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14-08-2017, 09:33
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#1
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Registered User
Join Date: Sep 2016
Posts: 503
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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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14-08-2017, 09:40
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#2
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Writing Full-Time Since 2014
Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 9,559
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Wckoek
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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)
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14-08-2017, 09:55
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#3
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Registered User
Join Date: Sep 2016
Posts: 503
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by thinwater
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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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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14-08-2017, 10:04
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#4
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Registered User
Join Date: Dec 2015
Location: Fort Pierce Fl
Boat: Gulfstar 36 MS
Posts: 210
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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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14-08-2017, 10:07
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#5
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Registered User
Join Date: Sep 2016
Posts: 503
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Outlaw7
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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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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14-08-2017, 15:19
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#6
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Senior Cruiser
Join Date: Nov 2013
Location: Port Moresby,Papua New Guinea
Boat: FP Belize Maestro 43 and OPBs
Posts: 12,888
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Wckoek
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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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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14-08-2017, 15:20
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#7
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Registered User
Join Date: Apr 2017
Location: Winnsboro, Texas
Boat: Catalina 30 MKII
Posts: 264
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Wckoek
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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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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14-08-2017, 15:44
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#8
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Writing Full-Time Since 2014
Join Date: Nov 2008
Location: Deale, MD
Boat: PDQ Altair, 32/34
Posts: 9,559
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Wckoek
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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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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15-08-2017, 08:55
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#9
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Registered User
Join Date: Mar 2011
Location: fl- various marinas
Boat: morgan O/I 33' sloop
Posts: 1,447
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by ValiantV
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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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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15-08-2017, 09:20
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#10
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Registered User
Join Date: Feb 2011
Location: Meandering about the Gulf of Alaska coast [NNE Pacific]— where the internet doesn't always shine... [Even Elon's...] Homeport: Wrangell Island
Boat: Nauticat 43 [S&S Staysail Ketch]
Posts: 1,662
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by Wckoek
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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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
__________________
SV Denali Rose
Learning every day- and sharing if I can.
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15-08-2017, 09:22
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#11
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,453
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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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"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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15-08-2017, 09:38
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#12
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Registered User
Join Date: Sep 2016
Posts: 503
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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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15-08-2017, 18:28
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#13
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Registered User
Join Date: Aug 2017
Location: Missoula, MT
Boat: Flicka 20
Posts: 94
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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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15-08-2017, 18:53
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#14
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Registered User
Join Date: Jul 2008
Posts: 86
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by CaptainJohn49
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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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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15-08-2017, 19:36
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#15
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Registered User
Join Date: Apr 2016
Location: Back on dirt in Florida
Boat: Currently in between
Posts: 1,338
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Re: Sizing the right air conditioner for your boat
Quote:
Originally Posted by CaptainJohn49
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
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Exactly! If it is 100 outside, 84 isn't so bad is it?
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SV Bacchus - Living the good life!
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