Loosing generator intake prime

[Norm Mayer]

My 1970 44' Hatt TriCabin originally had an Onan generator, replaced with a Westerbeke just before I bought it 6.5 years ago. Aside from being noisy, it has been flawless on the Ohio River. I recently relocated the boat to N Ft Myers (Yes, we DID it!) but had problems crossing the Gulf. If it was a little rough out, the generator would shut down from lack of water. I would reprime the hose, and it would run till the next time we took a bigger wave.
I have noticed that the impeller is several inches above the water level and I know that impellers don't like to SUCK water, just push it. I have a hard time believing the original Onan's impeller could have its impeller much lower than the one on the Westerbeke. The boat spent its first 20 years in fort Lauderdale, so surely if this had been a problem on the Onan it would have been corrected by now.
My thoughts are repositioning the intake well back on the hull so it doesn't pick up air (but then a long run for the inlet to the impeller) or install an auxiliary electric pump below the waterline. Any thoughts? Both solutions seem extreme. Thanks for any thoughts!

 

[67hat34c]

it should not matter on a self priming pump if above or below water line. my old onan mcck has raw water pump on top for ease of reach. never had the problems you are talking about.

Make sure you have the right size hose for the pump. too large of a hose can make it hard for a small pump to work right. also might want to change the pick up. guessing your pump should have 3/4" to 1" hose, check with westerbeke. also when was the last time impeller was changed? Is the wear plate warn out?

 

[Genesis]

Yeah, I'd check the pump carefully (including the impeller and cover plate) and also check the heat exchanger and rest of the system for possible obstructions.

Momentary loss of water won't cause it to shut down - if its overheating and shutting down you're either running VERY close to the edge all the time on temperature (thus the momentary issue causes it to trip) or there's some kind of problem with the raw water circuit that prevents it from picking back up.

 

[Norm Mayer]

It is not a momentary loss of water. When it quits, it stays quit. We got so any time we hit a big wave, we looked over the side to see if it was still pumping out water. If not, we scrambled downstairs and shut down the genny till I could reprime it. Not fun on a pitching boat and 118 degree engine room temperature!
Interestingly, after checking the output after big waves and shutting down within a half minute or so, we didn't burn up the impeller. I checked the impeller and saw no damage! I have been under the impression that impellers will be damaged in as little as 20 seconds, run dry, but ours is still intact. Also, I have believed impellers are NOT self priming. They need to be below the water line to prime themselves. What am I missing?

 

[Genesis]

Impeller pumps self-prime UNLESS there is an airleak (air is much less dense than water, therefore, a small airleak will prevent re-priming)

Check for one on the intake side of the pumping system.

 

[Norm Mayer]

Thanks, Genesis, I already did that, but it makes sense, and will do it again. I started using pipe dope a year or two ago to make sure everything was sealed, but maybe need a better way, plus I think I will try to overhaul the thru hull and strainer. Genesis, you are the best!

 

[Tom Ipock]

Not sure exactly where you have been, but I have had similar problems while in a lock. The turbulence of the water intering / exiting the vault will sometimes cause the pickup to lose it's prime. You'll have to bleed the air out and it will pick it back up.
Just one of my experiences.
Tom

 

[JCG]

It sounds like to me that you have wear on the impeller pump or in the pump housing that is not noticeable or a leak in the pick up side of the system. If the impeller is worn as long as there is water present it will pump in the same fashion as a centrifigul pump, if it sucks air it loses the water and quits. With a leak in the line, and a manual prime the impeller could still suck enough water to pump with the centrifigal action and pull some air. The genny would get some water maybe enough to keep it running but as soon as the wave trough hits and you suck air there is no water in the pump and it will not reprime. If correct these pumps are self priming. Try a new impeller.
You can check for a leak by using suction on the intake side. Devise a method of pulling water up the line and sealing it of the same way you can suck water up a straw and seal it off, if the water drains out you have a air leak.
My experience is based more on outboards but in theory they should be the same. By the way, a DRY impeller in a outboard for 20 seconds would probably be toast. It takes some time to remove all the water , so with the intermittant problems you have had the impeller probably never ran dry, just quit pumping.

 

[Pascal]

i've never had to prime an impeller cooling pump, it should start pulling water very quickly. in fact, I even wonder how you could prime it. If the pump is above the waterline and you fill the housing with water, the water will escape thru the thruhull and the level will drop back to waterline. If it's below water line, it will automatically fill up, pushing air up the line...

a brief loss of water shouldn't cause the coolant temp to rise quickly enough to shut it down... unless maybe there is a sensor on the exhaust side that woudl react right away to loss of water? i don't know...

how fast were you running / jumping waves ? these boats are heavy and i think it would take some BIG wave for the hull to come out... i assume the pick up is close to center line...

i 'd start looking for a small air leak on the intake side... has to be above waterline, otherwise it would leak in when genset is off. between waterline and pump.

 

[Norm Mayer]

Pascal, all you said makes sense. To prime the system, I closed the thruhull, took the inlet hose off the pump and filled it with water, and shot some dish soap in the pump. I then took the water hose off the exhaust, opened the thruhull, started the generator and waited for a few seconds for a good waterflow from the exhaust hose. I then shut down the gen and hooked up the exhaust hose and was ready to go. By the way the thruhull is close to the edge of the hull, not centerline. Thanks for all the input, everyone!

 

[OldHatt45]

I had the same (sounds like) problem on my 45C a few years ago. I have an old Onan 8Kw. Found out it was the actual impeller pump itself. Not the impeller! apparently the little gear on the end of the pump shaft had a problem or it was the housing, not sure, but replaced the while impeeler pump and haven't had a problem in 5 years.

Old Hatt45