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   Britannia had neither air-conditioning nor heating when I bought her one December in Fort Lauderdale. In Florida’s winters you can often manage without either. However, by April and throughout the summer, the oppressive heat and humidity can be unbearable inside a boat. I therefore set about researching the many makes and prices of reverse-cycle (heating or cooling) air-conditioning units available.

   I had never fitted AC in a boat, so I had no experience of what size unit(s) might be needed or how to install them. I first took detailed measurements and calculated the cubic capacity of all the areas I wanted to air condition.. I then mailed the drawing to different manufacturers for their suggestions.


AftAC   I received recommendations varying from a single large unit in the middle of the boat to three separate smaller units. Most seemed to agree the overall cooling capability needed to be about 30/35,000 BTU (British Thermal Units) per hour to be effective in the height of summer.

Prices and specifications were similar between manufacturers so, rather than try to choose which make to buy, I decided to buy from the sales person who had shown the most interest in my project. Mike Patrick worked for Pompanette Air, a company in Tampa, not too far away from Britannia. I went to see him and we began a conversation that resulted in a very satisfactory and efficient installation of two 16,500 BTU per hour AC units.

For people like me, who didn’t know much about boat air conditioning, a basic kit consists of the main air-conditioning unit, usually self-contained on some sort of base: An electrical box and digital controls and a 120-volt sea-water pump. Other items are purchased as needed according to the installation. These include water pipes, filters, ducting, and vents. Ducting is especially important because, if the tubes are very long or twisty, considerable efficiency is lost.

ACcontrol   Ducting was one reason I decided to buy two units instead of one large one, which would have been cheaper. I would have needed very long tubes to carry air to the front and rear of my 45-foot long interior. Another reason was that if one unit broke down we’d still have cooling from the other. In fact, a breakdown did eventually occur due to a blocked seawater pipe, yet the other unit continued to function while I unclogged it. This is a very practical consideration when it’s 100 degrees Fahrenheit outside and — without cooling — would rapidly become 110 degrees inside.

   The forward section of the boat consists of the port side cabin, the starboard side head, the forepeak cabin, and the forward area of the saloon. It’s better to have the cool air discharge as high as possible because cold air descends. That’s not so easy to do on a small sailboat. Luckily, I had a full-height locker forward that is 2 feet square. This space was once used for a washer and dryer. I built a strong shelf high in this locker and shoe-horned one of the 80-pound AC units into the space. This locker even had the heavy electrical cable left over from the washing machine, leading from the AC breaker panel so I utilized this and saved about $200 in wire.

crossover-ventCabin-trunking   It took a bit of scrambling to mount the large 120-volt water pump and filter and plumb it to a spare seacock, then wire it to a breaker. The water is piped through the air conditioner and discharges through an above-water seacock. The condensate overflow, which is the condensed water from the air-conditioning process, discharges through another small seacock above water level.

Once the electronics and plumbing were complete, the installation became a woodworking project, since I had decided to built my own ducting out of 1/2-inch plywood. This offered a much smoother flow than convoluted tubing and enabled me to direct the air in straight runs exactly where needed. The main duct was 8 inches wide by 4 inches high with ducts branching off into the two cabins. I also channeled some air overhead across to the starboard side bathroom and main corridor.  A further advantage of plywood was it didn’t need insulating like the thin flexible pipe normally used for ducting.

   I bought the more expensive teak adjustable vents, since they offer better air flow distribution and can be closed off completely if required. They also look nicer than plastic vents. I was advised to use 3-inch square vents for cooling the two front cabins and heads. This didn’t seem large enough to me but proved exactly right in practice. I used 7-inch by 3-inch vents for the saloon and aft cabin. I actually combined three vents to deflect air more evenly in different directions into the saloon.

Small-ventVent   It was all quite straightforward and I completed installation of the forward unit in a week. It was marvelous to finally press the “on” button and feel the machine blow a blast of cool air into the cabins and saloon. Mike arrived with flow and pressure gauges and declared this was the most efficient system he had seen on a small sailboat.

   I cleared a space for the aft unit by moving a couple of drawers in the aft cabin. Installing this unit was easier because I was now “experienced” and the seawater pump was already fitted. I again built the ducting as a straight-through plywood box in the aft cabin with branches supplying the aft head and rear area of the saloon. As part of the system, the dedicated vent blowing into the galley pleased my wife. The condensate was connected into one of the cockpit drain pipes.

ForwardAC   Both units work independently, except for the common seawater pump that feeds one unit, even if the other is switched off. This provides great flexibility. When there are just the two of us on the boat, we sometimes switch the forward unit off or close the vents into the two forward cabins. This has the benefit of blowing more cool air into the other areas.

   The electrical draw from both units is too high for a single shorepower cable, so I ran a second shore cable into a new AC breaker panel. I needed this anyway to handle our electric kettle, toaster, microwave, and washer/dryer. I also had to modify the wiring from the 6.5kv generator to feed both panels when running the air conditioners on generator power.

   In the summer, Florida gets to 95/105F with oppressive humidity. These two machines now keep the inside of the entire boat at a nice, dry 75F. Cool dry air or heat spreads evenly throughout the interior and no area is warmer or cooler than any other.

   The aft unit ducting passes just under the cockpit floor. As the cockpit has a fully enclosed Bimini, I now have an idea that I could deflect some cool air into the cockpit, using a waterproof vent. This would make it very pleasant for sundowners on hot sticky evenings.

   The total cost for both units was around $4,600, including pipes, plywood, wire, and vents. This, of course, did not include my labor, but on a boat that’s supposed to be fun!

   Even though we tend to take our air conditioning for granted now, it is still one of the best investments we have made on our boat

Articles by Roger appear regularly in these boating magazines, about improvements to Britannia and other nautical matters

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