well, the saga continues. I have fitted the extension piece to my "L" leg and also the spare gearbox that I very fortunately had laying around. Just been for a sea trial and I am still limited to 28 knots WOT one up with 300L of fuel onboard.
For anyone not familiar: Valiant DR750, Mercury 150 now XL so the correct length. Planing plate removed. 19 pitch lazer SS prop fitted.
I am getting 5600ish WOT which is about 200 less than I was getting before with the ventilating arrangement.
5000 RPM gives me 24 knots which I calculate still very roughly to be 40% prop slip.
I would very much appreciate any thoughts or advice on this setup. In case someone suggests upping the prop pitch, we have a 17 and a 19 as she will be lugging 6 to 12 divers with full kit so a "tug" prop will probably be best with full load. I still don't understand the reason for this much slip with this prop though.