Hi Charles,
Thunderbolt III is my 1956 3 point hydro. Bought this motor on spec off fleabay. As far as I have discovered there was a T550 monohull class on the continent that we didn't have over here (we have a T850 class) so it's possible this is where the motor came from. It is a hybrid of the 'cooking' Selva 25,but with twin dellorto carbs, and I guess a few mods inside, and appears to be a factory supplied race motor, hence the stamping XSR on the block and the 'racing' decals. I think the gear ratio is about 2:1 and if the same as T850 you started in gear. As I mentioned the motor ran a treat in neutral but I couldn't get enough forward motion starting in gear, the engine simply couldn't get the revs up.

So I think in the first instance I'll try some lower pitch props - the boat ran fine with my Merc 300 on a standard gearbox, but of course I had the advantage of going into gear whilst the engine was running. I feel if I try this approach with the Selva I'll wreck the gearbox!

thanks for your interest, all advice welcome!