Cavitation

[Deadline]

My Yanmar 2GM20F kept overheating despite replacing the mixing elbow and the thermostat. It even blew apart the exhaust hose to the muffler, wow.

My ace mechanic Steve Farris at York River Yacht Haven found that the water was reaching the water pump but not going beyond because of cavitation. I had put a paper gasket on the plate as well as the 0-ring, and the upshot was an air leak that prevented the proper vacuum. The pump didn’t leak because it wasn’t really filling with water.

I never would have figured this out. DIY has its limitations.

The mechanic said that one way to guage water going through the engine is to slowly disengage the line as it reaches the mixing elbow. If there’s no water or just a trickle, that narrows it back to the pump.

 

[Allan12210]

I had the same problem with never knowing if the sea water pump had lost its prime or the mixing elbow was blocked. Scrounging around a surplus store one day led me to this 10 gpm flow meter that I installed after the strainer. At 3200 rpm it indicates 7.5 gpm which is just what my service manual says is the capacity of the pump. At idle I see about 2.5 gpm. If it reads zero suddenly, that would indicate no water in the pump. If I see a general taper off of the expected flow it has been due to clogging in the strainer or intake through hull. I haven't experienced a blocked mixing elbow (yet). I try and ream that out every so often when I remember.

Allan

 

[Bill Roosa]

Pump working indicators

Without putting too fine a point on it you could gage the amount of water coming out of the exhaust thruhull.
Another way is to plumb a small (1/4") hose from somewhere between the pump output and the exhaust elbow to a place where it can drain overboard that is visible while in the cockpit. You may have to place a metering valve on it to restrict the flow. When you see water coming out of the hose you know that there is water flowing from the pump.