water pumping problems

[orion]

I recently bought a SO 32i and sold a Westerly Centaur.

The SO is doing fine however the Westerly (26ft) is having trouble with the engine water cooling and I was hoping someone out there may be able to help.

The boat uses a raw water system, the impeller has been replaced, the inlet filter is clean and hose leading to the engine has also been replaced.

On starting the sea water runs through the engine and exits with the exhause fumes. Everything appears normall, after about 30 minutes, and usually after high revving (to get off marina etc) the pump seems to loose its primming and the water stops pumping.

Any thoughts?

ORION

[rene460]

Hi Orion,

Any chance that it is the hull strainer on the intake below the intake valve?  

I had a similar problem once and could only think of the hull strainer on the hull intake.  It is a bit hard to get at if you do not want to swim, but I took the top off the filter, poured a little vinegar down the filter so it back flushed the strainer, followed by some chlorine bleach cleaner/disinfectant, then some hot water from the kettle just in case some little critter had found a comfortable home.  Then some cold water so the mixture would be well clear of the intake when I started the engine and no further problem.  I am not sure whether I sent the critter packing, dislodged a plastic bag or made a plant let go and washed it out.

rene460

[orion]

Hi, thanks for that, the theory is right but the inlet is perfevtly clear. People are suggesting back pressure from the exhaust may be the cause!!

Orion

[sitara]

Check the exhaust elbow where sea water is added to the exhaust gasses. If this blocks it could cause this problem and one would expect it to be worse at high revs.

[rene460]

Hi Orion,

certainly the exhaust mixing elbow is on the Yanmar service schedule at relatively short intervals. Definitely worth checking as water can get back into the engine if the mixing elbow fails and the result is not good.
rene460

[orion]

Bit of work for the week-end. Will check

Thanking you

orion

[orion]

The problem with the sea water pump failing at high revs was the fact that the exhaust outlet pipe had part collapsed inside and was causing back pressure which stopped the pump from priming when under pressure at high revs.

Thanks to everybody for the help.