Marine air conditioning sizing for the tropics
Thirty-degree seawater takes 10–15 % off a unit’s label rating before it has cooled anything, and the humidity is a bigger problem than the heat.
Seawater Temperature and Marine Air Conditioner Output
A unit rejects its heat into the sea. When the sea is 30 °C rather than 25 °C, the condensing pressure rises, the compressor works harder for less output, and the number on the box stops being true. Every figure here is sized up to account for it. The humidity matters just as much: only a refrigeration cycle removes water from the air, which is why a fan does not substitute this far north no matter how big it is.
In the sizer this is a single factor: 10 BTU per cubic foot of accommodation, against 7 for temperate water and 10 for the tropics. Everything below follows from it.
Boat Cooling Requirements by Length
| Boat | Cooling in the tropics |
|---|---|
| 32 ft | 12,000 BTU |
| 36 ft | 15,000 BTU |
| 40 ft | 18,500 BTU |
| 45 ft | 23,500 BTU |
| 50 ft | 29,000 BTU |
Where the boat lives: Cairns, Townsville, Darwin, the Whitsundays. A dark hull adds about 10 %, and a lot of glass another 10 % — both worth applying honestly rather than hoping.
Example Marine Air Conditioning Installation Cost
Forty feet, two cabins, in the tropics: 18,500 BTU, or 5.4 kW of cooling.
Shortlist: Velair Compact i10VSD SMART, Mabru SC12DC.
Supplied and fitted, ex GST, including the seawater side and the electrical work.
At the typical Australian tariff and a realistic duty cycle for a closed-up boat.
Reverse-Cycle Heating in This Climate
Reverse cycle is bought purely for cooling here. The heating side will never be used. Every unit compared on this site is reverse cycle, so the heating side comes free with the box — but it is worth knowing before you choose, because it changes which end of the argument the money is on. A reverse-cycle unit stops being efficient below about 7 °C seawater, which almost nowhere in Australia reaches.