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Cooling Systems
KEEL COOLING OR SKIN COOLING
Keel cooling or skin cooling is a closed circuit
method of cooling that uses only fresh water. Engine
heat is dissipated by passing coolant through tubes fi tted
to the outside of the hull (keel cooling), or incorporated
as part of the hull (skin cooling). Keel coolers protrude
from the bottom of the boat. The engine circulating
water pump is the only pump necessary.
The size and length of the tubes is determined by the
horsepower and water fl ow of the engine. When sizing
keel coolers for generator sets, they must be larger than
those for an equivalent propulsion unit because the
generator set normally sees maximum load while the
boat is at rest when no water fl ows past the coolers. Do
not combine the keel cooler for the generator set with
the keel cooler for any other engine on the vessel.
If a wet exhaust is desired on a keel cooled or a skin
cooled unit, a separate pump is required to draw raw
water from a thru-hull fi tting into the exhaust. A high
exhaust temperature switch is also required.
HEAT EXCHANGER COOLING
In a cooling system with a heat exchanger, and
with thermostats closed, a fi xed amount of fresh water
circulates through the engine's cooling loop. When the
thermostats are open, the coolant is directed through the
heat exchanger. It is cooled by raw water that is pumped
in by way of a thru-hull fi tting, sea cock and strainer,
then through the heat exchanger, and directed overboard
through a wet exhaust or another thru-hull fi tting.
Scoop type inlet fi ttings must not be used because
they can cause water to be forced up into the raw water
system when the generator is not in operation if the
vessel is underway. The waterlift muffl er will fl ood and
fi ll up the exhaust manifold with water.
The sizing of the thru-hull fi tting, the sea cock and
the strainer should be no smaller than the inlet to the raw
water pump. For raw water pump inlet sizing, please see
the appropriate installation drawing or call your dealer.
The raw water pump is a neoprene impeller type
pump, which provides a positive suction lift of up to 39"
(1 m). The thru-hull inlet fi tting should be a fl ush type,
located beneath the water level during operation. The
inlet water strainer keeps most debris from reaching the
raw water pump where it could break the impeller or
clog the heat exchanger.
Figure 5: Keel Cooling
Closing the sea cock enables the owner to clean
the sea strainer. The raw water pump impeller is a main-
tenance item and easy access must be provided. Make
sure to provide easy access to the zinc(s) located in the
heat exchanger. If the fresh water pump is belt driven,
access must be available for belt replacement.
Figure 6: Heat Exchanger Cooling
IM1000 02/13
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