Raw Water Pumps

[Rob19574]

Hi All -

I had a general question with regard to Raw Water pumps: Can you blow air OR does raw water of any amount flow past the raw water impeller when the pump is at rest?

Here is some background on my situation- I have an older Yanmar YSM12 and had it rebuilt about 5 yrs ago. Had issues with the raw water cooling system whereby I kept losing prime and had to constantly bleed the air out of the system to get the pump working again. It would hold water prime but after sailing (with turbulence and such running under the hull / thru-hull) - air entered the system and the pump would fail to re-prime on its own. I understood the issue was caused by air entering the system either through the pump cover plate (plate cover can wear down) or shaft seals. Rather than rebuilding an old pump I purchased and installed a new pump and the problem was solved. This spring however - I changed the impeller and the problem came back. Yesterday, I had everything apart trying to troubleshoot again and noticed that I could blow air through either side of the pump (at rest) and air would blow through at a restricted rate. Then when I had the pump reinstalled on the engine - a trickle of raw-water would flow through when the pump was not running (noticed this prior to installing the outlet line when I opened the thru hull). Odd as I thought the pump would hold back raw-water. The pump is 2 years old and looked in great shape - little no wear and the impeller matched the old one exactly. Luckely I have an inti-siphon system and all new hoses on the engine

In the sort-term, I close the thru-hull when I shut down the engine to allow it to hold its prime and then cloesly monitor the system upon re-start...but its not the way the system should functions.

Appreciate any tips on this one.

Thanks - Rob

 

[kloudie1]

Rob, the impeller will not make a prefect seal in the case so there can be a small amount of air or water flow through it at rest.. but they all do that; an older worn one will do it more than one that is new.It may be that the new one was sufficiently tight at first to stop the problem... sounds like an air leak in a hose or part of the raw water system that is above the waterline..Still, it seems that the pump should self prime and fill the system..?? I dunno. but I'd bet on an air leak .. raw water pump cover gasket, or hoses after pump or anti siphon check valve, or the hoses themselves or connections anywhere in the loop..

 

[Warren Milberg]

If you had a 2 yr old raw water pump that worked perfectly until after you changed the impeller, it sure sounds like something you did caused the problem. I'd check to make sure you used the correct impeller, installed it correctly, and that you properly seated and sealed the water pump cover gasket.

 

[Calif. Ted]

FWIW raw water pumps are "rotary vane" pumps a type of "positive displacement" pump which means it can pump both air and water. Positive displacement pumps are self-priming and are used as priming pumps for larger centrifugal pumps. So, as Warren said, something's wrong, leak somewhere.

 

[sailaway78]

Check your raw water strainer

Not too long ago, I had a similar problem. Found that the raw water strainer was leaking a little air, at the bottom gasket. It would not leak water, when the engine was runnning, but would suck enough air to allow the pump to get an air lock.

 

[Rob19574]

Thanks for the input guys. Yeah - I am thinking that the issue may be on the inlet side with a slight leak somewhere in the one of the hoses. The strainer had some older hoses that need an upgrade.

I will post back when I figure this one out.

Thanks - Rob

 

[Paul F]

Is your water strainer below the water line? In an older post someone had a similar problem because the strainer allowed air the enter the system. Mine is below the water line, which keeps air out of the line. When air enters and an air lock develops and the pump ceases to work.

 

[Stu Jackson]

KISS, check the new gasket you put on and check the fairness of the faceplate against the impeller. If the faceplate is scored, either fair it or buy a new one.

 

[Maine Sail]

Raw water pumps wear not just in the face plate but in the body as well. It is not unusual for a couple of year old pump to leak air or water but it should still self prime if the impeller is new. Perhaps you got the wrong impeller or as others have suggested have another air leak.

 

[Stu Jackson]

There's also the pump's GUTS

As Maine Sail noted. Read this: http://c34.org/bbs/index.php/topic,5073.0.html the pump references on page 2 show the blowup and all the parts. I learned about this in 1998 when we first bought our boat, and replaced the whole pump. If and when I have to do it again, I'll just do the seals and bushing.