Dear Boat Doctor,
The blistering heat last summer has convinced me that I want to install air conditioning on my boat. I want to try to tackle this myself over my winter haul-out. I am reasonably mechanical, but could use all the tips and tricks that you can give me.
Richard Hoyt
Boston, Massachusetts

Dear Richard,
Sailing purists will probably scoff at air conditioning, but it is very nice on a hot night. There are five things you need to worry about: getting the right sized unit, powering it, water cooling it, draining the condensate from it and managing the airflow to and from it.

An air-conditioning unit needs to be the right size for the space you want to cool. If the unit is too small it will not have the capacity to cool the area and may run continuously to the point of freezing up. Conversely, if it's too big, the unit will cool the space quickly but not run long enough to pull the humidity from the air. A good rule of thumb is to specify 14 British thermal units per cubic foot of space. An aft cabin that is 8 feet long, 12 feet wide and 6 feet tall contains about 576 cubic feet, and thus would require an air-conditioning unit of about 8,000 Btu.

An air conditioner consumes a significant amount of power, so you will likely need to plan for a second shore-power cord to be able to run the unit under all conditions. A modest air conditioner of 9,000 Btu will draw approximately 8 amps when running, but will need almost 15 amps when starting (the start current is typically 1.8 times the running current). If the air-conditioning unit tries to start at the same time the water heater or other large load is running you'll likely blow the dockside breaker. Your best bet will be to install a second 30-amp shore power inlet and breaker.

Marine air-conditioning units require seawater cooling. It is important for the intake to be well below the waterline and to flow "uphill" through the strainer to the water pump, which all must be below the waterline. The output of the pump will then flow through the air-conditioning unit and be discharged overboard. The output should be between 4 and 8 inches above the waterline; lower than 4 inches and the output needs to be considered a below-the-waterline through-hull with a seacock, and higher that 8 inches would make excessive splashing noises. It is important to use a strainer on the outside of the through-hull to prevent debris from clogging the seacock or internal strainer.

Finally, you'll need to route the cool air into the cabin. Air will flow about 6 to 8 feet so you'll likely need multiple outlets in a larger boat. Try to keep the runs as straight as you can, as every turn and elbow inhibits airflow.