Marine Air Conditioning: Sizing BTU for Tropical Boats
Almost every guide to sizing marine air conditioning was written for a boat that spends most of the year cold. The assumptions are buried so deep you never see them: a four-month summer, seawater in the low twenties, dry air, and an owner whose real problem is one hot afternoon in a marina. None of that describes a boat in Phuket, Krabi, Trang or Langkawi, where the air sits at 31 to 33°C, humidity rarely drops below 80%, and the sea is a bathwater 29 to 30°C every month of the year.
Size a tropical boat with a temperate method and one of two things happens. Either the unit is too small and never holds temperature on a still afternoon, or — far more commonly, because owners overcompensate — it is too big, and by March the cabin is cold, damp and smells of mould.
On this page
- How it works, and which architecture you need
- Why a 30°C sea changes the numbers
- A sizing method that works in the tropics
- Why oversizing is the classic tropical mistake
- Ducting, return air, and the commonest service call
- The seawater circuit and tropical growth
- Condensate and electrical reality
- Maintenance for year-round tropical service
- Where to start
How it works, and which architecture you need
A marine air conditioner is a vapour-compression circuit, the same as your fridge. The difference from anything domestic is where the heat goes: a household split system dumps it into outdoor air, while a marine unit dumps it into the sea, through a coaxial condenser fed by a dedicated raw-water pump. That one choice drives most of what follows, and determines which of three architectures suits your boat.
Self-contained units put compressor, condenser, evaporator and blower in one chassis, installed under a bunk or beneath the sole with a short duct run to a grille. No refrigerant pipework runs through the boat, so installation is far simpler and a competent owner can often manage it. This is the right answer for most boats to about 15 metres, and for individual cabins on larger ones. Our DM-MAC range is built around this configuration.
Split gas systems separate the condensing unit from the air handler, so the hot noisy part can live in a lazarette. The penalty is that the refrigerant lines must be run, brazed, evacuated and charged by someone who knows what they are doing, and a leak six months later is a far bigger job than swapping a self-contained box.
Chilled water systems cool a loop of fresh water at a central plant and pump it to fan-coil air handlers in each cabin. Simple pipework, centralised servicing, and it scales gracefully — which is why almost every boat above roughly 20 metres uses one, and why below that the complexity rarely pays for itself.
Why a 30°C sea changes the numbers
A refrigeration circuit’s capacity depends on the temperature it rejects heat into. Warmer condensing water means higher condensing pressure, a higher pressure ratio across the compressor, and less cooling delivered per hour — while the compressor simultaneously draws more current. You lose capacity and pay more for it at the same time.
Manufacturers publish a nameplate capacity measured at a stated seawater temperature. American manufacturers commonly rate at around 29.5°C (85°F), close to Andaman conditions, but European and Asian makers frequently rate at cooler water and some publish no basis at all. Rating bases are not standardised across the industry, so confirm the stated seawater temperature on any specific unit before you compare nameplate figures. Find the rating condition before you compare capacities: a unit rated at 21°C seawater will give noticeably less than the box says in 30°C water, while one rated at 29.5°C derates only slightly here.
A sizing method that works in the tropics
Forget single-multiplier rules of thumb. Build the load up from its parts, space by space — air conditioning is sized per conditioned space, not per boat.
One: base envelope load. Measure the internal volume in cubic metres — length × average beam × average headroom, deducting nothing for furniture — then apply a base factor:
| Space | Base load |
|---|---|
| Fully below deck, hull largely below the waterline, minimal glazing | 400 BTU/h per m³ |
| Main deck saloon or deckhouse, moderate glazing, deck above | 600 BTU/h per m³ |
| Flybridge, sky lounge or heavily glazed above-deck space | 800 BTU/h per m³ |
These already assume tropical ambient — roughly 33°C air, 30°C water — and typical marine insulation. Add 25% for a bare, uninsulated GRP deckhead in the sun; a dark uninsulated deckhead in Andaman sun reaches 55 to 60°C and radiates straight into the cabin. These are our working rules of thumb, derived by converting established area-based marine load factors to a volumetric basis. They are a sound first pass, not a substitute for a proper load calculation on an unusual boat.
Two: glazing. Glass is the biggest variable on a modern boat. Add roughly 150 BTU/h per m² for conduction through single-glazed marine glass, then add solar gain for any glass seeing direct sun for a meaningful part of the day: around 500 BTU/h per m² for tinted glass with a reasonable overhang, up to 1,000 BTU/h per m² for clear unshaded glass or a horizontal skylight at midday. External screens or an awning take most of that away — shade is far cheaper than a compressor.
Three: people. Allow 450 BTU/h per person at rest. Roughly a third is latent load — moisture from breath and skin — which matters more here than anywhere temperate.
Four: equipment. Allow 600 BTU/h for each fridge or freezer compressor discharging into the space. Do not try to size against cooking; no sensible amount of air conditioning holds a saloon against a wok in full use. The answer is a decent extractor and an accepted hour of discomfort.
Five: openings. Add 10 to 20% where a companionway, sliding door or hatch is opened regularly — at 85% humidity every opening admits a slug of moisture the unit must then condense out. On a yacht where the companionway lives open, no unit will win; fit an insulated washboard or a proper curtain and treat it as part of the system.
Total the five and select the nearest standard capacity. Falling between two sizes, take the smaller for sleeping cabins and the larger for a heavily glazed saloon with a door that keeps opening.
Worked example: a 12 m flybridge cruiser
| Space | Volume | Base | Glazing | People | Other | Openings | Total | Unit |
|---|---|---|---|---|---|---|---|---|
| Forward cabin | 15.2 m³ | 6,080 | 550 | 900 (2) | — | +10% | 8,280 | 9,000 |
| Guest cabin | 10.0 m³ | 4,000 | 100 | 900 (2) | — | +10% | 5,500 | 7,000 |
| Saloon / helm | 27.2 m³ | 16,320 | 3,850 | 2,700 (6) | 600 fridge | +15% | 26,990 | 24,000 |
The saloon is the interesting one. Nine square metres of glass and six people put it just past the top of a typical single self-contained range. Three honest options: accept that it runs a degree or two warm on the worst afternoons; split it into two smaller units on separate thermostats; or attack the load — reflective screens and an awning over the flybridge overhang will take several thousand BTU/h out of that column for a fraction of the cost of a second compressor. We would usually do the third first.
Worked example: a 40 ft sailing yacht
A yacht is a different animal: far less glass, much more hull below the waterline acting as a heat sink into 30°C water, but usually poorer insulation under the coachroof and a companionway that lives open.
The saloon at 22.5 m³ with a blended base factor of 500 comes to 11,250. Add 3.2 m² of coachroof windows and hatches — 480 conduction plus about 1,400 of solar on the two deck hatches — four people at 1,800 and a fridge at 600. That is 15,530, and 17,080 once you allow 10% for the companionway on the assumption it is properly closed off. A 16,000 BTU unit is the sensible choice, with the honest caveat that on a windless April afternoon it will be working flat out. Forward and aft cabins come out around 5,500 each and take 7,000 BTU units.
That is 30,000 BTU/h for a 40 ft yacht, which at tropical conditions is roughly 3.5 kW, or about 15 A at 230 V. That assumes an effective EER of around 8 to 9 BTU/Wh at tropical conditions including pump and blower; check the specific unit’s published input power. Most Thai marina pedestals are 16 A or 32 A. On a 16 A pedestal that boat’s air conditioning has consumed the entire shore supply, leaving nothing for the water heater, the charger or the kettle. Better to discover that on paper than at two in the morning.
Why oversizing is the classic tropical mistake
An air conditioner does two jobs: it removes heat, and it removes water. In a temperate summer the moisture load is a minor part of the total. At 85% humidity it is a very large part, and it determines whether your boat smells right in six months.
Moisture only leaves the air when the evaporator coil is cold and wet and air keeps passing over it. That takes time. An oversized unit satisfies the thermostat in five minutes and shuts down, and the water sitting on the coil evaporates straight back into the cabin. Repeat all night and you get 22°C at 80% humidity — cold, clammy, with mould quietly establishing itself behind every cushion. A correctly sized unit runs long cycles, keeps the coil wet, and drains litres over the side.
If you take one thing from this guide, take that. Undersizing costs you comfort on the worst afternoons of the year. Oversizing costs you the boat’s interior. Variable-capacity DC inverter units sidestep the problem by modulating compressor speed and running continuously at partial output; where budget allows, they are the better answer in this climate.
Ducting, return air, and the commonest service call
More “the aircon is broken” calls turn out to be airflow than refrigeration, and the usual culprit is the return.
Air that goes into a cabin must come back. That means an unobstructed return grille of generous free area — at least equal to and preferably 25 to 50% larger than the supply outlet area it serves a working design rule rather than a published standard, so follow the manufacturer’s grille sizing where it is given — and a genuine air path from every conditioned space back to it. A closed cabin door with no undercut and no transfer grille strands the unit: it starves, the coil ices, airflow drops further, and it trips. Owners describe a unit that “works for an hour and then stops”, which is exactly what a starving evaporator does. The other frequent finding is a return grille buried behind sail bags. On the supply side, use insulated flexible duct, keep runs short, and treat every 90-degree bend as a real cost in airflow.
The seawater circuit and tropical growth
Every unit needs its own adequate flow through a dedicated seacock and a proper strainer, with the pump below the waterline so it stays primed. As a sanity check on pump sizing: carrying away the heat rejected by 12,000 BTU/h of cooling, with a 3°C rise in the seawater, needs roughly 1,300 litres per hour.
Then there is growth. In Andaman water a strainer basket goes from clean to significantly restricted in weeks, and barnacles will colonise a seacock that sits unused through a layup. Clean the strainer weekly in continuous use and always after the boat has sat; check the overboard discharge at every start, because a weak stream is the first warning long before a high-pressure trip; and descale the condenser annually with an inhibited acid cleaner circulated through the raw-water side. A fouled coaxial condenser produces precisely the symptoms of a failing compressor — poor cooling, high current, high-pressure trips — and gets a lot of healthy compressors condemned.
Condensate and electrical reality
A 16,000 BTU unit in 32°C, 85% air produces roughly two litres of condensate an hour. That is estimated from a latent fraction of roughly a third of rated capacity, so treat it as indicative rather than measured. Use a generous drain — 19 mm (¾”) minimum — with continuous fall and no sags, and a trap where the pan sits on the suction side of the blower. Drain to a dedicated sump with a float switch rather than into the bilge; condensate is fresh water, and fresh water standing in a bilge is a corrosion and odour problem you have created yourself. A blocked drain flooding the cabin sole is the second commonest service call after a blocked return.
Thai shore power is 220–240 V at 50 Hz and units for this market must be specified accordingly; a 60 Hz-only unit runs its blower and pump slow. The bigger issue is starting current. A conventional compressor draws locked-rotor current several times its running current for a fraction of a second, and on a 16 A pedestal shared down a long run, or on a small generator, that surge either trips the breaker or drags the voltage down far enough to be bad for the compressor — low voltage is harder on a motor than high. Fit a soft-start device on any unit above the smallest sizes, stagger the starts where there is more than one unit, and remember that a DC inverter compressor avoids the problem altogether by ramping up rather than slamming on. Since shore power plus seawater also means galvanic exposure, check your bonding, anodes and ideally a galvanic isolator before adding a new raw-water heat exchanger.
Maintenance for year-round tropical service
| Interval | Task |
|---|---|
| Weekly (in use) | Clean raw-water strainer; confirm strong overboard discharge; check return grille clear |
| Monthly | Wash return-air filters; check condensate pan and drain flow; check for salt weeping at hose joints |
| Quarterly | Inspect raw-water hoses and clamps; exercise seacock; clean evaporator coil face; check pan for biofilm |
| Annually | Acid-flush the condenser; service the pump; check suction and discharge temperatures and all electrical connections; disinfect drain lines |
| Before layup | Flush the raw-water circuit with fresh water, drain the pump, close and label the seacock — see our monsoon layup guide |
Where to start
Work through the five steps space by space and write the numbers down. It takes twenty minutes and usually tells you something surprising.
Drake Marine supplies a house range of seawater-cooled marine air conditioning — our DM-MAC range — from 7,000 to 24,000 BTU/h, specified for 220–240 V 50 Hz tropical service, alongside pumps, strainers, ducting and spares. Send us the boat, the spaces and how you use her, and we will do the load calculation and quote a system rather than a box, as part of our yard period planning support and at no cost. Talk to us at hello@drakemarine.com, or browse the marine air conditioning range.
