The rubber impeller pumps are inherently self-priming, they increase the
volume of the gap between vanes as the impeller rotates past the cam,
pulling fluid into the chambers, thus they can work at elevations above
the outside waterline. That said, they need enough liquid around the
vanes and side plates to form a seal. If the pump is completely dry it
may not prime, and if you're feeling it heat up that may be happening.
It could also be the shaft seals behind the impeller as you look in to
the pump.
Neil Gallagher
Weatherly, 35-1
Glen Cove, NY
On 6/5/2025 10:34 PM, David Knecht via CnC-List wrote:
As I always told my students, there are no bad questions or dumb
questions or insulting questions. Just questions. So I appreciate
everyone trying to help. It is why I love this community. Knowing
you are all out there gives me confidence to tackle problems that
don't have YouTube videos to walk me through.
To sum from some earlier questions:
1. I verified that the pump shaft is turning.
2. I am pretty sure the impeller is turning because the pump housing
got very hot very quickly from a brief test engine run with no water
moving.
3. I can find no obstructions to flow from the seacock to the pump.
You comment actually makes me wonder if I have been working under some
bad assumptions in doing diagnostics. I had presumed that water would
flow rapidly into the raw water pump due to water pressure and the
pump would move it along. But as you pointed out, the pump is quite a
bit higher than the entry point, so maybe the pressure is fairly
neutral there. If I open the seacock, with the cover off the pump,
should I see water flowing into the pump? Does the pump simply move
water that is already there due to pressure flow, or does the pump
actually draw the water into the seacock/strainer by sucking it in?
If the latter, then many of my tests have been beside the point and
nothing is blocking the flow of water into the boat. It is more that
the pump is not drawing the water in, which gets back to the air leak
issues mentioned.
1. If it is air leaks in the pump seal, I would propose to use a
gasket sealant line Form A Gasket in addition to the o ring to seal
the cover? Any reason that is a bad idea?
2. I asked earlier, but is there possibly a pump priming issue? I
have never heard of someone having to do anything to prime there raw
water pump. I did read an online engine thread where people had this
problem with a particular engine, but simply revving the engine got
the pump primed and pulling water. Dave
David Knecht
S/V Aries
1990 C&C 34+
New London, CT
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