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Old 01-10-2009, 05:16   #1
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AC and a 40ft cat

What size AC is needed to make life bearable on a 40ft cat, I have looked at a couple of sites, and reckon that a 16000 btu could be run easily from a 2.5kw generator, although the output would need to be boosted during the start phase by a parralleled inverter.

a 24000btu would really make a 2.5 kw generator work hard.
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Old 01-10-2009, 05:29   #2
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Well lets see, my 55 ft monohull probably has similar space...three 16,000 AC's,
AC watermaker, battery charger, washing machine, AC freezer, electric water heater, toaster... My 14.5 kw generator struggles...

You need 4kw minimum and probably a 6-8kw.

If your wife is willing to sail the world and go out in North Sea gales at least make life comfortable in the Caribbean.
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Old 01-10-2009, 05:40   #3
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Quote:
Originally Posted by s/v Moondancer View Post
Well lets see, my 55 ft monohull probably has similar space...three 16,000 AC's,
AC watermaker, battery charger, washing machine, AC freezer, electric water heater, toaster... My 14.5 kw generator struggles...

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Old 01-10-2009, 05:51   #4
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Sizing Air-Conditioning:

A rough “rule of thumb” recommendation for sizing marine air-conditioning is about ± 14-to- 15 BTU/h per cubic foot of cabin volume.

More glazing, pilot house, etc = more heat gain; so perhaps up to 17 BTU/h per cu. ft.

A professional will size the air conditioner based on the latent cooling load (which considers the relative humidity of the air), as well as the sensible cooling load (which considers the outdoor & indoor design air temperatures) for your boat & location.
A (theoretically) perfectly-sized air conditioner will run continuously during the hottest 2.5% summer design (outdoor) temperatures.

Don't buy an oversized unit. An over-sized unit short-cycles (turns on & off too rapidly), so doesn’t properly de-humidify*, and uses more energy (starting currents).

On the other hand, make sure any ducting is sized large enough to allow low-velocity air distribution.

* The ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain. For short cycles (< 10 min, or so), the coil does not have time to operate at the low temperature, and when the unit stops, the moisture on the coil evaporates back into the indoor air. Thus, in humid climates, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units.
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Old 01-10-2009, 06:19   #5
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Our new (to us) boat has an A/C unit on it. Since we anchor out pretty much all the time, it is useless and occupies valuable storage space. I want to chuck the thing overboard, it will be a PITA taking it out. We have always found good cabin fans and open hatches to be sufficient, even in the Carib. Amp draw per fan over 8 hours is hardly enough to even register on the monitor. We don't want no stinkin' genset. A/C on a boat just seems so wrong.
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Old 01-10-2009, 07:14   #6
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jdoe71...I agree but I don't argue with the 'Senora Capitaine', who is Puerto Rican from the Bronx...not a safe pastime. I just make this boat sail...check the pic on the profile.
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Old 01-10-2009, 08:18   #7
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Our new (to us) boat has an A/C unit on it. Since we anchor out pretty much all the time, it is useless and occupies valuable storage space.
While you are at anchor, you have a very valid point. However, if you are in a marina, there is a higher percentage chance that the boat is "hidden" from the cooling breezes. Furthermore, with a long term plan for liveaboard, the additional reverse cycle capability (if water temperature stays above 10 deg C) means that I would also have heat when needed.
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Old 01-10-2009, 09:02   #8
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While you are at anchor, you have a very valid point. However, if you are in a marina, there is a higher percentage chance that the boat is "hidden" from the cooling breezes. Furthermore, with a long term plan for liveaboard, the additional reverse cycle capability (if water temperature stays above 10 deg C) means that I would also have heat when needed.
Gord has it right (again!).

FWIW, we have two 9K BTU units on our MC41. Located amidships under the main berth in each hull, they are noisy and take up a bunch of space - ugh! (Only runs off shore power, and we are trying to avoid adding a genset).

IMO, you have to really love reverse cycle heating to put up with it! We have hydronic heating that runs off a diesel-fried boiler - that's heat!

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Old 01-10-2009, 11:22   #9
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We have one 16000 btu that services the saloon and two fwd cabins and two 5500 units for the stern cabins on a 44ft cat. Here in the Caribbean, we only run the 16000 unit when there are just the two of us aboard and then only for 3-4 hours in the evening to cool and dry the boat. After a year out here we can sleep comfortably without it running overnight.

16000 will do you just fine.
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Old 01-10-2009, 12:21   #10
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We have one 16000 btu that services the saloon and two fwd cabins and two 5500 units for the stern cabins on a 44ft cat. Here in the Caribbean, we only run the 16000 unit when there are just the two of us aboard and then only for 3-4 hours in the evening to cool and dry the boat. After a year out here we can sleep comfortably without it running overnight.

16000 will do you just fine.

Thanks.

Where have you located the unit?
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Old 01-10-2009, 13:23   #11
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I'm sort of with jdoe71 on this, we have a 16,000 unit on our boat and EXCEPT for in a marina, it is just taking up space. In a marina occasionally, don't know how we would live without it.
Almost a case of dancing with the Devil!
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Old 01-10-2009, 14:51   #12
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jdoe71...I agree but I don't argue with the 'Senora Capitaine', who is Puerto Rican from the Bronx...not a safe pastime. I just make this boat sail...check the pic on the profile.
I did check the photos. Something seems to be wrong with your boat in the photo. I swear, it appears to be leaning to one side.

Does this happen often?
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Old 01-10-2009, 15:21   #13
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Another data point for Talbot:
The Princess* insisted on having A/C in our 43 foot F-P cat. We had a Carrier rep visit the boat to consult with the installers to ensure adequate capacity. We ended up with one unit for each hull and a third for the main saloon area. I'm not on the boat at the moment so I can't look up the sizes of the units but as I remember, the units for the hulls are 12,000 BTU/hr and the one for the main cabin with much higher heat absorbtion from sun is 16,000 BTU/hr. All these are powered by a 6KW Northern Lights gen set. We compromised on the gen set size, though, so we can run only 2 of the 3 units at a time. The units are located in the port engine room.

* [We skipped the rank of Admiral and went right to Princess, mostly because her name is "Lela" and the kids lovingly called her "Princess Lela" while Star Wars was popular.]
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Old 01-10-2009, 16:28   #14
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Thanks.

Where have you located the unit?
It is under the saloon settee. Original fitment at Privilege. Big outlet in the saloon and ducted through to both fwd cabins. All three units + watermaker + 100 amp battery charger run comfortably off our 110v, 7kva Northern Lights.

Main unit could easily be run off 2.5kva generator provided, as you say, you have a nice inverter to kick in for the startup load.
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Old 01-10-2009, 16:30   #15
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I installed 2 12KBTU units on my 36 foot Mahe, I can run them with my 3.5 Kw generator with a 70% load. Living in Florida the twin 12K units are just fine.
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