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Relieving the exhaust above the cavitation plate would definitely help with an over-hub prop as far as getting on plane. As long as a much louder exhaust isn't a problem.

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Loudness is not a problem.
have you had a similar thing happen and have better results with exhaust relief ?

Tell us more about the boat - or did I miss something?

boat is a Orkney Dory 4 metre, 13ft 3 in long and 6ft wide, designed max speed 33mph
designed max hp 50hp longshaft .weight of empty boat is 230 kilo =507 pounds.
that weight is not including motor fuel battery hydraulic steering controls, and hand rails etc.
It looks a bit like a classic boston whaler but it is very very different underneath, and built stronger.
A pronounced tri-hull V at front tapering to rounded shallow v at transom !!!!

here is a pic of the original first boat out the mould back in 1973 and a pic of my transom (blue)
and a short video showing pull away with 13.5x15 alloy prop (my toy pulling prop)







.

nobody here is going to nail your set up perfect out of the gate. You have to experiment. if the slip at top end is high the motor is likely too high IMO. With T-boats we see a difference in slip with as little as a quarter inch in jack plate height, especially once you get near the high end depending on the prop you are using. You said that prop hooked up great on the transom but didnt improve top end. It didn't hook up when it was up high, either for take off or top end. So try in between, get a result, record the top end slip number. If you get under 10% i say you are getting close. once you get there try some smaller adjustments up and down to try and hone in on the perfect height. Then if the holeshot still cavitates too much i would consider exhaust relief up top to try and limit the accel slip, or try a different prop. Have you considered a bottom mod to try and make a flat pad on it?

the top end was not brilliant but I did not expect it to be.
I know I need more power to get the prop spinning up a bit faster, for this I am getting the bigger carbs to get another 10hp and hopefully more rpm's.

with the engine jacked up I only saw an increase of 2mph over it being down deep.
But I am think with more revs the slip figures will start to drop off rapidly (just my theory on that).
its the pull away I was so baffled about, at first I thought the hub had spun or a plastic bag caught around the prop, this is the reason I asked about the over hub and the exhaust relief.

Over hub props are something I have not had on any engine that was a thru-hub exhaust before.

getting answers about this type of prop and how they like to run is not easy.
all I ever hear is
" jack the engine up as high as it will go " !!!

But at last I am getting some feedback on this site and some sensible answers to what might be
a simple problem to those who know.

I did think about a pad on the bottom, but this would be adding even more weight to an already heavy boat.
Also I need a lot more bow lift not stern lift, the back comes up easy once it gets going, then the front drops a bit too much for what I want to be honest.
I think if I could get more bow lift and a bit less boat touching the water then speed might increase.
again the reason for a jack plate with a touch of setback !!!

I hope all this makes sense ha ha .

By the way when using a sst 13x19 prop straight on the transom with engine up just 1 hole the
slip figure drops to just 4% or 5 %.
5900rpm , 2.42-1 gear ratio 19 pitch = 42.3mph .
I think that is good for what it is, but I need more speed and to get the bow up. so I thought about
the over hub having so much more pitch might be the answer ?

What were these hulls orginally rated for power out of curiosity?

when new/original the boat was rated for a maximum 60hp.
mainly see these older ones with 2 cylinder 50 omc or 4cyl merc 50's.

Now the makers rate the new boats for 50hp 2 or 4 stroke.
the company who build them will sell with a Honda a Yamaha or a mercury/mariner 50hp 4 stroke.