I have a 1990 Scarab 26cv the engine is a 461 bbc that made 627hp at 5900. Bob at Rm builder did a custom cam in the boat. I am running a 25p Mirage plus now and the boat is lugging down at 5200 rpms. i am looking for ideas on where to start with a baseline for a prop. With the 25p around 5200 i was getting 73gps with a full tank and 1 passenger. The boat has a bravo 1 at the stock height.

thanks,
Mike

A lot of times, using the same cid but camming and heading for extra hp, the original best prop will still be the best prop as you are now able to spin extra rpm.

This is a whole new setup. The boat had a stock 420hp 454 before. On the first day out of owning the boat it jumped timing and burned a hole in 3 valves. I am basically starting fresh

I would think with 627 HP you should be able to turn a 25 Mirage more than 5200 RPM. I think something that is different from when it was dynoed is holding you back. Have you checked your tune in the boat?
I had a 251 Checkmate with a 454 producing the same HP and I spun a Mirage 25 to 5700 RPM. DON'T go to a 4 or 5 blade - they won't work.
You might just have the drive buried too deep. Try a -2 IMCO shortie. Where do you live?

Hey Tinkerer, No i havent checked the tune with it in the boat, what should i check timing and stuff like that? I know from the dyno sheet at 5200rpm its making 588hp so it deff needs more rpm. I know the drive is pretty deep in thoese boats. I was deffently thinking about a -2 shorty. I live in Upstate NY area.

thanks,
Mike

588hp at only 5200rpm ?

Timing shouldn't have changed. Are you running the same exhaust as it was tuned with on the dyno? If not you are probably running rich. You need to check the plug color or if you have a bung for an O2 sensor get an A/F ratio meter.

My thoughts as well. It makes 588hp at 5200 and only gains 40 more hp with another 700rpms???

Yea i got the dyno sheet there but i cant figure out how to post it. At 5900 it made 627 and 558tq it was still climbing. 454 .030 over with 305 afr heads and a quick fuel 950. Rmbuilder did the custom cam.

-Mike

Yea the engine was on the dyno with different manifolds on the boat also. i did notice when i got the boat back from testing there was some black soot on the back on the boat, im guessing its running a little rich.