetic field. Another, more complex type orients itself with respect to the earth’s rotation. This is the ultimate device for measuring direction at sea, yielding true rather than magnetic directions. Such units, however, are generally too large and expensive for small craft. But even gyrocompasses are not perfect; they must occasionally be reset, and adjustments are required for changes in latitude. Interested readers are referred to Publication No. 9—Bowditch—of the National Geospatial-Intelligence Agency, or to Dutton’s Nautical Navigation, published by the U.S. Naval Institute.

The signals that GPS receivers use to determine position can also be used by a GPS COMPASS (also called a SATELLITE COMPASS) to determine the heading of a vessel. This can be done whether or not the vessel is underway. Ordinary GPS receivers can display course made good (also called course over the ground), but this is not the same as an instantaneous heading and cannot be determined when the vessel is stationary.

In a satellite compass, two or more GPS antennas are mounted on a fixed baseline, either in a small dome or in the open. The GPS signals from satellites arrive at these antennas at the same time only for a specific orientation; at other times, there is a difference in the phase of the signals. Sophisticated computer technology uses the phase difference to determine the direction that the signals are coming from, and hence the heading of the vessel. As only phase information is used, these signals need not be differentially corrected. Three-axis solid-state gyro rate sensors ensure the continuity of heading information during periods of signal dropout for any reason, such as passing under a bridge. Some models use a third GPS antenna to reduce the negative effects of vessel motion in pitch, roll, and yaw. Headings are displayed to tenths of a degree; accuracy is typically better than 0.5°. The output of a GPS compass is in true headings; these can be changed to magnetic values by the introduction of variation information.

GPS compasses are not affected by the vessel’s speed, acceleration, latitude, or magnetic conditions on board. The heading information can be used by autopilots, course plotters, and radar displays. GPS compasses also provide the output information of an ordinary GPS receiver, and benefit from having multiple sources of data.

HIGH-TECH COMPENSATION—USING GPS FIXES

If your craft is equipped with a GPS receiver and you have access to a body of water that will permit straight runs of a mile or two on cardinal headings, you can adjust your compass for deviation using the following procedures.

1. Select a starting point and establish that as a GPS fix; designate this as Waypoint “A.”

2. From “A” run 090°(or 270°) by your magnetic compass for a distance of about one mile if you are using GPS readings corrected by ground or satellite signals. If you are not using differential corrections, make your run about two miles long. Most GPS receiver models now are accurate to within a few yards or meters.

3. At the end of this run, record your GPS position as Waypoint “B”; drop a disposable marker, if necessary, and make a sharp turnaround. Read from your GPS display the magnetic bearing of Waypoint “A”; if this is not 180°from your outbound compass course, adjust the E-W compass compensator to remove one-half of the difference. Using the direction of “A” (GPS bearing), make the run back to “A”; an autopilot can be used if available. (Do not use course-made-good (course-over-the-ground) information from the GPS receiver; it will not give as good results as using GPS bearings to established waypoints.)

4. At “A,” turn around and read from your GPS the bearing of “B”; if your compass does not read the same as the GPS bearing, again use the compass compensator to remove one-half of the difference.

5. It should not be necessary to repeat the above steps to eliminate the E-W deviation error, or reduce it to the minimum possible, but a similar second run can be done as a check.

6. Follow similar procedures to eliminate, or reduce to a minimum, the N-S deviation.

7. Follow Steps 1 through 3 on intercardinal points—NE, SE, SW, NW (in any sequence) to measure the deviation—the difference between the GPS bearing and the compass reading. Do not change the compensators; record the deviation. If deviation is greater than 3°, consult a professional compass adjuster.

It will be noted that the above procedures are much like those described previously but do not require physical objects for steering objectives. Other actions, such as aligning the compass with the craft’s centerline, temporarily fastening it down, and permanently securing it in place, are the same with this procedure as covered earlier.

Magnetic	Deviation	Compass
133°-144°	0°	133°-145°
145°-153°	1°E	146°-153°
154°-163°	2°E	154°-162°
164°-176°	3°E	163°-174°
177°-187°	4°E	175°-185°