Marine air conditioning sizing for temperate water
Sized for Sydney and Hobart summers — and this is the band where the reverse cycle is often the better half of the argument.
Seawater Temperature and Marine Air Conditioner Output
Cooler seawater means a unit makes close to its rated output, so the same boat needs meaningfully less capacity than it would in Cairns. The interesting part is winter: a reverse-cycle unit is an efficient heater down to about 7 °C seawater, which covers Hobart and Melbourne. Plenty of southern owners fit air conditioning and use it mostly in July.
In the sizer this is a single factor: 7 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 temperate water |
|---|---|
| 32 ft | 8,500 BTU |
| 36 ft | 10,500 BTU |
| 40 ft | 13,000 BTU |
| 45 ft | 16,500 BTU |
| 50 ft | 20,500 BTU |
Where the boat lives: Sydney, Adelaide, Melbourne, Hobart. 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 temperate water: 13,000 BTU, or 3.8 kW of cooling.
Shortlist: Webasto BlueCool S13, 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
The heating side is frequently the main reason to fit one at all. 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.