It should be noted that engine manufacturers pay special attention to the
design of engines for direct cooling in order to minimise corrosion in the
cooling passages. However many small craft owners marinise standard car
Raw water (river or seawater)
is pumped directly through the cylinder block, exhaust manifold, exhaust
silencer (if fitted) and exhaust pipe. To ensure efficient engine performance
it is essential that an optimum operating temperature is being maintained by
regulating the amount of cooling water through the cylinder block, either
using 'Manual' or Automatic temperature control.
A hand valve is fitted in
parallel to the engine block. When the engine is cold, the valve must be
opened to reduce the flow of water through the block. As the engine
temperature increases, the valve is closed gradually until optimum temperature
IMPORTANT. Do not regulate the pump flow by fitting the by-pass valve between
the pump discharge and suction or by restricting either the pump discharge or
A Marine thermostat is fitted
after the cylinder block and is closed when the engine is cold allowing most
of the cooling water to be pumped via a spring loaded back-pressure valve to
the exhaust. As the engine temperature increases, the thermostat opens until
the required operating temperature is maintained.
A pump recirculates Fresh
Water on a closed circuit through the cylinder block, thermostat and around
the tubes of a heat exchanger (Primary Cooling Circuit) which is often an
integral part of the vented expansion - or header tank. Cold Raw Water is
pumped by a second pump through the heat exchanger tubes (Secondary Cooling
Circuit) and maintains the fresh water in the primary circuit at an average
temperature of 80-90oC.
Keel Cooling is basically the
same as heat exchanger cooling, except that the fresh water is recirculated
through the keel cooling pipes which are fitted outside to the bottom of the
boat. The heat generated by the engine is dissipated directly to the raw water
flowing around the pipes.
Raw water cooling systems
demand a great deal from circulating pumps. They must have the unfailing
ability to self prime at various engine speeds and must be able to pass such
solids as sand and silt without impairing the pump's flow.
On average, petrol engines and fast running diesel engines require a raw water
pump flow of approximately 8-8½gpm (36-39l) for each 100BHP for Direct Cooling
systems and 14½-15½gpm (65-70l) for each 100BHP for Heat Exchanger Cooling
These simple actions will only take a short while to carry out, they may save
you a lot of money.
When you leave your boat for the winter, to avoid frost damage, drain your
pressurised water system.
The easiest way is to turn the pump off, open the taps and allow the water
to drain out, then, open the drain valves to empty the rest. If your system
doesn't have drain valves, disconnect a pipe joint at the lowest point in the
system and allow to drain.
It may also be necessary to drain the pump supply tank and hot water storage
Before you leave check you have drained as much water as possible.
If your system is frozen and you find it in this state check everything.
Pumps can crack, water storage heaters can split, pipe can split, joints can
break. If you are using Hep2O then the pipe should be undamaged, it is however
not uncommon for a joint to split.
Do not turn on your pump until you are confident there are no leaks in the
system. You may pump the contents of your freshwater tanks into the bilge, if
your bilge pumps are frozen this may have knock-on effects.
Troubleshooting a Pressurised Water System
Insufficient pressure or flow:
Check the pump is running correctly and in the right direction.
Is the correct size pipe installed for the size of pump? We do not
recommend using smaller pipe bore than the bore of the pump ports.
Check for blockages in the pipework.
Check the pump specification, is the pump large enough for your system?
Check the pipework layout, are there restrictions or is the layout over
complicated? Could you or should you consider a larger pipe size? Larger
vessels and longer pipe runs can often benefit from larger bore pipe
particularly for main runs and up to showers.
Motor runs but no flow:
Check the pump is priming -
Is there water in the tank? If there is no water the pump can't generate
pressure so it won't turn off.
Are there any air leaks in the suction line? Check all joints.
There may be an airlock in the inlet line, ensure there is a steady
gradient to the pump without any dips.
Check for closed valves in the inlet or blocked pipes.
Check the valves in the pump (or the venturi) are not blocked, dirty or
Motor runs hot:
Motor is overloading - check the fuse size, check the motor condition
The pump maybe cavitating due to a blocked inlet - check/clear the inlet
The pressure switch cut-out is set too high and the pump can't reach the
pressure - check and reduce the pressure if required.
Pump wont turn off:
Voltage is low at the pump - check the voltage at the pump, if it's low
check the batteries, check the wire size and cable run against the pump
Water tank is empty - check/refill the tank
Blocked orifice in the pressure switch - clear the blockage or replace
Blocked or worn jet pump impeller
Airlock resulting from lack empty water tank