Airlock in raw water cooling.

[Epicurean]

On my catamaran while sailing in rough seas it appears that gradually the foam from breaking waves passing under the hull aft, where the salt water pick up is, fills up the container basket with air and then to the pump and an airlock develops and the pump looses its prime, then the engine overheats. It's almost always on the high, windward side. The Johnsons pump doesn't seem to be able to self prime.
Anyone else have a similar experience? Any solutions?

[Tin Tin]

Is the pump below the waterline? If so a solution may be to replace the filter basket with an inline strainer that is rated for below the water line or one that is located at the sea-cock.

[alamoana]

Happened to me too…the strainer lid was not closing properly. A little Vaseline on the Oring and care when closing the lid fixed it. Good luck

[Tsuru]

Originally Posted by alamoana

Happened to me too…the strainer lid was not closing properly. A little Vaseline on the Oring and care when closing the lid fixed it. Good luck

I agree, Look the entire system over carefully for an air leak (I'd recommend silicon grease over Vaseline fir O rings). Also examine the pump faceplate. If it is scored it will reduce the pumps efficiency. This happens slowly and shows up with similar symptoms.
Good luck,

[Epicurean]

Tin Tin. I'm thinking about that.
Alamoana and Tsuru, leaks in the system are definitely issues that need to be addressed. That's not what I'm talking about here, though. These are air bubbles generated by wave foam that enter the system through the raw water intake. A perplexing problem

[skipperpete]

Both engines or only one? I’d advise you to follow Tsuru’s advice regarding the water pump">raw water pump end plate.

[Epicurean]

Originally Posted by skipperpete

Both engines or only one? I’d advise you to follow Tsuru’s advice regarding the raw water pump end plate.

Both engines and only when waves are breaking and the engine on the side of the boat exposed to the breaking waves.
The plate is pretty smooth.

[Tsuru]

I respect that you are working through the problem in a logical manner and believe you are on track for finding a solution.
That said, at least in my opinion we are looking at the problem from a different perspective. What we know is that your engines overheat in certain conditions and you see air or foam in the strainer.
While it certainly could be a design problem it could also be an equipment problem.
Rubber impeller vane pumps are a pretty good choice for the raw water cooling application. They are simple, robust, and if they are in good condition they will draw an amazing amount of air, foam, or emulsion through the system. Simply, they are very good at self priming.
As the pump wears it becomes less efficient reducing its ability to draw these burps of air or foam. This could witnessed as air or emulsion in the sea strainer.
Pump wear is normally visually apparent during inspection, but not always.
Most engine manufactures have a recommended guideline for rebuilding raw water pumps (bearings, seals, impeller, inspect shaft, faceplate, not just impeller). In most cases it’s around 1000 hours. If your pumps have 1000 hours on them I’d start with an overhaul. If it fixes the problem you’re golden. If it doesn’t, it needed to be done anyway.

Good luck finding a solution.

[Epicurean]

Originally Posted by Tin Tin

Is the pump below the waterline? If so a solution may be to replace the filter basket with an inline strainer that is rated for below the water line or one that is located at the sea-cock.

Thanks. Actually, I thought about this. It may solve the problem. I'm willing to try it. Yet it's very unsatisfying in that it doesn't tell me why am I (almost) the only one having this problem.

[Tin Tin]

Originally Posted by Epicurean

Thanks. Actually, I thought about this. It may solve the problem. I'm willing to try it. Yet it's very unsatisfying in that it doesn't tell me why am I (almost) the only one having this problem.

Most cats would have sail drive with pick up well below the surface. Other boats with shaft drives may have the raw water pick up further forward. Most planing inboard power boats would need a solution for this, ours was a Groco strainer below the waterline. Also it could be that your water pumps are getting tired and not have the same lifting capacity when new. Could be a combination of factors. Start with new impeller, then new end plate these are not that expensive, then new seal set, failing that get a new pump for one motor and see if it makes a difference. Worst case is you have a spare pump.

[Epicurean]

Originally Posted by Tin Tin

Most cats would have sail drive with pick up well below the surface. Other boats with shaft drives may have the raw water pick up further forward. Most planing inboard power boats would need a solution for this, ours was a Groco strainer below the waterline. Also it could be that your water pumps are getting tired and not have the same lifting capacity when new. Could be a combination of factors. Start with new impeller, then new end plate these are not that expensive, then new seal set, failing that get a new pump for one motor and see if it makes a difference. Worst case is you have a spare pump.

Thanks for your analysis. The impellors are new. The end plate looked fine; not scored. The pumps were recently rebuilt with few hours.so the seals are new. Putting the strainer below the waterline worked for you. But I can't see why. It's a sealed system under vacuum with no air leaks (that I can detect). So having eliminated everything(!), I agree with your suggestion to buy a whole new pump. That makes sense. I wanted to do that before but couldn't get one in Malaysia.