Tony Abbott wrote:

On the installation of a fresh pair of marine engines how do you prime the
raw water pumps. Each engine has a Sherwood (G7 I believe) with 1 1/4"
inlet/outlet. The boat is currently out of the water as we are finishing a
long restoration process. I have a garden hose with the water intake
adapter hooked to the intake under the boat (yes the correction piece for
raw water pickup). The path the water goes is intake from under the boat,
through a raw water filter, through a transmission oil cooler then to the
pump. There is about 5' worth of hose and the filter and cooler prior to
reaching the pump.

Thanks in advance for the priming answers/suggestions...


Presuming all this rigging is below the waterline, the only problem
might be air trapped in the pipes.

I would expect most vane pumps or displacement pumps would be able
to allow that air to escape during operation, even to the extent of
pumping it through the system.

This business should be self priming. It might now contain some
trapped water, since you have seemingly tuned the engines.

You should probably not try to prime the pumps by putting a water
hose into the exhaust.

If you are worried, after launch, open the water line about at the
level of the waterline, and allow water to fill the pipe as you
release the air. If there is no air in there when you crack the
valve, there would be water already priming the pipe, and that would
indicate the air to be expected found trapped has already escaped.
The carpet was wet when you got there, right?

Only centrifugal pumps require proper priming, other than spitting
on a well pump's leather to enable it to seal and suck out
antipriming air for a few feet.

Of course, a leak in the piping / filter covers would permit the air
to escape, as it was replaced by water under pressure, but a small
air leak above the waterline could break prime on a pump.

You will be checking down below for leaks before removing the
trailer winch cable?

Most automotive engines use a centrifugal pump; it is one more snag
encountered whilst marinizing daddy's old truck engine.

Even if the pipe is full to 1/2" below a centrifugal inlet, there
would be not enough centrifugal sucking out of the air to prime the
loose flippered pump. Rubber vane pumps can work for a few moments
sucking air, but not for long. They get hot. They get torn, they
get loose in the housings. The vanes melt and shred and pill and
plug up engine blocks's passages if run dry for minutes.

Instead, moniter the engine discharge. If there are no leaks below,
and there is no water discharged in the exhaust after it starts,
shut down inside ten seconds and find the problem. Don't wait for
overheat warnings. By then rubber impeller pumps with air leak
induced loss of prime would be toast. If all the exhaust discharge
is below water and you can't normally observe a water discharge from
you engines while afloat, consider a valve on top of the engine to
allow hot wash engine to be discharged and observed for quantity and
temperature once the engine is hot. Do not plan to bleed off water
quantities needed to cool exhaust parts downstream, though such can
be circulated through domestic water heat exchangers, the flow must
be returned to the exhaust cooling discharge. Flow meters can be
connected at this point to positively indicate and alarm water flow
before failure or blockage cooks pumps and exhausts.

Or, use a bucket and hose. Elevate the bucket so the water in it is
even with the boat's waterline or a little below, jamb the hose
airtight into the engine water inlet possibly using a cork with a
hole in it, with the other end in the water in the bucket, keeping
the hose as short and level as possible, and spin the engine out of
gear. The engine should suck the bucket dry quickly, primed or not,
unless the hose gets sucked flat sucking on the bottom of the
bucket. If you want to "prime" the bucket system, stretch an inner
tube rubber over the top of the bucket and lash a hole in it tight
to the hose. Also lash another hose through another hole, and the
rubber to the top of the bucket, reasonably tight. Prime the pump in
the normal manner by turning on the hose, and once the engine starts
and starts spitting water, turn off the garden hose tap, or remove
it from the rubber seal, so the bucket is emptied. Stop the engine
inside ten seconds or so of a dry bucket or hose suddenly above
water level in the bucket.

Caution: this assemblage may leak a little on the testing
personnel, if the engine will not evacuate the entrapped air and
water quickly enough. The rubber bladder may burst, inundating the
persons employed doing the testing. This might be a happy explosion,
since it could indicate that at least two rubber pump vanes were in
servicable condition, and in correct positions within the pump body,
even if the engine does not start or crank.

If you can get it to spit water with the hose priming, but not with
the garden hose off, then your pump, or the other one, is bad. One
pump on each engine, right? One inlet? two engines?

If it sucks the bucket dry, it should do so before we get to here.

But if one supposedly tight rubber vane pump is leaking air, it
could break prime and starve the other pump of prime if the hoses
are both hooked to the same sea chest, and open to the exhaust
discharge. Of two, which one would be bad?

There is a good reason to use seperate inlets and filters for each
engine.

A strong enough pump will shred seaweed plugging a filter, which
actually then becomes a mascerator, and fills your engine cooling
system with insulating debris only a little less long lived than
rubber vane bits. I take it you have a plasic bag / seaweed trap /
macerator, and a sand filter?

Of course, if a centrifugal water pump is mounted on an engine so
what it sits below water line, it would prime itself only once
immersed, afloat, refusing to evacuate air, or our bucket until
forcibly immersed in prime by our garden hose.

This is the one circumstance where you need to ensure the pipes are
full of water, or primed, and tight for air. If so you need a
fitting at the water works to evacuate air and replace it with
water, using a bucket full of water and a funnel. In the best of all
such worlds, a leaky bilge will bathe the engine in cool water,
which it will suck up centrifugally for cooling purposes about as
fast as it comes in, hopefully passing sand enough to keep the
channel clear.

A worn vane pump can still pump some water, but can't suck air well
enough to prime itself, or ensure no air leakage and no loss of
prime in the other engine connected to a common feed.

No water cap on top of the old truck tranny / engine rad?

Terry K