APPENDIX

STABILITY IN SAILING YACHTS

The question of stability in yachts should always be seen in light of the fact that very few are actually ‘knocked down’ while cruising in coastal waters. Some of us venture further afield, however, and become involved with conditions from which, for us at the time, there can be no escape. Even for coastal sailors, the chance of encountering heavier seas than were expected, in a tide-rip, for example, remains a real one. Stability therefore concerns us all. Nonetheless, working stability in active situations is awkward to define, slippery to calculate, and dangerous to pontificate about.

THE GZ CURVE

Under theoretical conditions, a vessel’s ability to stay ‘on her feet’ arises from two basic forces: the form stability that she possesses by virtue of her beam, and the pendulum effect of her ballast. The results of form stability are noticeable at relatively small angles of heel, while the righting effect of a ballast keel is at its greatest when the boat is on her beam ends. For a yacht to be properly seaworthy, her stability must stem from a healthy combination of the two.

When a yacht heels, her centre of buoyancy shifts to leeward as more of her volume is immersed on the downwind side (Fig A.1). Her centre of gravity (G), however, stays more or less where it was. This means that as her ballast is pressing down, the buoyancy is pushing up. Both are trying to return the boat to an even keel, and an equilibrium is reached with the sails, which are attempting to do the opposite.

The length of the ‘lever arm’ between the centre of gravity (G) and the moving centre of buoyancy (Z) varies with the angle of heel. It is known, logically enough, as GZ. A graph of the length of GZ (which, for a 35-footer with good stability characteristics, may rise to between 2 ft and 3 ft) against heel angle gives a useful indication of a boat’s range of stability in flat water. It demonstrates, among other things, the angle at which the value of GZ becomes negative, and at which the yacht’s stability vanishes.

Fig A.2 shows clearly that the moderate-shaped Contessa 32 remains positively stable until she is virtually inverted at almost 160°. The flat-floored ‘fin-and-spade profile’ yacht of similar waterline length becomes negatively stable if she is rolled to just under 120°. Should she be knocked down to that angle, she will actually try to float upside down, because her centre of buoyancy will have passed on to the ‘wrong’ side of her centre of gravity.

Fig A.1 Stability of heeling yachts.

It is important to understand that while a boat’s range of positive stability provides a vital clue to her potential safety in heavy weather, it is by no means the whole story. Her actual ability to resist capsize depends directly upon her displacement. A big boat with the same range of positive stability as a small one will be more likely to remain upright in a given set of circumstances. Similarly, if two boats are the same length while one is of greater displacement than the other, she may well have greater resistance to capsize, even if her GZ curve were to prove less advantageous.

So far, we have considered stability only in a static sense. We have looked at yachts in a flat sea and have measured performance in relation to a steadily increasing heeling force. The sea rarely delivers such ideal circumstances. The wind does not generally capsize 30 ft yachts. In nearly every case, it is the waves. A wave is a moving force. It arrives in the yacht’s vicinity and, especially if she is beam-on, it causes her to roll and rise, or be knocked down if that is what circumstances warrant. After a brief encounter, it continues on its way.

If a yacht can generate enough short-term resistance to a wave’s efforts, her angle of vanishing stability is less likely to be put to the test than would otherwise be the case. Resistance of this type is offered by a yacht’s inertial unwillingness to begin rolling, which is determined, among other things, by her roll moment of inertia. How effective this is depends upon her weight and its distribution. It is, for example, ironic that if she has a heavy mast, the force needed to start her rolling is greater than for a lightweight equivalent. Yet a massive metal spar would have an entirely negative effect upon the yacht’s GZ curve. A wooden one has buoyancy characteristics of its own which cannot be discounted should it hit the water, and so on. A dismasted vessel has a far worse roll moment of inertia than an intact one. Ask anyone who has lost his stick in rough water. The sparless yacht’s range of positive stability as plotted on a GZ curve will appear greater than before, yet she is undoubtedly less seaworthy.

Fig A.2 Two contrasting GZ curves.

From these comments about masts alone, it will be obvious that however important the dynamics of capsize may be (and in the USA they are considered extremely important), they are difficult, expensive, and often unreliable to assess empirically. It has also been said with some justification that a big enough breaking wave will knock down any boat afloat. A GZ curve is an excellent starting point for making an assessment of a yacht’s likely performance in potentially capsizing conditions, but it is never more than that. This is why some traditional working craft have such a remark able record for seaworthiness, despite unimpressive static stability predictions.

If you anticipate ever being at sea in really bad weather, your assessment of a boat’s chances if she meets a big breaking wave beam-on will affect your choice of survival options deeply. It is therefore important that any serious seafarer studies this matter as fully as he may. There are a number of excellent books specialising in the topic. It is wise to read and consider at least one of them. In the final analysis, however, the experienced, unprejudiced seaman can tell a great deal by the way a yacht looks and feels. There are few surprises lying in wait for the sailor with sensitive ‘gut reactions’.

Recreational Craft Directive

The European Recreational Craft Directive (RCD) sets out standards for, among other things, ranges of operation for yachts. While these may be superficially helpful, like all specific rules dealing with imprecise subjects the results sometimes leave much to be desired. Boats are being categorised as ocean-worthy that no sane skipper would take across the North Atlantic north of 40°. As in all seafaring, the final decision must be down to the skipper. If you don’t like the feel of a boat that the paperwork says is up to the job, trust your judgement. It’s your crew who are on the line, not some bureaucrat sitting in a nice warm office.

A quiet sleep and a sweet dream when the long trip’s over.