Marine air conditioning sizing for the subtropics
Sized for a Brisbane or Perth summer afternoon with the boat closed up — the case that actually decides whether a system is big enough.
Seawater Temperature and Marine Air Conditioner Output
The subtropics are the honest middle. Seawater in the high twenties, air in the mid thirties, and enough humidity that dehumidifying is half the value of the machine. Most of the Australian cruising fleet lives in this band, and most of the installed prices published on this site were computed for it.
In the sizer this is a single factor: 8.5 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 subtropics |
|---|---|
| 32 ft | 10,000 BTU |
| 36 ft | 13,000 BTU |
| 40 ft | 16,000 BTU |
| 45 ft | 20,000 BTU |
| 50 ft | 24,500 BTU |
Where the boat lives: Brisbane, the Gold Coast, Perth. 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 subtropics: 16,000 BTU, or 4.7 kW of cooling.
Shortlist: Dometic Turbo GTX, Webasto BlueCool S13.
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
The reverse cycle earns its keep for a few weeks a year. Useful, not the reason you buy. 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.