20 passagemaker.com July/August 2018
ENGINE SEAWATER PUMPS
Almost all marine engines rely upon an engine-driven pump to
provide the seawater needed to maintain proper engine tempera-
tures. Taken for granted until they fail, these simple pumps can
bring your cruise to an abrupt end. Servicing a failing impeller-
driven pump is one of the skills the prudent cruiser should have.
Once you successfully troubleshoot your seawater impeller, you
will be underway again with engine temps where they belong
and with your boat blowing soap bubbles out the stern—more
about that later.
Two companies, Jabsco (a Xylem brand) and Sherwood (a
Pentair brand) dominate the market, but JMP Marine (a Korean
company with a U. S. division) is making gains. The following
information applies to all of these brands.
MOVING WATER
The seawater pump moves water from the intake strainer into the
engine’s heat exchanger. The engine spins this pump by means of
a belt or gear. Knowing which type of pump you have is critical. If
it is belt driven, it likely shares this belt with the alternator. If the
alternator seizes and you cut the belt, then you lose the seawater
pump and therefore the engine. If it is gear driven, however, it
might be possible to cut the alternator belt and keep cruising.
The belt or gear spins a flexible impeller inside the pump
housing. The impeller consists of neoprene vanes attached to a
center hub. Neoprene, a synthetic rubber, retains its flexibility
over a wide range of temperatures. Neoprene has good chemical
resistance and seawater poses no problem. Changing the impel-
ler might expose the neoprene to other chemicals, however, and
we will discuss that shortly.
It might seem obvious that spinning vanes will induce a flow of
water, but that alone does not suffice. Inside most pump housings
a cam—a thick metal or plastic insert—rests between the inlet and
outlet ports. The flexible vanes ride over this cam, which
bends them away from the inlet opening where seawa-
ter enters. When the vanes pass over this opening, they
spring back, creating a low-pressure area that draws the
water in. The spaces between the vanes fill with water.
When the vanes come to the outlet port, the cam com-
presses them, forcing the water into the outlet port.
So far we have identified the pump housing, impeller,
and cam. A cover plate and gasket or O-ring provide the
seal needed to contain the water and maintain pressure.
The cover plate is more than simply a cover: The edges
of the impeller vanes ride against the inside of the cap,
forming a seal. The same happens on the other side of
the impeller where it rides against a wear plate.
Finally, we have a seal or two. Gear-driven and belt-driven
pumps have a seal that fits around the pump shaft to keep water
from leaking out. In addition, gear-driven pumps have another
seal that keeps oil from leaking out around the shaft.IMPELLER DETAILS
Prudent preventative maintenance calls for replacing the impel-
ler annually. Sure, it can last longer, but why take the risk? As
it depends not just on hours of use but also a range of other
conditions (how long it sat unused, abrasives in the seawater,
temperatures, and other factors), impeller longevity cannot be
easily predicted. Impellers can also suffer from extended periods
of not being used. How long is too long? That depends on many
factors, including temperatures and impeller size. While sitting
unused for several months (through the winter, for example)
should not be a problem, if the impeller has not moved in a year
or more it should be replaced.
Many boaters go years without replacing the impeller, but
the prudent path would be annual replacement or, at a mini-
mum, annual inspection. Preventative maintenance aside, you’ll
want to know how to replace an impeller in case of an overheat-
ing engine. If your engine temperature starts to increase and the
strainer is clean, the impeller would be the next place to look.
Whether performing a preventative replacement or trouble-
shooting an overheating engine, the first step is to close the raw
water intake seacock and place a reminder tag at the key switch:
You don’t want to go through this whole process only to start
the engine with the seacock closed and toast the new impeller.
Next, you will have to remove the cover plate to access the
gasket. This seemingly simple task can often be far too challeng-
ing. In many cases access can be shockingly restricted, leaving
you to cuss the engine manufacturer or boatbuilder in disbelief.Maintain your Seawater Pumps
Troubleshooter STORY & PHOTOS STEVE ZIMMERMAN
Who thought this was a good idea? The wiring connections
and hard piping will greatly complicate what should be a
simple task of removing the impeller and special tools will be
needed. Learn how to remove the impeller while you are still
at the dock so you’ll be prepared if it fails when underway.