The Boat Improvement Bible

Basking under a hot sun, the temperature inside a boat can rapidly rise, especially if it has a lot of windows. However, air-conditioning units are now so compact that even the smallest of cruisers can fit one.

In today’s market, there is a wide choice of compact air-conditioning units that can be fitted to craft as small as 8m (26ft) making venturing into the hottest cruising areas not just comfortable, but positively enjoyable too.

Most marine air-conditioning units use sea water as a heat sink (heat exchanger), and the cooling process not only lowers the temperature, but also strips a lot of the moisture out of the air too, reducing the humidity and preventing mildew and the growth of mould.

While portable battery-operated units are available, most cruising boats fit a more powerful self-contained unit. All the major components are attached to one basic chassis, which can be installed under a bunk or settee, or in a locker. Usually, one unit is required for each cabin, but to save cost or space the air can be ducted into two or more cabins if necessary.

To choose the right unit, you need to know the British Thermal Units (BTUs) required to cool the cabin space. Used in different countries, one thermal unit is equivalent to around 1,055 joules. There are several different formulas for calculating BTUs, but one, used by Dometic, a US marine company is as follows:

1 Below decks: Where the cabin slopes in towards the keel and there are very few windows.

A diagram showing the through-hull skin fitting for the inlet sea water below the waterline which is connected to the seacock, strainer and sea water pump. The sea water outlet through-hull skin fitting is for draining sea water and condensation and is sited between 100–200mm (4–8in) above the waterline.

A typical fixed air conditioning unit in place, suitable for a larger vessel. These units need a solid base, AC power (DC units use a built-in inverter) and good drainage.

The outlet grills should be placed as high as possible in the cabin. Aiming the cold air at the deckhead will help to displace the warm air and aid convection. The average distance travelled of a forced cold air draught is 2–3m (8–10ft).

The temperature control panel is usually very neat, and can be set positioned conveniently in the cabin.

Measure each cabin and living space to give you the total floor area in square metres for the deck. Use the ‘mid-deck’ only for smaller boats.

Decide which type of climate you will be cruising in, either temperate (air temperature maximum 27°C/80°F) or tropical (air temperature exceeds 27°C/80°F).

Determine the right ‘factor’ to apply. Typical factors for a temperate environment would be: 60 (below decks) 90 (mid-deck) 120 (above deck). Tropical factors are 80, 120 and 150 respectively.

Multiply the factor by the floor area in square metres to find the BTU, then add 10–15 per cent to the total, but no more.

The final calculation will be: Square metres x Factor = xBTUs. xBTUs x 10% = Y xBTUs +Y = total BTUs.

Having totalled the BTUs required, choose the unit with the best size to match. If between models, then go for the bigger size.

Ensure that your generator can meet the high surge current needed when the air-con units first starts up (although modern AC units are becoming less demanding than they were).

Air intakes should be straight back to the unit through a grill design that won’t drop the pressure. Treated air should pass through a filter to stop dust clogging up the fins of the heat exchanger.

Always ensure that the air intakes are only drawing air from inside the boat for recirculation, avoiding bilge air and engine fumes. It takes several passes over the chilled pipes to drop the air to the required temperature so the unit must be protected from receiving any hot air from directly outside.

Keep the galley and heads compartment slightly depressurised with an extractor fan. This will help to prevent odours from being recirculated.

The air-conditioning unit must be in direct communication with the room it is treating. If the unit is in one cabin, but treating another, a closed door between the two could cause problems with the return, unless there is a void such as a vented locker common to both cabins.

While DIY fitting is possible, professional installers will have experience of most problems you are likely to encounter, so the fitting of a reliable unit by a professional team should be very straightforward.