MOTC Manual - edited

Cooling System – Boat motors are not equipped with radiators as cars are, but still must
somehow dissipate the heat generated by the friction of the moving parts.
Outboards - Outboards pull in water through an intake on the lower unit. A water
pump consisting of a rubber impeller inside a stainless steel housing circulates water
around the motor and out the exhaust which exits around the propeller hub. This
discharge has two benefits. It cools the exhaust and also muffles the sound of the
exhaust. Outboards have a “weep hole” that discharges a stream of water to
indicate when the cooling system is functioning.
New outboard engines come with ports for connecting to a hose to flush engines
after use in salt water (refer to manufacturer owner’s manual).
For older engines you can attach a flusher connected
to a 5/8 inch garden hose, and operate the engine
for about 5 minutes.
Do not start the motor without water being
supplied to the cooling system.
In outboards, the water pump impeller will burn out
very rapidly without water to cool and lubricate it and
you risk the possibility of the engine seizing because
of overheating.
Make sure when you initially start your vessel that water is being discharged from
the exhaust system. This indicates that the cooling system is operational.
Inboards - An inboard
cooling system
functions like an
automobile engine in
that it has antifreeze
circulating through the
motor. However, rather
than cooling the
antifreeze using air that
passes through a
radiator, raw water
cools the antifreeze by passing through a “heat exchanger”. Inside the heat
exchanger there are small tubes through which the antifreeze flows, surrounded by
the raw water that dissipates the heat, and then is expelled through the exhaust.
Boat motors often have an alarm that sounds when an engine gets overheated, and
the engine will sometimes automatically reduce in RPMs to idle. This is one of the
reasons it is good to carry a copy of the owner’s manual onboard the boat.
DOI – Motorboat Operator Certification Course Chapter 2 – 10