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Old 14-08-2017, 09:33   #1
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Sizing the right air conditioner for your boat

Does anyone know how to choose the correct BTU of air conditioning in terms of cooling for your boat?
Is there a formula for length or different weather?
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Old 14-08-2017, 09:40   #2
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Re: Sizing the right air conditioner for your boat

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Originally Posted by Wckoek View Post
Does anyone know how to choose the correct BTU of air conditioning in terms of cooling for your boat?
Is there a formula for length or different weather?
Not really.

Variables include:
  • location
  • deck and topsides core construction
  • liner
  • window area
  • use of awnings and window covers to reduce solar gain
  • how quickly you want to cool
The down sides of over-size include:
  • additional electrical installation cost
  • power problems in some marinas
  • noise
  • poor humidification performance (cycles are too short)
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Old 14-08-2017, 09:55   #3
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Re: Sizing the right air conditioner for your boat

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Not really.

Variables include:
  • location
  • deck and topsides core construction
  • liner
  • window area
  • use of awnings and window covers to reduce solar gain
  • how quickly you want to cool
The down sides of over-size include:
  • additional electrical installation cost
  • power problems in some marinas
  • noise
  • poor humidification performance (cycles are too short)
So how would people know if it was the right size, was it through trial and error or experience?
How about say a 38 feet steel/aluminum boat with some larger windows at pilot house at a tropical climate?
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Old 14-08-2017, 10:04   #4
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Re: Sizing the right air conditioner for your boat

Take the cubic footage of the area you want to cool and multiply it by a factor of 14 if below decks and use 16 for above decks. Then add 200 BTU for every square foot of non vertical windows like a catamaran. If you have shades or tinted windows you may not have to account for the windows.
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Old 14-08-2017, 10:07   #5
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Re: Sizing the right air conditioner for your boat

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Take the cubic footage of the area you want to cool and multiply it by a factor of 14 if below decks and use 16 for above decks. Then add 200 BTU for every square foot of non vertical windows like a catamaran. If you have shades or tinted windows you may not have to account for the windows.
Thanks, square footage is easier to figure out, I have no idea how to calculate cubic footage of interior space inside a boat like times it over the headroom?
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Old 14-08-2017, 15:19   #6
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Re: Sizing the right air conditioner for your boat

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Originally Posted by Wckoek View Post
Thanks, square footage is easier to figure out, I have no idea how to calculate cubic footage of interior space inside a boat like times it over the headroom?
Yep, square footage multiplied by headroom is close enough. Depending on the boat, you may need to do that separately for different parts of the boat and add the results together.
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Old 14-08-2017, 15:20   #7
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Re: Sizing the right air conditioner for your boat

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Thanks, square footage is easier to figure out, I have no idea how to calculate cubic footage of interior space inside a boat like times it over the headroom?
Length times width times height.

Then again, that only works if your interior has straight sides and square corners.

I'd say you need to multiply the "average" L, W, H. Those should be somewhat easy to measure (?)

I'm in the same boat (...) As I likely need to replace my 25 year old AC. The formula someone furnished earlier sounds like a great starting point.
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Old 14-08-2017, 15:44   #8
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Re: Sizing the right air conditioner for your boat

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Originally Posted by Wckoek View Post
So how would people know if it was the right size, was it through trial and error or experience?
How about say a 38 feet steel/aluminum boat with some larger windows at pilot house at a tropical climate?
Formulas will get you slightly oversized with an error bar of ~ 50%. In the case of a metal boat, it will depend strongly on insulation and liners. You're going to be on the high side.

For example, that formula (ft^3 x 14) would give about 22000 BTUs for my cat. However, I have a thick core in the deck, a padded liner separated by an air gap, and window covers, and so I get a long fine with 10,000 BTUs. If I take off the window covers, the boat gets warmer. Farther north I could have less, farther south I might like more. A poorly insulated boat might require 3 times as much.

So the formulas are somewhat conservative.

You can calculate a better value using measured insulation losses (heat the boat with a space heater and measure the delta T at night) plus window gain. For a metal pilothouse boat in the tropics the formula is probably be low, depending on the liner and any hull insulation (probably none).
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Old 15-08-2017, 08:55   #9
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Re: Sizing the right air conditioner for your boat

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Originally Posted by ValiantV View Post
Length times width times height.

Then again, that only works if your interior has straight sides and square corners.

I'd say you need to multiply the "average" L, W, H. Those should be somewhat easy to measure (?)

I'm in the same boat (...) As I likely need to replace my 25 year old AC. The formula someone furnished earlier sounds like a great starting point.
It will vary with the boat design. For mine I treat the fore peak as a trapezoid with height measured up from the 'v' birth. The rest I approximate as rectangles with headroom as the height. My Morgan 33 O/I calculates to 1190 CF. My 16000 BTU Cruiseair keeps me comfortable when the air temperature tops 90 and and the water is close. I do not have any canvas shading. Interesting that multiplier of 14 works out a little over 16000. Of course, a metal boat would make a giant difference.
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Old 15-08-2017, 09:20   #10
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Re: Sizing the right air conditioner for your boat

Quote:
Originally Posted by Wckoek View Post
Does anyone know how to choose the correct BTU of air conditioning in terms of cooling for your boat?
Is there a formula for length or different weather?
This can be a very complicated question because of all the interdependent variables.

Start with assessing the Degree Days for heating/cooling requirements in the latitudes/seasons you plan to travel, then assess your vessel's heat gain/loss, volumetric and air exchange requirements, etc., etc.

From the practical side, I can offer our vessel as an example. [Nauticat 43; no deck or hull coring; insulation R10 minimum.]

The previous owners used our boat in the tropics for 15 years and therefore installed 3 independently controlled and regulated air conditioners [A/C: 9k BTU fore cabin, 18k BTU pilothouse, and 18k BTU in the master cabin. All are raw water cooled using a single pump and plumbing circuit.]

Since the A/C units each have a dedicated circuit breaker and thermostat, we can run 1, 2, or all 3 as needed. [Note: when at the dock in winter we use them for heat since our current hydro electric power rates are cheaper than putting hours on our generator or Esbar diesel heater...]

In case this is useful.

Best wishes sorting out a composite system that will yield the results you are looking for.

Cheers! Bill
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Old 15-08-2017, 09:22   #11
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Re: Sizing the right air conditioner for your boat

It depends on how cool you really need. On a cat one problem is getting cool air to the 4 staterooms. You could have the salon chilly and still be hot in a state room.
Not sure if you are talking mono or cat? Mono ought to be easier.
Also are you talking an installed type of just a common one for use at the dock?
A $165 x 6500-8000 btu unit cools my small 1100 sq ft house well enough. We just finished a 55 day run of no rain and hot weather. No it doesnt get chilly, and we dont try to cool all rooms, but cool enough.
I have no doubts that little unit would cool a 45 ft mono to chilly if located centrally on say the skylite hatch.
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Old 15-08-2017, 09:38   #12
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Re: Sizing the right air conditioner for your boat

The figure are still plenty high and use a lot more energy than I like.
I think I would have it configure in a way that is cold for the stateroom and merely enough for the saloon.
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Old 15-08-2017, 18:28   #13
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Re: Sizing the right air conditioner for your boat

Great topic. I have a boat and an RV. The boat has no air but I'm interested. The RV I just removed the Coleman AC and replaced it with an AirCommand made in Australia and rated at 15,000 BTU's. I found both the Coleman 13,500 BTU unit and the AirCommand 15,000 BTU unit just reduced the temperature by a certain amount on the inside from what it was outside. The drop seems to be 12-15 degrees difference. So the point is, if it's 80 outside I can get it 70 inside no problem, if it's 90 outside it's 76 inside, if it's 100 outside then it's 84 inside. So what?

Well my point is that 15,000 BTU's is the max my generator can handle. I think you should be looking at the power you are willing to devote to the AC then figure if you are happy with a certain temperature drop from that setup. I know that I'm happy with 12-15 degree difference. Sure sometimes it's a little hotter than I like but it's still a big improvement. A larger AC unit means a larger generator, more fuel and larger wiring.

Maybe next year I'll look at AC for my boat.

Hope this helps
John
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Old 15-08-2017, 18:53   #14
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Re: Sizing the right air conditioner for your boat

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Great topic. I have a boat and an RV. The boat has no air but I'm interested. The RV I just removed the Coleman AC and replaced it with an AirCommand made in Australia and rated at 15,000 BTU's. I found both the Coleman 13,500 BTU unit and the AirCommand 15,000 BTU unit just reduced the temperature by a certain amount on the inside from what it was outside. The drop seems to be 12-15 degrees difference. So the point is, if it's 80 outside I can get it 70 inside no problem, if it's 90 outside it's 76 inside, if it's 100 outside then it's 84 inside. So what?

Well my point is that 15,000 BTU's is the max my generator can handle. I think you should be looking at the power you are willing to devote to the AC then figure if you are happy with a certain temperature drop from that setup. I know that I'm happy with 12-15 degree difference. Sure sometimes it's a little hotter than I like but it's still a big improvement. A larger AC unit means a larger generator, more fuel and larger wiring.

Maybe next year I'll look at AC for my boat.

Hope this helps
John
Great advise John: another boat in my club in VA had what I considered a half sized unit, his boat was a whole lot more comfortable than a boat without AC in the day time, and I am certain his boat was wonderful at night.
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Old 15-08-2017, 19:36   #15
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Re: Sizing the right air conditioner for your boat

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So the point is, if it's 80 outside I can get it 70 inside no problem, if it's 90 outside it's 76 inside, if it's 100 outside then it's 84 inside.
John
Exactly! If it is 100 outside, 84 isn't so bad is it?
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