Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Where

Btu= British thermal units

Disp. tons= boat displacement in long

tons or metric tons

The standard method for determining re-

quired air-conditioner capacity more pre-

cisely than Formula 15-1 permits is to use the

vessel’s cabin volumes below deck and above

deck as a basis for calculating the required

BTUs, as in Formula 15-2.

Formula 15-2. Recommended Air-

Conditioner Capacity

Below-Deck Btu= length, ft.× width, ft.×

headroom, ft.× 11 Btu/cu. ft.

Mid-deck Btu= length, ft.× width, ft.×

headroom, ft.× 15 Btu/cu. ft.

Above-Deck Btu= length, ft.× width, ft.×

headroom, ft.× 17 Btu/cu. ft.

or

Below Deck Btu= length, m× width, m×

headroom, m× 388 Btu/m^3

Mid-deck Btu= length, m× width, m×

headroom, m× 530 Btu/m^3

Above Deck Btu= length, m× width, m×

headroom, m× 600 Btu/m^3

Add 600 Btu for each person in a compart-

ment after the first two people, if more than

two people will be in it for any length of time.

Add 4,000 Btu for compartments with a

fully equipped galley, and add 6,000 Btu for a

large galley (or a galley with a diesel stove) as

might be found on a yacht or small-passenger

vessel over 100 feet (30 m).

Add the requirements for all compart-

ments to get the total Btu for a system for the

entire boat, or treat individual or groups of

compartments separately if that makes more

sense. For instance, you may want separate

air-conditioning systems for forward and

after accommodations areas, and a third,

smaller system for, say, the wheelhouse. This

way, you can run the air-conditioner units

only in the areas being used by the crew.

Add 10 or 15 percent to the preceding Btu

requirements for tropical waters.

Example:If our boat, Kool Kat, were,

say, a 32-foot (9.7 m) express cruiser, we

might find:

Below-Deck Btu= 13 .75 ft.× 11 ft.×

6 .4 ft.× 11 Btu= 10,648 Btu

Above-Deck Btu= 6 .5 ft.× 8 .5 ft.×

6 .4 ft.× 17 Btu= 6,011 Btu

or

Below-Deck Btu= 4 .2 m× 3 .35 m×

1 .95 m× 388 Btu/m^3 = 10,645 Btu

Above-Deck Btu= 2 m× 2 .6 m× 1 .95 m×

600 Btu/m^3 = 6,084 Btu

Total= 16,659 or 16,729 Btu

A 16,000 Btu air-conditioner would do

the job in temperate climates, while two

12,000 Btu units would be about right for the

tropics. You wouldn’t want to install much

more, because you would end up with an

oversized, inefficient system.

Note that if the above-deck area had a

table for four and also a full galley, you would

add 4,000 Btu for the galley and 1,200 Btu for

two additional people. This would bring the

above-deck total to 11,200 Btu.

In large compartments requiring 12,000

Btu or more, it’s best to have two air handlers

on opposite ends (or as close to that as pos-

sible). 11,200 Btu is almost 12,000, so it would

be a good idea to have a pair of 6,000 Btu air

handlers (or a 5,000 and a 6,000 Btu combi-

nation) installed at opposite ends.

AC Power for

Air-Conditioners

This brings us to the question of where you’ll

get the juice to power your air-conditioner.

Except with small units (7,500 Btu or less),

battery banks and inverters simply aren’t up

to this job. Air-conditioners must be run off

AC electric systems. In fact, even a standard

120-volt AC circuit can handle only up to

about 20,000 Btu. After that, you have to go to

240-volt AC power. Our Kool Kat, with 16,000

Btu, is just under the limit here.

If you have a moderately fast boat and

usually anchor at marinas, where you can

plug into shore power, you could install an

air-conditioning system without a gen set

and run it only when you’re at the marina

overnight. Underway—on all but the hottest

days—good natural ventilation should keep

you at least reasonably cool. For continuous

PART FIVE: VENTILATION, AIR-CONDITIONING, AND HEATING

Formula 15-2.