Hey, I have an 1984 Scarab III and after a whole series of trouble over the last 5 years I finally headed out and had EVERYTHING working like its supposed to for about 5 minutes..haha. Even with trim adjustments I was only able to spin about 3000 rpm at 50 mph WOT. Speed was okay but it should spin more then 3000 right? Still has stock TRS with BW drives and 454s are probably in the high 300s for HP. Any suggestions? It has mirage props (23s I think).

Try a smaller prop. 23's don't sound too big, but it doesn't sound like you're certain that they're 23's. Try a 21p and see what happens.

Do you know for sure what Hp you have? If you have the 330hp Mercs you should probably be spinning something smaller than a 23p anyhow in that boat with that set-up.

They were 330s when I started. When I went through the motors I added a cam with a little more lift, high performance exhaust manifolds, had the stock quadrajets tweaked a little and bored them .60 over so I don't know exact HP but I assume its a little over stock 330. I cant find any markings as far as pitch on the props. But less bite makes sense if I max out at 3000. I just wasn't sure how much less.

I would start with something that you're sure of. If you can, borrow a set of 21ps and start there. If you can't find a set of 21ps but you can borrow a set of props that you know are 23ps, that would be worth a trip out too.

What do you have your timing set at? Have you had the boat running properly since you installed the cam? What did you set the cam at? What does the motor sound like and the boat feel like when you reach 3000rpms? Does it feel like the motor is running properly with a strong load on it, or does it feel like the motor is missing?


FYI - you may get more responses if you post this under "General Q&A" in the "Technical" section of the forum.

It sounds like its running right with a heavy load. I really appreciate your input. This boat has just about ruined the sport for me. Its old, big and has alot of moving parts. Previous owner killed just about everything in it and I should have been more patient when I bought and kept looking. Too late now. Nonetheless, the engines are running pretty good, tuned and running even. I think its just a prop issue at this point. Ill see if I can pin down what I have as far as pitch and repost to tech q&a.

verify what prop is on that. Something doesn't sound right,I dont think a 23" mirage is gonna do 50 at 3K with that set up (or any really). Can you get us cam specs or maybe brand and part #?

I have an 89 Scarab III and i am turning mid 4K at fifty with 23 inch Mirages.

I agree. It sounds like the prop that's on the boat is more like a 28-30p prop. The good news is that its a relatively inexpensive fix!!!

That's 13% slip with 30 pitch props. Like others have said, rather inexpensive fix.

Do you believe your tachs? Are they both reading the same?

On the backs of the tachs is a dial to be set at 4-6 or 8 cylinder engines. Maybe somebody changed the setting? Just a thought. Borrow a photo tach from a surveyor and check your RPMs.