boat owner

E

ngine cooling
systems vary in their
complexity, beginning
with the cheap and
nasty direct or raw water
cooling systems where sea
or river water passes directly
through the engine block for
cooling purposes.
There is only one advantage
to this system, and that is
cheapness. It is seldom seen
on modern engines, although
the Americans still seem to like
it on their big V8s. Raw water
cooling shortens the engine’s
life and also signifi cantly
reduces engine effi ciency as
the engine always runs well
below its designed running
temperature of 80-85°C.

Fresh water cooling systems

For raw water-cooled engines,

the normal recommended

running temperature is about

52°C as this cuts down the

amount of salts and sediment

deposits that cling to the

cooling system walls while

running. Over a long period

the cooling system gradually

becomes clogged and engine

temperature rises, causing

hot spots in the areas where

the water can pass through

least effectively. Having said

all this, thousands of engines

are still running well despite

being under-cooled

throughout their lives!

There is very little that can

be done to prevent silting of

the engine block. It is possible

to occasionally backfl ush

the engine by removing the

hoses and running clean

water through the system

as this will remove loose

debris, and if done regularly

from new it does make a

difference to the engine

cooling passages.

The far more effi cient indirect

or fresh water cooling systems

allow engines to run at their

correct temperature of around

85°C, and by allowing the

introduction of coolant inhibitors

and antifreeze into the cooling

system provide for long and

effi cient engine life.

Unfortunately it is not all good

news as a regular service regime

is required to keep the cooling

systems of fresh-water-cooled

engines working correctly. This

mainly involves cleaning the small

tubes in the tube stacks located

within the heat exchanger, the

engine and gearbox oil coolers –

and also, on more powerful

engines, the intercooler that cools

the turbo and supercharger

charge air before it enters the

cylinders. In this latter case, it is

not only engine temperature but

also engine performance that

suffers when the tubes become

blocked. Heat exchangers, oil

coolers and intercoolers come

in many designs, so the method

of dismantling prior to cleaning

the stacks will vary slightly

between engines. The engine

featured is a home-marinised

BMC 1.5 diesel (one of the best

little diesels of its time) with less

than 500 hours of running.

Raw water-cooled engines

Thermostat

Bypass

Cam

Raw water

pump Strainer

Fresh water

Raw water

Heat

exchanger

Water-cooled

manifold

Thermostat

housing

Relief valve

Exhaust manifold

Strainer

Raw water pump

Water circulating

inside block

Cam

Bypass allows

water to

recirculate

when the

thermostat

is closed

PRACTICAL

Is your boat’s engine beginning to run hotter than normal? It may

well be time to service the cooling system, suggests Tony Davies

STEP

BY

STEP

How to service your

cooling system