Cavitation or something else?

[Bronteboxer]

I have my boat on this forum restoration under ‘Wooden boat rebuild’ I’ve had it out twice now here on the Norfolk Broads.

First time it was flat calm, no wind no swell. Everything seemed good. The boat performing really well.

Yesterday we took it out again and on calm water when we pushed the throttle, the boat stood up and went. On waters heading into wind with a light swell, on opening up, the boat would start to stand up to go, but the prop would spin up, as we could hear the engine revs rising, but the boat would not pick up, refusing pick up speed as it should.

Given we are on the Broads, most of the time we are pootling about at 4/5mph, so it’s not a major issue, but out in open water in the centre of a big broad. out of season with no other boats about, we thought we would open it up to check everything was good.

It’s a short shaft 4 stroke, Yamaha 15hp on a light 12’-6” runabout.

Is this a matter of adjusting the engine tilt on the adjusters in relation to the transom? If so which way, towards the transom or away from it. Or it’s is something else I need to look at. I’m a novice at this, so help and advice would be welcomed!

[haventaclue]

If I'm reading right, the engine is revving up but boat going nowhere, I would check for a spun hub first.
Put mark across the hub, take it for spin, see if mark aligns. A spun hub will hold at lower revs, "the get me home" speed

[Bronteboxer]

Thanks for that advice. It’s odd, because it will pick up and go, I can get it going well (as much as I dare on a supposedly restricted Broad) and it does sit up and start to push on when the throttle is opened when in choppy water, but then we can hear the revs rise and the boat just doesn’t carry on picking up speed, in relation to engine revs. So it’s back off time and everything returns to normal revs for the mandatory 5mph. As I said, it only happens in choppy water

[se7en]

I assume the boat is supposed to have a standard/short shaft motor ?

[Bronteboxer]

Yes definitely short shaft.

Should the anti-cavitation plate be angled so it follows on the same lateral line as the boat hull?

Having now checked mine, it seems to be set slightly up at the front down at the rear, so it's not exavctly following the hull lateral line.

I didn't do any adjustment (lack of knowledge here) when I dropped the engine on and it had been on a rib I think before, so maybe that's part of the issue.

I've also very recently joined our Facebook site (or my wife has!) and posted shots of the boat. It's now had a significant amount of views with over 80 likes and very positive comments. We've included shots of it on the water. We do not get much chance to get out with it, so it's been very pleasing to get in the water again this week. One & half our cruise, lunch at a nice cafe in a Potter Heigham. Cruise back across Hicking Broad, recover & home. We are very pleased with it generally and it gets lots of waves & smiles.