Air lock? - Problem solved

He was built in 2007 and we bought him second hand in 2010, the Yanmar instrument panel isn't the best and as such the hour meter stopped working as did the tachometer and I've since replaced the whole system with my own instruments and warning lights and ignition.

The plate is original.

Since ownership the engine has mainly been used to move in and out of marina's/ anchorages with occasional long runs on windless days, 2,200 rpm cruising speed.  We did get caught out rounding Alderney and had the revs about 3,500 for several hours rounding the corner into the harbour. But with Covid and one thing or another the last few years we've not really been anywhere.

I checked the filter basket housing and that was solid no cracks or air leaks from the O ring, jubilee clips or cuts behind the clips all proved negative.  I took the inlet off the basket and fitted it directly to the inlet of the impeller, careful as this is below the water line, so expect water ingress. This removes the need for the pump to created a negative pressure to draw water in and with the positive pressure from being under the water line the pump worked happily pumping water out of the exhaust.

If you're struggling to get water through, having learnt what I have, I'd smear Vaseline over the front side of the plate and switch it round, so the outside is in, yes there's inscriptions engraved on the outside, but these are shallow compared to the 'gouging' my plate has suffered.  This should improve things but will guide you to locate the issue.

Yanmar provide a wear kit, a spacer washer to push the impeller against the plate, but at a sixth of the cost of the plate itself, just replace the plate.  I'll keep the old plate as I might just polish out the inscriptions and have a spare.

Obviously its a slow process of wear and so you don't notice it.  The usual is to remove the basket at the end of a trip and clean any muck out, so engine off.  Just see what the level of water is next time your on the boat for a set rev, at 1500rpm mine is now almost full with water splashing the top and keep an eye whether it starts to drop as the vacuum within this negative pressure side of the system drops.