Properly sizing your boat is a critical part of planning a successful air conditioning system that you will enjoy for many years.
The thermal gain of your boat varies greatly during every 24 hour period, and peaks when the sun is at its maximum strength. You must at least equal this maximum BTU gain, otherwise the temperature inside your boat will go up rather than down when you close it up and turn on the a/c at noon.
If you undersize your load you will be very uncomfortable and unhappy with your system when you need it the most!
Significantly oversizing the system will result in system “short cycling” – not giving the system adequate run time to satisfactorily dehumidify the vessel.
The general rule of thumb for sizing a pleasure boat is using the formula of 14BTU’s/cubic foot (480 BTUs/cubic meter) of air conditioned space.
This factor is for typically constructed pleasure boats and should be adjusted if non-standard construction techniques have been employed.
It is prudent to use a factor of 16 to 19 BTU’s/cubic foot (550-650 BTUs/ cubic meter) for areas that are used during the heat of the day that have a lot of sunlight coming in – for example a pilothouse, especially if the roof is not shaded or well insulated.
For areas below deck that are primarily used after sunset such as a stateroom, you can usually drop this factor to 10 – 12 BTU’s/cubic foot(380-410 BTUs/ cubic meter).
If you plan to air condition the vessel only in the evening hours, or if your boat is under a canopy and out of the sun, you can typically use a factor of 10-12 BTUs for the entire vessel.
Exceptions: Catamarans and boats with large “greenhouse” like glass that allows lots of direct sunlight into the boat: Unless you can shade these areas so they are not in the direct sunlight, add about 600 BTUs of a/c capacity for every square foot of this glass (not the vertical panes as well). Tinting the glass helps a lot, however a simple tarp a few feet above the glass is the best solution while not eliminating your view. The darker the fabric the better for dark colors also block most of the UV radiation unlike a white fabric. Keep in mind that tinted glass is not permitted in the navigation station because it almost eliminates your nighttime visibility.
In applications where there is an unusually high heat load from many passengers such as a party boat, you should also increase your factor accordingly. The general rule of thumb is 250 BTU’s for inactive occupants and 500 BTU’s for active occupants such as working or dancing.
Please note that these formulas are for maintaining the desired temperatures of a typical vessel. Don’t arrive at your boat at noon after it’s been baking in the hot sun for hours with an inside temperature of 120°F, and expect it to be at 72°F in a half hour! You usually have thousands of pounds of furniture, fiberglass
We have a lot of experience sizing different vessels and will work with you in making this determination.
Equally important to sizing the vessel is determining how much individual temperature control is desired. In individual