Air lock? - Problem solved

The trip from lowering into the water to the windward side of the pontoon at the marina, would've been 5 minutes including a change from forward to reverse and an increase in revs until control was mine. Removing the impeller to check showed no signs of damage, but I did coat it in the grease that came with it.  The trip from the marina pontoon down the river to my home berth was about 10-15 minutes, I noted no change in engine noise, which without the water going through the exhaust was not 'normal'.

I'll not get to the boat till next weekend.  I learnt years ago from an old mechanic, spark, fuel, air and trained as an Engineer to think methodically, so I'll work from the sail drive intake to the exhaust outlet. I know that the sail drive is not blocked as water poured out when the hose was discharging, so its from the strainer to beyond.

The kit comes with a new O ring, which as a matter of course I change, I did look at back the of the plate and noticed some unevenness. On the yanmar the plate has inscriptions written on it so turning it over would probably be worse.

When I return at the weekend, I'll inspect more closely, see if there's anything that may cause an air leak first, as that's an easy check. I also tighten the bolts in an opposing sequence so not to load one corner at a time.

Thanks for your response.

Last time I was in Bradwell I arrived at 0500 helping my friend deliver 'Escalon' and I may be back with his new purchase.

I thought I had resolved the problem but we went out the other weekend and had to run back to tie up with an engine reaching 105C.

On inspection the new impeller I had replaced during the winter maintenance had shredded with no splines.  I spent a couple of days removing them from the heat exchanger, can never understand why manufacturers use Allen key bolts in places that is difficult to get to.  At least with a traditional bolt you have a chance to get a socket on it.  However, I still couldn't get water through the system.

Turns out that Yanmar provide a wear pack!  This is a space washer that fits behind the impeller to push it against the plate keeping the vacuum!  I bought a new plate as well.

This I fitted and the system is back to normal.

So the top plate is the new one and the bottom the worn one, worn by the actions of the rubber impeller, who says stainless is stronger than rubber.

I now have a filter basket almost full of water at 1,500 rpm with discharge out of the exhaust almost hitting the pontoon.