Airlock in Raw Water Cooling System

[vonchris7]

I took my boat out of the water recently and redid the bottom paint. While out, I took out the old impeller, looked great, so I put it back in. When the boat hit the water and I started the engine there was no raw cooling water coming out. Shut er down...

I'm wondering if I have an airlock in the raw water cooling system now and if anyone has had a similar experience, and how I might go about resolving the problem.

It's a Perkins 4.108 with a closed loop freshwater system, heat exchanger to transfer heat from the freshwater cooling with raw water.

[model 10]

I have that problem sometimes. I remove the water hose at the elbow and water starts flowing. No idea why.

[Tkeeth]

If you can remove the hose up near the pump and water gets to that point, it will probably pick it up. I also have a 2 inch to 3/4 inch pvc adapter with a water hose connection that I can screw into the top of the strainer and just turn on the water hose. Works great for cleaning and flushing.

[vonchris7]

Tkeeth,

I have water flow at the impeller... when I take off the impeller cover water flows even with the seacock closed. So I'm guessing I also have a problem with the seacock valve since it's in the closed position with water still coming in.

[redbeard1]

Hello,

On most boats, the raw water pump is below waterline. This makes it simple to prime the pump as water will flow into pump as long as seacock is open.

With the seacock closed, water will still pour out from your open raw water pump as any water downstream of the pump that is in hose that is higher than your pump will want to drain out. The amount will depend on your hose set-up (height over pump, loops). On our boat, we usually drain 20 oz with seacock closed.

If possible, put a large pan under your impeller cover, close seacock, and loosen cover screws. Water will begin to dribble out. If the pot begins to overflow, you may have seacock problem. Hopefully, water will stop flowing and you can continue to debug your other problem.

Regarding other problem and maybe stating the obvious. Could the painters have plugged your exhaust tru-hull?

[Captsteve53]

So as i understand it :

1) You probably have a non functioning intake valve in a partially opened or closed position ?
2) May be enough to leak out but not supply the pump when running

3) I have numerous rubber impellers (even new ones) that look great but have come un bonded from the shaft so dont turn when installed and or the drive pin has fallen out, some have splines so drive pin wont apply

4)With the impeller removed check to see the drive shaft is turning(turn over the engine with stop button engaged or just flick it over with some one watching the shaft,
5)If the impeller and housing/cam plate are in good condition these pumps should suck water very well with no priming required,

6)Clogged up heat exchanger/oil cooler or exhaust riser blocking the flow of water,

But reckon you start with the sea cock/valve issue


Cheers Steve

[Captsteve53]

Forgot to mention that some systems have the gearbox cooler in the suction line between the sea cock and the pump, which can cause a severe restriction when trying to prime,


Cheers Steve

[bongo]

The Raw Water Pump is a Positive Displacment pump, and if it is working properly it should be able to pump water whether or not it is slightly above or below sea level. It should also be able to draw water through an air pocket.

Our Westerbeke 40-108 is basically a Perkins 4-108. The raw water goes through the strainer, refrigeration, v-drive, transmission, jacket water, and lube oil coolers before it goes overboard through the exhaust elbow. The water has no problem navigating all of these heat exchangers. Of course, if there is mud, growth, or an obstruction anywhere in this system, then you are going to have problems.

I once had a small octopus living in the cage at the thru hull suction. If one opened the thru hull valve, water would flow. If one attempted to suck water with the raw water pump, the flow would stop, as the octopus would block the inlet. This had me baffled for a while

Try sucking water from a bucket, or from the suction strainer. It may be your thru-hull valve itself, or an obstruction. It is a process of elimination.

[vonchris7]

Well I figured it out with some help from a mechanic at the marina...

There's a small metal plate that fits into a notch in the impeller. I found the part in the bottom of a bucket I put below the impeller housing but had no idea what it was for. So basically without the little metal plate in place the impeller wasn't spinning.

Thanks for all the great advice, always love this forum for that!!

[bobnlesley]

Originally Posted by vonchris7

Well I figured it out with some help from a mechanic at the marina...

There's a small metal plate that fits into a notch in the impeller. I found the part in the bottom of a bucket I put below the impeller housing but had no idea what it was for. So basically without the little metal plate in place the impeller wasn't spinning.

Thanks for all the great advice, always love this forum for that!!

That would pprobably be a woodruff-key, they certainly help.

How about the leaking/failed seacock? How long did you allow it to leak water out of the pump housing; if only for 10-15 seconds, then it might well be that this was just seawater already in the system draining out and that the seacock too is fine?

[vonchris7]

Originally Posted by bobnlesley

That would pprobably be a woodruff-key, they certainly help.

How about the leaking/failed seacock? How long did you allow it to leak water out of the pump housing; if only for 10-15 seconds, then it might well be that this was just seawater already in the system draining out and that the seacock too is fine?

Yes... I googled the woodruff key and that's it... guess I should order a couple of these as spares. Looks like something to keep on hand.

Regarding the seacock... it seems there were a couple of reasons for the water flow. First, I had the seacock turned past 90 degrees allowing some of the flow. The mechanic also pointed out since I have a dripless shaft seal it was contributing water to the same system.

[model 10]

Make sure you get a stainless key.