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Old 08-06-2005, 09:50   #1
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Marine Air-Conditioning

I've noticed several requests for A/C information (on other forums) - so here's a little basic information on Marine Air-Conditioning:

A-C Cooling Pumps:
Centrifugal type water pumps offer several advantages over positive-displacement types. They typically run quieter, longer and consume less power. Unfortunately, centrifugal pumps are not very forgiving when it comes to poor installations ~ they will NOT run ‘dry’ (check & clean filtres often). If you are installing one of these pumps make sure you understand what is required to ensure a good self-priming installation.

The sea water requirements for cooling Marine Air-Conditioning equipment is about 200 gallons/hour (3.33 gpm) per ton of air conditioning(12,000BTUs= one ton) at a maximum ambient temperature of 100̊F.

In the ideal installation, the cooling water enters the vessel via it's own through-hull located deep on the hull. The incoming water then rises to the sea strainer and continues smoothly uphill to the inlet of the water pump (The through-hull, sea strainer and pump are positioned so as to remain under the water line and continue this uphill sloping attitude at all angles of heel). The discharge outlet of the pump points upwards with the connecting hose continuing to rise smoothly into the Condenser Unit inlet. From the Condenser Unit outlet, the connecting hose then continues to a discharge through-hull located above the water line and above the level of the water pump.

Sizing Marine Air-Conditioning (BTU's Needed)

Two “Rules of Thumb”:

Below Deck: 14 BTUH/Cu Ft
Length x Width x 6' average height x 14 BTU
10' x 7' = 70 sq.ft.
70 sq.ft. x 6' = 420 cu.ft.
420 cu.ft. x 14 BTU = 5880 B.T.U. + 1000 BTU


Topside (area above main deck): 17 BTUH/Cu Ft
Length x width x 7' average height x 17 BTU
18' x 11' = 198 sq.ft.
198 sq.ft. x 7' = 1386 sq.ft. 1386 x 17 B.T.U. = 23,562 BTU + 1000 BTU

“How to Select Air-Conditioning for Your Boat”
“DIY Air-Conditioning” (Practical Sailor)

Sizing Refridgeration:

Use the chart below to estimate the total 24 hour heat load experienced by a couple living aboard in a tropical climate. First, measure your box to determine it's interior surface area[1] (including partition dividers) in square feet. Then multiply it by the numbers below.

Insulation "R" Value [2] (Insulation Thickness) - Refridgerator Box - Freezer Box
R10 (2") - Fridge 150 BTUs - Freezer 280 BTUs
R15 (3") - 120 BTUs - 225 BTUs
R20 (4") - 100 BTUs - 185 BTUs
R25 (5") - 90 BTUs - 170 BTUs
R30 (6") - 80 BTUs - 160 BTUs

[*1] While some authorities prefer to estimate heat load based on the volume (ie. cubic feet) of the ice box, others have found it more accurate to make the calculation based on the interior surface area. This method better estimates the additional heat load which occurs when an odd box size gives reduced volume but a large surface area.

[*2] The performance given is based on the typical "R" value for good quality polyurethane foam board. Two-part "pour-in" foam and wet foam of any kind will have a lower insulation value.

The very best website I’ve ever seen for Marine Refrigeration is Richard Kollmans’s sites at: ~and~

Gord May

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Old 27-06-2005, 15:12   #2
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We had a Taylor Cruisair AC system installed last summer on a 1980 Seafarer 30 sloop. The AC seawater pump is a March which is simple to install and maintain. This pump MUST always be below waterline to assure a supply of water or the AC system will go into shutdown mode to protect itself. The system includes an optional reverse heat pump for chilly mornings. All in all it has been one of the best improvements to date on this Great Lakes boat. I would strongly recommend installing an air bleed valve at the high point in the line between seawater filter and March pump inlet as you will have to bleed the air out of this line if you lose prime (for example each time you recommission in the springtime, or each time your powerboat goes up on plane.) This system also saves a lot of hassle of pulling off all the intake hoses to the water pump to bleed the lines. unfortunately Taylor/Cruisair don't tell you about installing such a bleed system in their installation instructions.

MitchM and the Wild Rover, out of Erie PA

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Old 27-06-2005, 19:48   #3
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After considerable debate, I am installing an air conditioner on my boat now. This weekend I worked on mounting the unit and plumbing the water side. I followed all of the stated recomendations except one. I wanted to avoid drilling another hole in the hull, so I tee'd off of the head intake for the water supply. I also tee'd into the shower sump overboard discharge for the AC raw water discharge. After trials and some consideration, I am going to make some changes...

The water pump sucks a fair amount of air into itself, backwards through the head. It's enough air that I am worried about damaging the pump and the head. I chose the head through-hull since it was 3/4". There is another through hull near by that is used for a wash down pump, and has a future use for a water maker. It is only a 1/2" through hull, but I am going to switch to it and then swap it out for a 3/4" at the next haul. I don't envision running the AC at the same time as the wash down or watermaker. Although it is smaller, I think I will have better flow as the head intake has a perferated cover over it that restricts the flow area to about half of the total area. I forgot about this fact until after I was done with the install.

On the discharge side I plan to install a new 1/2" through hull. Sharing the discharge with the shower sump makes me nervous , I am affraid that I will forget to shut off the valve to the shower and at the same time have the check valve fail, causing me to pump raw water into the boat. That would be a bad day.

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Old 27-06-2005, 23:49   #4
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March Pumps:
March pumps are centrifugal, low pressure pumps, with a magnetic motor-impeller coupling - hence, no valves or diaphragm to wear out. The pump motors are hermetically sealed in epoxy. They are liquid cooled, allowing them to be run either submerged or in open air - but must be mounted below the waterline for proper pick-up and lubrication. Do NOT run these pumps dry !!!
They are fairly expensive little pumps (ie: > $225 for most)
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Old 18-07-2013, 11:10   #5

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Re: Marine Air-Conditioning

Good info here. How is hose run calculated into total head?
Eg from my pump to the outlet is a 3' head, however there is about 30' of hose from the pump to the ac. What is my total head?
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