Need to replace air conditioning unit.

[Wild_Bill]

I recently bought a Beneteau 331 (34') sailboat. The boat has an aftermarket Mermaid 12,000 btu a/c unit that did not work when I bought the boat. I need to replace it.
I have two questions:
1) I plan to start living aboard soon, and I plan to stay in South Florida. Will a 12,000 btu a/c cool my boat sufficiently?
2) How difficult would it be to upgrade to a 16,000 btu unit? Do I need to install additional duct work? Is the unit likely to freeze up? With a moderate level of skills, can I do this myself?
I appreciate your input.

[ranger58sb]

Ref ducting, I think you have to do the actual measurements to decide if you can increase BTUs. Might also be some slight power requirement differences, but I'd guess that's not so critical (but you should check) and more likely about start-up amperage versus actual running amperage.

In that climate, my opinion -- as here, too -- more BTUs is better.

FWIW, I just replaced and old but still working well Marine Airrr Vector Compact with a new Dometic Turbo DTU... and the newer unit seems better in all ways. Quieter, more air flow, better condensate drainage, etc. Don't know how it might compare to other brands/models, though.

-Chris

[Gary S]

Intake grill size, duct size, and discharge grill size will likely need to be increased if going to 16k from a 12k. The risk would be insufficient airflow causing freezing of evaporator.

Whether 12k is enough is a loaded question. What temperature do you want to cool the boat down to? In the middle of the day in Florida summer it is very difficult to keep a boat at 74 degrees. Are you okay with it being 80 during the day and cooler at night? Covering hatches and portlights can make a big difference.

[Flagship Marine]

Wild Bill,
The difference between a 12 and 16 in regards to duct work is more about the size of the existing duct and the vent at the end. Both a 12 and a 16 require about 36 square inches of venting and 140 square inches or about return air grill size. Amp draw is about a 2 amp difference running and probably 6 amp starting draw. As to is it big enough remember most Marine AC units are targeted to a 74 degree temp and a 15 degree split. but empirical data says your choice of a 16 should be enough. Please take a look at our website for more info.

[Cpt Mark]

Contact tech support of the unit you will be purchasing and ask them. As others have noted, ducting may be an issue. If it's simply adding more output ducts, look and see how they are distributed now. Is it easy to add a couple more or do you have to make a new air distribution box? If you need to add more ducting how will you run it? Get all the answers before you decide.

[OS2Dude]

We had a Webasto FCF 24000 BTU self contained
A/C on our Catalina 30. It would cool the boat from 95 to 72 in 20 minutes. It kept it cool even with the companionway hatch open. Still, it was too big for our boat and short cycled on all but the hottest days so it did not get the humidity down. We just replaced it with a Webasto 16000 unit. It now takes about 3.5 hours to go from 95 to 72, but once there keeps it there and the humidity is much better. I can't leave the hatch open for long, but other than that it is doing the job.

I'd be surprised if a 12,000 unit wold do anything more than take the humidity out on a 34. Depending on manufacturer, you may need to go up in size on your water hoses (assuming water cooled), larger strainer and possibly pump if you go to a 16000, maybe a slightly larger breaker too . (Worst case is the need for a larger thruhull as well) If you are happy with the vent locations, then the most you may need to do is get an adapter from the unit fan to the existing ductwork. You can always go 16000 for the main saloon and later add a 6000-10000 BTU unit for the v-berth area later if needed.

I did the install myself. If you are handy and understand how and why things work you should be able to do your own as well. (May need help manhandling the old unit out and new one in)

Good luck.

[Flagship Marine]

In regards to pump and water flow you will probably be OK a 12000 BTU unit needs 3 gal per min and a 16000 needs 4.2 GPM. You will probably be surprised and see that the same pump will work.

[GordMay]

Greetings and welcome aboard the CF, Bill,


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

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.

Make sure any ducting is sized large enough to allow low-velocity air distribution.

[zengirl]

I have a 32' Freedom. I had a 8,000 BTU from Walmart (Emerson Quite Cool) no hoses intstalled in hatch over salon. Works great. I'm in Norfolk, Va now but temps have been in the 90's. I should think th 12,000 is perfect. I love this AC just plug it in and good to go!

[ebsail]

Had a 12,000 BTU in an Ericson 35 and it cooled nicely

[Quadrille in JB]

I'd look closely at Flagship marine. They use a lot of stainless steel in their evaporator and the unit is compact and quiet. I purchased an evaporator from them and later a condensing unit for a Bertram motor yacht and they were very good to work with. When I needed help the office personnel all pitched in and gave me all of the info I needed, even technical drawings. They even deeply discounted some parts that I needed later. They do a lot of work for the US Coast Guard.

[Ken Z]

The difference in fl. temp. from 86 -92 is big on a boat is big, heat index. You need to realize what temp. you can handle, can you sleep at say 80 maybe a fan backup. What color is your boat? Do you have a binimi? or a shade cloth that covers a large portion of your boat, ie. over your boom. 12000 would probably work depending on your tolerance to heat. will you be going in and out of your boat a lot? I would go with the 12000 but I am heat tolerant.

[Quadrille in JB]

Shading your deck makes a huge difference. I converted an old jib into a shade that stretches from my mast fwd to the bow pulpit. It is secured with bungee cords both at the bow and mast. The mast sides are connected to the stantions.
Some of my friends use this setup as a rain catcher also.
If it’s really hot I use a silver tarp over the boom.
On my 34’ O’Day I have and old but hardly used 9,000 btu Marine Airr unit. It sat in a friends garage for 30+ years.
As I didn’t want to give up locker space I mounted it in a box under the aft end of the v-berth. It makes a great step up into bed. The return air is fwd and the supply is through a 5” 90 degree elbow aimed toward the companionway.
No duct loss and max airflow cool the boat fine.
On a 95+ deg day it will hold at 80 deg which is very comfortable for us and later in the day it cycles at 76 deg.
I was doubtful that it would cool the boat but the price was right and the unit was in great shape.

[Quadrille in JB]

I know it looks like the Beverly Hillbillies but it keeps the boat cool

[a64pilot]

I’m going to argue that it’s tough to oversize a boat AC, first you turn the fan down, then unless it’s something like a 24,000 BTU unit on a 30’ boat as was posted then that is oversized.
However do this to get an idea what your facing, get a good IR thermometer and shoot the deck temps on a hot day early afternoon, mine runs as high as 150 degrees.
Then shoot the interior underside of that deck, mine runs as high as 105 degrees on the side that is getting direct sun.
That shows you just how much shade can do. It likely cuts the heat load almost in half.
We don’t usually use our shades in a Marina as it does look a little Beverly Hillbilly. On our 38’ boat it takes both a 16K and a 6 K running wide open 100% duty cycle on a hot day, a single 16K would never do it.
So I do not think a 16K is too big for you, duct work etc should work fine, and your water pump may, but check. They are not inexpensive, usually about $300 for a March pump.

Of course a windless hot Summer day in Fl is going to be. A lot different than a windless hot day up North, so location makes a big difference.