Marine AC Sizing, Pump, Seacock Questions

[Troutslayer]

Newbie here but longtime lurker. I am purchasing a 30' Allied Seawind Ketch and bringing her down to New Orleans from Maine. The heat is well, hot and the humidity here is brutal to yacht interiors so I am installing marine AC. I will preface by saying I believe in doing things the right way the first time in all cases.

I have been talking to manufacturers, using online calculators, etc and although I would like use a smaller unit it seems that 12-14k BTU is the smallest I can specify. I calculate about 920 cubic feet of interior space and am factoring 15btu per cu/ft. Any feedback on this would be greatly sppreciated!

The next problem is seacock sizing and seawater pump size. There is less information on the interwebs on how to get this right. Is a 250gph March pump an adequate choice?

On Seacocks I am going to use Groco BV series seacocks. I am manufacturing backing plates from G10 sheet bonded to the hull with bronze studs rather than going through the hull.

3/4" seems to be large enough but I read that a larger size will help reduce fouling from sealife. Should I go to a 1" BV-1000 and scoop intake? Will this affect pump priming (will be self priming)

On the discharge side is it OK to use a smaller 3/4" seacock? Can I plumb in a condensate discharge line with inline check valve?

And finally am I right going with real bronze or should I be looking at Maralon?

Many thanks for any feedback! I will document and share the project once I get started.

[a64pilot]

Do NOT go smaller with an AC unit. Go with a 16K, also by far the most available, and common size
If it’s slightly oversized, well then you can enjoy it being on low fan and not having to listen to the thing so much.
These things depending on installation can be louder than house units.
Why go smaller, money saving is insignificant.
I bought a complete install kit from Defender, yes there were parts I didn’t need, but having the kit saved having to source little pieces that I may have forgotten if I didn’t have it, and I think I may have saved some money too.


I have never heard someone complain that their AC was too big, but I have seen many that cover windows, and wet the outside of the boat etc cause theirs isn’t quite as big as it should be.

[rourkeh]

I have lived in New Orleans so I understand the summer heat and crazy humidity
I would not even consider a unit less than 16,500 btu as you will be surprised at the amount of radiant heat your boat creates in direct sunlight. All the normal calculations for the number of btu you require are great if your boat is in the shade, and had any kind of insulating properties. I live in the Caribbean and run my A/C full time year round and have a lot of practical experience with installing A/C in my boat. I bought units from Flagship Marine in Florida and have run them non stop for years and have been very pleased with the simplicity and ease of repair. Their customer service is the best in the industry and replacement parts are cheap, probably 50% less than other manufacturers.. At some point you are going to need a new compressor or blower and you will be glad you did not buy marineair or wabasto, or similar. Also the unit comes with everything you need for the installation, sized correctly which makes it easy to install.. Try and make sure you have easy access to the sea strainer and through hull. And do your hose runs as short as possible. DO NOT use a pick up scoop on the outside of the hull.

[Troutslayer]

Ok, Thanks for the feedback everyone!

[Otia]

I guess it is a question of YMMV
I installed a Dometic 12k BTU on my 40ft boat last summer; I sail in the Antilles which is a more temperate climate than eastern US, max temperature here is 32C but the sun in intense. It cools the boat easily in the galley and v-berth. I did not run ducks to the aft cabin.
I used a 1/2 in seacock.
I wish my boat was made like an Amel, with one single seacock for all systems (engine, genset, a/c...)
Do plan a slowstart if you intend to use a/c with a genset.

[HopCar]

I rarely disagree with A64Pilot, he is one smart dude. The problem is that if you go oversize with an air conditioner, it might not run long enough to remove the humidity before the temperature drops low enough to shut it off. My 28 Ft power boat did great with a 5K btu air conditioner. My gut feeling is that a 12K unit would be right for a 30 Ft sailboat. I’m sure there must be a company that specializes in marine refrigeration in New Orleans. Talk to them.

I don’t think you want to use an external scoop strainer on an AC intake. I recommend the Groco RSC series strainer to keep the big stuff out. It is a round strainer that is hinged so you can open it to clean out any growth inside.

Good choice on the seacocks. I suggest you just buy the matching Groco fiberglass backing plates. Much easier than making your own. By the time you pay for the G10, not much more expensive.

As to the pump, follow the AC manufactures recommendation. I think the Depco website has a table where you can figure out how much water you need for the btu of cooling.

The March pumps are excellent. I’m guessing you’re going to need the 500 gph pump, LC-3C-MD. It’s not self priming but I like to mount AC pumps below the waterline anyway.

[sailorchic34]

I tend to agree with Hopcar. The AC needs to run long enough to pull the humidly out of the air. Too large a unit will short cycle on cooling and not do as good a job on reduction of humidity.

Also agree on a inline strainer. AC water inlets have a nasty habit of sucking "stuff" up into the line. An inside filter will be easier to clean.

As to water flow, allowing for additional compressor heat rejection on the condenser loop, A 12k btuh unit would need 3.2 gpm or 200 gph for a 10 degree F delta on discharge temp. A very common approach is to allow no more then 10 degree F rise on cooling water temperature on the condenser loop. I'm allowing for an additional 3000 btu's of heat rejection to account for compressor cooling (common on 200 to 2000 ton units scaled up of course.

Engineering Formula is GPM=BTUH/500/DeltaT rise.

Where GPM = gallons per minute
BTUH = British thermal units per hour
Delta T = difference between entering water temp and discharge water temp. For a condenser loop, conservative number is 10 degrees F.
500 = 60 minutes x 8.33 (weight of a gallon of water in pounds)

One Note: for areas with very hot sea surface water, over 80 degrees (say the forida keys), Use 5 or 7 degrees F Delta.

For a 18k btu, 4.2 gpm wound be the minimum. In either case, a 500 gph pump would be more then adequate for the dinky heat rejection.

[sailjumanji]

Yep, my AC was oversized for the sleeping compartment and head at 8k BTU. It would get temp down to 65 deg. But then not run long enough to really get humidity and moisture level down. That's in Texas, and it gets hot here on southern coast. Don't oversize.

[boeing1]

also agree with not over sizing unit,

[Sailmonkey]

We have an allied seawind II, so more volume than the original.

We installed a 5k btu unit, during the day it’s a great dehumidifier and keeps the cabin cooler than it would otherwise be. At night it is perfect.

Do not oversized the unit, it will short cycle and make the cabin cold and wet.

3/4” throughhull is fine for this, I would not install a scoop. The constant flow of water breeds sea life.

What are you hoping to accomplish with the AC? Comfortable sleep?
Ice box temps in the cabin in August?

I would also not recommend running it all the time. Use a dehumidifier for the times you’re not aboard. It’ll save the AC and you from constant line and strainer cleaning.

[Troutslayer]

Many thanks to everyone. Good to hear from someone with a Seawind also. I was afraid of oversizing as well as I have seen the effects of short cycling in homes with humidity problems. I am going to with a smaller unit and seacocks.

[OS2Dude]

Too big is worse than too small. Too large will cycle really fast, aand besixes ot pullig the humidity out will harm the unit. That said, when we were sizing one for our 1985 Catalina 30 the best size would have been 20 BTU. The issue was they did not have a 20, just a 24 or 16. We wet with the 24 since it had dual fans.

[a64pilot]

I have a 38’ Boat, with a 16K and a 6K.
For us it takes both in the heat of Summer to Cool the boat, and then it’s a just barely thing.
Boat came with one 16,and that didn’t cut it, worked great at night, but by 2 PM it was overwhelmed until after dark.

Then remember a boat AC fan runs continuously, as long as the evaporator is cold there is dehumidifying going on. I don’t think you will shock Cool a boat in the middle of Summer, no matter how big the AC, cause if you could, then all of us with proper sized ACs would have high humidity in the boat every night, when the heat loss is way less than half.


Plus he was saying he was going with a 12 or 14,a 16 is not all that much larger, and I bet on low fan with duct losses etc you would be doing very good to get 12K.

[a64pilot]

Oh and 12and 14 K but are not the smallest, I have a 6K Webasto in our forward Stateroom. I think that is about the smallest.
I treat the Webasto’s as throwaway systems, they are inexpensive enough and seem to last long enough so I don’t worry about replacing a compressor etc. I doubt that would make economic sense.

[Sailmonkey]

Originally Posted by a64pilot

Oh and 12and 14 K but are not the smallest, I have a 6K Webasto in our forward Stateroom. I think that is about the smallest.
I treat the Webasto’s as throwaway systems, they are inexpensive enough and seem to last long enough so I don’t worry about replacing a compressor etc. I doubt that would make economic sense.

Don’t forget the OP has a 30’ boat with a 9’ beam. Half the cockpit is long so the cabin is quite small.