cooling is appropriate only for small, inexpen-
sive runabouts and day boats, and only for
vessels operated in temperatures well above
freezing. For such craft, the savings in cost
and some weight may be worth it. Corrosion
and silting problems can be reduced some-
what by flushing the engine with fresh water
after every run. This is accomplished by
installing a valved T-fitting after the seacock
on the raw-water intake. The seacock is
closed after every run and a freshwater hose
is connected to the T-fitting (see Chapter 17).
The fresh water is turned on, and the engine is
operated for several minutes at idle to flush
the system.Heat-Exchanger Cooling
Most boats use heat-exchanger cooling(also
calledindirect coolingorclosed cooling;see
Figure 8-2). This is similar to the cooling sys-
tem of a standard car or truck, with one big ex-
ception. In cars, the water-cooling system is a
closed loop. The cooling water is mixed with
antifreeze to protect from freezing, reduce
corrosion, and optimize operating tempera-
ture. The coolant path is similar to a raw-
water system except that the water fills the
system and part of a header tank (also called
an expansion tank) and circulates round and
round. (Note: The rule of thumb is that
header tank capacity should be 10 percent of
total cooling system coolant, but follow thePART THREE: EXHAUST SYSTEMS
Figure 8-1. Raw-
water cooling Figure 8-2. Heat exchangers (Courtesy
Caterpillar, Inc.)
manufacturer’s recommendations.) In this
circuit—in a car or truck—the hot coolant
passes through a heat exchanger that is
called the radiator (Figure 8-3). This literally
radiates heat to the air, and it requires a fan
to increase the airflow over the radiator’s
cooling surface.
Radiators won’t work on a boat. Deep in
the hull, there’s no practical way to use an air-
cooled radiator. Instead, a second cooling-
water system is used to transfer heat away
from a water-cooled heat exchanger rather
than an air-cooled radiator (Figure 8-4). This
is quite efficient because seawater is denser
and often colder than air and is easily avail-
able from right outside the boat. The second
raw-water cooling circuit enters the seacock,
goes through a sea strainer, and then enters
the raw-water pump (see Chapter 17). This
circulates the raw water into the engine
cooling heat exchanger. Most commonly,
the raw water flows to the oil cooler and
reduction-gear oil cooler prior to entering the
engine heat exchanger or cooler. Finally the
raw water—in the illustration, the raw water
coming out of the heat exchanger—is injected
into the exhaust and exits the boat, as we saw
in the wet exhaust system in Chapter 7.