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Old 05-02-2018, 04:30   #16
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Re: Sizing Air Conditioning Units

We have a 5000 btu window-type unit ($100 from Home Depot) that I fit on a board that slides into the companionway opening. We only use it when on shore power, but have tried it running on our Honda 2000, and it runs fine without any issue. This unit cools our Endeavour 37 nicely when it's quite hot outside.
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Old 05-02-2018, 04:48   #17
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Re: Sizing Air Conditioning Units

Check what the manufacturer specs as the starting point. Using area or volume runs into a problem as how well the area is insulated and how much of a greenhouse the interior is, makes a huge difference.

If you only want to cool your cabin during sleep, you can get a smaller unit that is just ducted to that space.

If you like air/con now, you will like it on the boat. Yeah, you can survive without it but why be uncomfortable.
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Old 05-02-2018, 04:51   #18
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Re: Sizing Air Conditioning Units

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Originally Posted by pathlesschosen View Post
Iíve always wondered about buying a cheap Home Depot portable AC unit exhausting it thru a porthole. Powered by the Honda generator. Or shore power when at marina or in the boat yard.
We did that one summer when the built in unit died. It worked pretty well but it wasn't the prettiest option. The critical thing is to have a place to put it where you can route the hose to an open porthole.

If you do get one, splurge on the dual hose unit. The single hose units use interior air to cool the condenser, so it's constantly sucking the cooled air out of the cabin. The dual hose ones use outside air, so even though the BTU is technically the same, they cool better.
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Old 05-02-2018, 07:55   #19
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Re: Sizing Air Conditioning Units

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Originally Posted by pathlesschosen View Post
Iíve always wondered about buying a cheap Home Depot portable AC unit exhausting it thru a porthole. Powered by the Honda generator. Or shore power when at marina or in the boat yard.
There are some threads about that here. Window units on a cat might be hard compared to mono with companion way hatch. The free standing units might work better. But these units are not Marine rated, so might not last long. There are some Marine units that you put on deck and the air feeds through a hatch over the bunks. I think west Marine has them at about 1k.
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Old 11-01-2019, 08:49   #20
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Re: Sizing Air Conditioning Units

I bought a Lagoon 380 S2 in 2015 in the Caribbean and sailed it to Thailand we had no AC only really.needed it a few times..Catamaran unlike monohulls have better ventilation and cool quicker after dark.
but we often get addicted to AC
We have allot of fans
You picked a great boat..we have traveled more.than halfway around the world and our Lagoon 380 has been perfect, a joy
good luck with your new boat
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Old 12-01-2019, 05:38   #21
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Re: Sizing Air Conditioning Units

Thanks, Rickwest. We did not install any AC units and had a very pleasant first season in the NYC area. Only a couple of really sticky times.
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Old 12-01-2019, 06:05   #22
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Re: Sizing Air Conditioning Units

The problem with the "rules of thumb" is that they are intended to cover the installers butt. You may be able to get along very well with half that or less, with compromises.
  • How well insulated in the boat? Thick core really helps.
  • Window covers and external shade help a lot.
  • Do you need to AC the entire boat during the day (the sleeping cabins may only need it at night)?
  • How quickly does it need to cool down? 5 minutes, like a car, or an hour, like a house? This makes a BIG difference.
I would try to find out what other use and what the run cycle is like. In general, small units dehumidify better because they run longer.


I focused on efficiency, and as a result I could run off batteries for many hours. I'm pretty sure I could have run off solar with a few more panels and a switch to lithium batteries. But since I only needed it in windless marinas, that wasn't worth it to me. But I can see it.
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Old 16-01-2019, 14:11   #23
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Re: Sizing Air Conditioning Units

Hi all , new to the forum ,we are ordering a New L42 cat and plan both for extended Med sailing round the year and cross Atlantic later to tropical regions . Shipyard unit recco is 48k BTU , at ridiculous cost as all extras , so we are exploring after sale servicing a smaller unit . Our dealer advises for 40kBTU multiple self contained units for a ďlike home ď full conditioning year round . There are 4 spaces ( three cabins and salon) . From all comments above it seems oversizing . Good to have more points of view from real experience .
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Old 16-01-2019, 14:37   #24
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Re: Sizing Air Conditioning Units

For those who say AC is not needed, spend some time in the Southeast US and Northern Bahamas in July to September. In a marina it can get very steamy but even anchored out it can push the limits of comfort. Also, if you plan to leave your boat at a marina for extended periods, AC will keep the boat mold free and ready to go. For me, it would be a worthwhile expense.
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Old 16-01-2019, 14:42   #25
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Sizing Air Conditioning Units

I donít see the gain in trying to get away with the smallest AC you can.
If you oversized a bit, you can run it in low, which is usually less noisy, and if you wait and decide itís just too hot, Iím turning on the AC, then youíll likely have success cooling down an already hot boat, but if your AC is just big enough, itís going to get cool about 10 PM at night, cause itís just enough to maintain cool, but not enough to remove all the excess heat from an already overheated boat.
Think about it, if itís already hot, you have to absorb all the excess heat from all the interior furnishings etc., before you really begin to cool the boat.

Multiple, independent units are best I think, because the are less expensive for each, if one goes bad, real easy to replace and you donít break the bank, plus you still have the others to stay cool with, and you only have to run the ones you need to, not cool the portion of the boat that doesnít need it.
We arenít a Cat, but I can run the little one in our stateroom, a 6K off of just a little Honda generator in eco mode. It keeps us cool for sleeping, but we rarely use it.
To run the 16k and the 6K requires the Diesel generator
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Old 16-01-2019, 14:47   #26
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Re: Sizing Air Conditioning Units

Greetings and welcome aboard the CF, Fernando.
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