losing prime in cooling?

[shorebird]

Went down to the boat yesterday, opened the raw water seacock, then fired up the Yanmar. As per my usual routine, I checked the exhaust to insure water was coming out, but couldn't see any. I quickly checked the water pump to make sure it was turning, then looked over the transom again to find lots of water, but it took about a minute or more before it began exhausting the water. The pump was just rebuilt last year, and I have never had this happen before. I'm assuming the cooling system lost prime. Is there a way to check? I thought I might check the raw water strainer next time if the engine has not been started for some time. Is this normal? Is there a way to prime the system without disconnecting a hose? I thought closing the seacock would keep the system primed.

[Sailmonkey]

What size yanmar? It's not unusual for the smaller ones to take quite some time to fill up the muffler.

Either way it's normal, but if your impeller is more than a year or two old it should be replaced. As the impellers age they become stiff and less efficient.

[djmarchand]

A minute is a bit long but probably acceptable. An impeller can often look good, but has tiny cracks on the edges which make it difficult to prime. Or maybe the internal cam has worn and it isn't pumping like it should. And yes, closing the seacock is a good way to keep the prime. Just don't forget to open next time ;-).

You can temporarily pull off the hose from the outlet of the heat exchanger to the injection elbow, start the engine and see how long it takes to get the first flow. If you had left the seacock closed previously then you should see flow immediately. Any delay of more than a few seconds and I would replace the impeller.

David