Pump Up the Volume

Serenity is equipped with a Westerbeke (now Universal) M-35B diesel engine.

It’s a wonderful power-plant–at 35 HP more than powerful enough to drive the boat at hull speed in rough seas and very economical.  Our engine has seen light duty, with a total run time so far of 171 hours.  (It’s a sailboat, after all.  The whole point is to not use the engine.)

I have only one complaint: the raw water pump is a crappy design. The job of the raw water pump is to circulate sea water through a heat exchanger to cool the engine. A simple but important task. Inside the pump body is an impeller that needs to be checked and changed periodically, at least annually.


The picture above is of the Sherwood G-908 pump specified by the factory. My first issue is that the body of the pump is made of a material that rusts.  I understand that the body of the pump is not supposed to come into contact with the seawater (the pump chamber proper is bronze), except that it does. If the seals start to leak (as occurred with this pump) sea water comes out the weep holes, by design.  And the pump is installed in a marine engine room, a harsh environment.

The second issue I have is that, to inspect or change the impeller, you have to remove three bolts that are on the back side of the pump body. It’s very, very difficult to get a wrench on the bolts with the placement of the pump.

Just a bad design. But unfortunately, this is the only pump made to fit this engine. In the picture below, you see some of the challenge. The mount plate is small and diamond shaped, and there is little clearance.  In the picture above, the keyed shaft at the top of the pump fits into the slot in the picture below.


Oberdorfer makes a pump that doesn’t have these design issues and has an appropriate pump volume and compatible shaft. It’s also $100 less expensive than the Sherwood and merely ridiculously expensive.  But it has a different shaped base that won’t fit the M-35B (see picture below).  The good news is that was easy to fix.

First step was to make a template of the mounting plate by taking a rubbing and then transferred that pattern to the base of the pump.


Then I broke out the power tools.  I used a rotary tool (basically, a more powerful Dremel) with a cut-off wheel, which went though the bronze like butter, then cleaned up the edges with a grinder and a mill file.  I used a hand-held drill (the tolerances are not tight) to enlarge one of the bolt holes.  Total time was about 30 minutes.   A few minutes more with some bulk gasket material and scissors and I had a stack of custom gaskets.


As an aside for those who know me well, you may have noticed in the picture above that the top-right side of the pump base is still a little larger than the gasket.  This side of the pump base did not need to be altered to fit the engine.  After considerable thought (there may have been twitching), I decided not to give into the OCD and that good enough was preferable to perfect.

One other modification was needed. Because the pump fittings are a little closer together on this pump compared to the the Sherwood, I had to grind one of the hose fittings so that it would clear the other one while screwing it in.


The modified pump was easy to install and generated an equivalent flow of water.  A sea trial verified that the engine temperature is stable under load.

(March 2014 checkpoint:  the pump continues to operate as intended.)



As a final step, I was going to rebuild the seals on the old Sherwood G908 pump so that I would have a spare pump.  Judging by the picture below, I waited too long!  I’ve removed the seals, but the bearings have fused into a lump of rust and the shaft is scored and pitted.  A slight weeping was noted in the original marine survey, so it’s likely that the bearings were toast even then.


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