air conditioning sizing

I have had my boat in the PNW but now I am ready to head south and need air conditioning. Finding someone that gives me confidence in air conditioning sizing in the PNW has not been fruitful. Using the various tools on web sites provides a range that doubles - also not confident building. At that the tools do not describe how to include volume in the boat that might be otherwise occupied - furniture, beds, etc. How would one go about making sure he gets what he needs without going to excess? Thanks.

Bill Fleenor
Double-Wide

What boat do you have? I would see what other owners of the same boat recommend.

For a house it is easy. Use the total square feet of the exterior of the box, divide by the R value of the walls and multiply by the delta T (outside minus inside temps), and that is the total BTH/hr you need to remove.


Boats aren't so easy and your boat in particular due to its irregular shape. It looks like it will have a bunch of exterior area. Also the R value of outside walls is hard to figure. In many cases the R is very low about 1 due to a single glass layer. Cored superstructure panels help and double or triple the R. There is usually no inside insulation. Also the solar load is very significant.


So rules of thumb and experience is what is generally used to size boat A/C systems.


My guess from looking at your avatar, is that you will need three 16,000 btu/hr units: one in each hull and one in the central cabin and pilot house. You might get by with smaller units in the hulls and maybe go to 24,000 in the cabin. From the geometry that looks like where your most cooling load will be.



David

Although some will say " bigger is better" it is not true with air cond.

The idea is the unit should be on for modest to long periods to dehumidify the interior.

A too large unit that goes on for a few min and cools the cabin then shuts down for a short time will not dehumidify so the cabin may get cool, but be clammy .

The hassle with boats is a lack of insulation and huge glass areas , compared to houses.
The cooling requirements will vary greatly night or day.

Insulated shades can help to balance the load.

What similar boats use would be a help in sizing if from the area you will be in..

Personally I would chose a 120V mini split as they can increase/reduce their output easily.

Please list length and width of the air conditioned space of each hull, and enclosed structure (length and beam) of the main cabin. Also what percentage of the hulls is isolated from the main cabin. How hot an environment do you plan to cool the boat for. Can you / will you curtain the windows when air conditioning or do you plan to air condition underway.

Ted

Do you have covers for all thise windows in the salon? They will add an enormous heat gain in the sun.

more info on AC needs

Well, I guess I was hoping someone would point me to some magic guru, like Peggy for heads or Jim for electrical, but I guess I need to supply more information and take the best collective advice I can get.

The boat is a custom built power catamaran, 48-ft LOA and 23-ft beam, so not much to compare. Yes there are loads of windows but the saloon/galley windows have blinds and also some shade. The pilot house will likely need to have exterior shading installed on the 11 windows of approximately 32"x24" size. Also an exterior pilot house door but I have a way to shade it.

I have attached rough drawings of the main deck, the pilot house, and the lower head deck in each hull. The engine rooms and steering rooms will not be air conditioned. All guidance greatly appreciated.

Bill Fleenor
Double-Wide

Seattle area boat HVAC contacts

Hi Bill, these two companies up in Seattle may be able to help you:


https://nwmarineair.com/


Welcome - Sure Marine Service Inc

Can you calculate an approximate cubic foot measurement? It is going to take a bunch of air conditioning to do this beamy of a boat. I would have exterior covers for the salon windows made out of white Stamoid, it will let in light but help keep out the heat and UV. In white Stamoid is much easier to keep clean than Sunbrella since it isn’t woven like Sunbrella. It will make a huge difference in the way your A/C perform.

Because of it's size, this might be a good application for a chill water setup. Basically you would have an air handler in each bedroom, 2 in the saloon and one in the pilot house. The compressors, heat exchangers and pumps could be in one engine room. 2 insulated plastic pipes circulate chilled water from the engine room around all the air handler coils. The only noise you hear at each air handler is the fan (no compressor noise). I see you needing a lot of capacity, and this type of system allows you to direct all of the capacity to selected areas. As an example, you could let the staterooms be warmer while underway to have more capacity for the saloon and pilot house. At anchor or dock, you could choose to not cool the pilot house for more capacity in the saloon and staterooms. The other advantage in this system is that since there would be 2 or 3 compressors, you get some redundancy and in low capacity needs, only one compressor would be running.

Ted

Thanks, everyone for the input. I will eventually post the outcome.

Bill Fleenor
Double-Wide

A second advantage of a chilled water system is the compressors can be removed easily by a non freon tech.


The compressors only cool or heat the circ water so taking one out and bringing it to be serviced is easy , and cheap.


The charter boats may keep a spare unit to swop out to maintain onboard comfort.