Re: 35hp evinrude on 15' am I doomed?
If your prop is a 15" pitch, I'd drop back to 13". That should bring the revs up 1000 RPM at least. The thing is, with the 15" prop, you're not near the power band of that engine, so it just doesn't have the poop to spin it up any faster. The 13" prop will get you into the power band right away and will then be on plane and finish winding up.
Again, though...it could just be that a 12" pitch prop is the right one. If it were me, I'd try to find both, eventually and use the one that works the best, and keep the other as a spare.
Or, use the 12" pitch when the boat is heavily loaded and the 13" when it's not. Changing props is a snap. I'm sure the 11" pitch prop isn't what you want, though. It might be ideal for pulling a tube around or dragging two skiiers out of the water, but for general use, you'd be underpropped and wouldn't get the most out of it.
All of this, of course, is assuming that your trihull isn't waterlogged in the foam. If that's the case, then it's difficult to predict.
My 14' 6" Glastron was perfect with the 13" pitch. It jumped on plane super fast, and ran up to around 25-26 mph with 2 guys on board. And that was with a 1958 Johnson 35.
Your boat is a little heavier, but the engine is actually a little more powerful, at least at WOT RPMs. The old one probably had more torque at lower RPMs than yours.
12" or 13" Either one will do the job.