Yachting and Boating World forums: Air lock in my cooling system..... I THINK???

I am the proud owner of an old wooden yacht which is fitted with a Yanmar 2QM 15 hp engine. A strange thing has happened recently involving the raw water cooling. Last month I was sailed for about 7 hrs in a good following sea before starting the engine. The overheat alarm sounded and no water was being pumped. After being towed to a nearby mooring, we checked the seacock and strainer and they were fine. The impellor had been changed 1 week earlier. After about 20 mins, I fired it up again and water was pumping out fine. I have used the boat again since with no problem until yesterday. To clarify a point, the yacht has a shoal draft keel with bilge plates so can take the ground. I anchored on a sand bank which caused the boat to lay over to the starboard side more than normal. The engine seacock is on the S/board side and has a strainer built into it. Sure enough when I started it up... No water was being pumped. I turned off the engine checked seacock and strainer as before. All fine. A friend towed me for about 100 metres to clear the sandbank. I think started her up again... hey presto... water was pumping out fine. I then motored for 7 hours without a hitch.
ANY IDEAS? As for the set up, there is a pipe running straight from the seacock/strainer to the waterpump. There is no non return valve fitted as far as I can see. My guess is an air lock maybe in the strainer which clears itself. On the last occassion, I opened the seacock up just before starting the engine so not allowing enough time for the water to push out the air. I really don't know.

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