How Many BTUs Air Conditioning?

[Chotu]

I have a 50' x 25' cat I'm trying to cool part of.

The deck house is about 20' wide by 12' long. And has 360 degrees of very large windows all around it. The aft windows (on the doors) get no sun because there is a hard dodger over that area.

I'm trying to cool an area on the bridge deck level that's 8' x 12' as well as an entire hull. I used plastic to seal off the other 2/3 of the bridgedeck and other hull.

I'm using 18,000 BTUs between 2 air conditioners. It's only getting it down to 77 degrees. It's currently 79 degrees and nearly 100% humidity. Partly cloudy.

How many BTUs does a boat like this need to feel nice and cool??

NOTE: The boat is in the water and under construction. Interior work.

Also the ACs are right side up. The pic is 90 degrees out.

[a64pilot]

Have you tried blocking the windows with foil? Putting sun shades on the boat? Heat gain is what is killing you of course, lower that and you may be OK, or at least better.
But your question is hard to answer, by that I mean what’s your location? If its South Fl for example and you also want to be able to cool in July and August, then I believe you will need probably twice the BTU that you will need in Feb.
Of course that’s just a SWAG.
People really want to size AC’s so that they just barely make it, under the theory of course that they are more effective dehumidifiers that way and big AC’s will cool too fast leaving the air moist but cool.
I disagree, any AC that I would buy has more than one speed, I want to size it so that they work fine on low, that way I can come in one day with them off and turn them on high and by the time it’s time to sleep it’s cool enough to do so. If they are sized to just barely get by on high it’s not until day two that it’s cool enough to sleep comfortably, and who wants to listen to the things on high?
Plus with multiple AC’s you can turn one off when it gets comfortable, so I say go big, you’ll be glad you did.

On a house where the AC is always on and conditions don’t change much is different, then you can size to where it runs nearly continuously and just gets by, and that is most efficient energy wise etc.

[GordMay]

A rough "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 (below decks) to 17 (above decks, glazed) BTU/H for each cubic foot of interior conditioned volume.

[Chotu]

Thanks, guys!! Using Gord's rule of thumb, I had just barely enough.

As it turns out, it simply took a long time to get down to temp. I was able to get it down to a chilly 66F in 88 degrees (outdoor), mostly sunny conditions. Plenty cool enough to work.

Yes, I do have the windows shaded - with tarps so as not to damage the real shades during construction.

So, it's working! I was able to make progress yesterday on the boat even though it was hot and humid outside. $12 or so for an 8 hour day, but increased productivity and intelligence seems well worth $12/day. Ha ha

When I get the real air conditioners I'll go big with multiple units, definitely, A64. These are a couple old window units I had laying around.

[sailorboy1]

Just reading that got it down to 66 made me cold

[Chotu]

Originally Posted by sailorboy1

Just reading that got it down to 66 made me cold

Absolutely. It was freezing. Ha ha. A successful test.

[kas_1611]

Try adding window tinting film to your windows. Reduces the heat gain and protects the interior from UV degradation.

I had 35% smoke fitted to my car when I moved to SW France (the legal maximum I could fit) and it made a noticeable difference to the interior temperature. As long as the windscreen wasn't in direct sunshine of course.

Anything that helps reduce the heat gain goes a long way to reducing the energy you need to burn to power your AC.

[joelhemington]

Originally Posted by Chotu

I have a 50' x 25' cat I'm trying to cool part of.

The deck house is about 20' wide by 12' long. And has 360 degrees of very large windows all around it. The aft windows (on the doors) get no sun because there is a hard dodger over that area.

I'm trying to cool an area on the bridge deck level that's 8' x 12' as well as an entire hull. I used plastic to seal off the other 2/3 of the bridgedeck and other hull.

I'm using 18,000 BTUs between 2 air conditioners. It's only getting it down to 77 degrees. It's currently 79 degrees and nearly 100% humidity. Partly cloudy.

How many BTUs does a boat like this need to feel nice and cool??

NOTE: The boat is in the water and under construction. Interior work.

Also the ACs are right side up. The pic is 90 degrees out.

If your windows are glass, I'd recommend the 3M P18AR window film - it'll drop your temps by 20 degrees. Caution, this stuff acts like a two way mirror so you need to keep the interior lights very low if you need to see out. Shades are only really effective for glare control - once the heat penetrates the glass, your A/C will have to deal with it. Controlling the heat penetration will really save on the A/C costs downstream and will easily offset the installation cost of the film. Plus the film has a high UV blocker which will protect your interior as an added bonus. This product will not work on plastic as plastic out gasses and causes the film to lose adhesion.

[Brewgyver]

Originally Posted by Chotu

Thanks, guys!! Using Gord's rule of thumb, I had just barely enough.
(snippage)

When I get the real air conditioners I'll go big with multiple units, definitely, A64. These are a couple old window units I had laying around.

With a properly installed system, the boat will cool much faster with the same tonnage (cooling capacity that is, not boat tonnage). The problem with using household window units is they can't distribute the air well enough. In a ducted system, 18 K BTUH would be moving air at a nominal 600 CFM. As A64 referenced, a system with higher than nominal flow (or an undersized evaporator) will dehumidfy better, albeit sometimes taking a bit longer to initially pull down the temp when you're starting with a very warm space.

[GordMay]

Originally Posted by Brewgyver

... 18 K BTUH would be moving air at a nominal 600 CFM. As A64 referenced, a system with higher than nominal flow (or an undersized evaporator) will dehumidfy better, albeit sometimes taking a bit longer to initially pull down the temp when you're starting with a very warm space.

Some A/C Rules of Thumb:
400 CFM per ton (12,000 BTU/h) of cooling is needed, for normal comfort applications, 500 CFM per ton of cooling, for heat pump and high sensible heat applications, and 350 CFM per ton of cooling, for high latent heat applications.
One CFM is needed per square foot (1 cfm/sq ft) of floor area. There is an assumption of an 8-ft ceiling, no unusual window areas, and average insulation. This rule of thumb provides about 7.5 air changes per hour.
Six to 10 air changes are needed per hour.