Raw Water Air Lock

[captainfrank]

Ok, so, I'm new here as a registered user although I have used the threads here for many years as an education to particular problems I've had. Here is a issue that has me stumped.

The set up: 1976 Tartan Tock with Ford Lehman 80 aux eng. 7500 hrs to the best of my knowledge. Engine runs GREAT.

I have hanged nothing about my raw water cooling system to have started this issue.

The issue: Occasionally The raw water system will lose prime and the pump will not clear it. Yes I have checked and changed the impeller twice now. The air lock occurs at the raw water strainer which is below the water line. The supply hose to the RWP is below the water line. If, on these instances, I crack open the strainer the air burps out water flows in and I'm back in business. It does seem that there is a vacuum on the strainer when I open it.

No water is leaking from the strainer, thruhull, seacock, or any hoses before or after the pump.

What in the world is happening?

Thanks in advance and I'm sure glad there is a world of folks out there smarter than me.

Captain Frank

[scoobert]

could be a clogged intake, clean and report back.
my ac was doing this last week.

[Sailmonkey]

At 7500 hours it is quite possible that the impeller pump housing has worn beyond spec. Once that happens the pump will move water (with a small reduction in total flow) but no longer be able too be air.
The worst culprits for this are usually the end covers over the impellers. Check it for wear compared to the edges whee the gasket sits.

[sartorst]

I made the mistake of moving my strainer from a hard to access lower location to a higher easier location that was only 4 inches higher all was ok until taken offshore in much bigger waves, after about 30 minutes or so of operation it would air lock just as yours is doing and could be cleared the same way, good to go for another period of time dependent on conditions. What I found was small bubbles would start to collect in the top of the strainer which is where the pump pulls water from also this would in turn air lock The cure was to make sure there was a constant upward travel for the air bubbles to continue through the system and out and not collect in strainer or a high spot in the hose My AC would air lock also if I had been sailing pretty hard the easy cure was to force water from a hose back through the outlet

[River Cruiser]

Also make sure the gaskets on the strainer are in good condition & not allowing air into the system.