5200 Rpm 50 Mph??
 

First post. Thanks to all for the good information going back and forth. I have searched for a few hours before posting my question...

Last week I bought a 1986 Scarab III twin 330 HP BB. It came with 23P cleavers. We GPS'd the boat at 50 MPH with the tach (one does not work) at 5200 RPM. That seems awfully high for the size of the prop and the stock configuration of the motors. I feel that we should be at least 54-55. The boat is slipped and has antifouling paint on the bottom.

Is my prop selection wrong?

We set the base timing at 14 degrees.


Thanks.

Brandon

Re: 5200 Rpm 50 Mph??
 

you gave prop pitch but not diameter.

Like you, I would think that 5200 is too high.

That usually means too small of a prop.

But... when I had my Nova, I had a cleaver that would spin 400rpm faster than a cupped prop on a SBC. You might be picking up some revs by virtue of the cleaver cavitating.

If you have any other props it would be interesting to see what effect each has. Some prop shops will let you try out props on a "you bend it you buy it" basis. That can be a real help in selecting a good balance between WOT top speed and cruising efficiency/planing time.

Re: 5200 Rpm 50 Mph??
 

Oh, stick a tach in the other side.

Even an old one that doesn't match or a 60 dollar boat store special would be better than guessing rpms.

Good luck, and welcome to OSO.

Re: 5200 Rpm 50 Mph??
 

Thanks for the response. I will measure the diameter this evening. I will also look into getting 2 new tachometers......

Re: 5200 Rpm 50 Mph??
 

5200 is pretty high for those motors. They are 4200 to 4600 motors I believe, but I would put a shop tach on it first. Seems like a lot of rpms for 23's on that boat. Bravo One four blades will be a lot better out of the hole and also total performance but you must find correct rpm's first. What drives do you have, I'm figuring TRS but maybe there Alpha's, which I wouldn't recommend Bravo Ones on.

Re: 5200 Rpm 50 Mph??
 

weren't they cast cranks, some were two bolt mains, stamped rocker arms?

Definetely sounds high to me.

Re: 5200 Rpm 50 Mph??
 

I guess cleavers have their place in boating somewhere but it's very limited. Generally on our type of boats ...they suck :(
Heavy deep Vs usually cause a lot of slippage. Bravo 4 blades work great and will vastly increase cruise speed (3000 to 3500 rpm).
Just for reference a friend ran a 23 Mirage at just about 5000 with 1.5 gears and got 60/61 mph. I had 25 Mirages at 4800 and 65 mph with 1.5 gears.

Re: 5200 Rpm 50 Mph??
 

The merc tachs are notoriously inaccurate.

I have checked mine, and sometimes it is right on, other times its 500+ RPM off

Re: 5200 Rpm 50 Mph??
 

5200 is high for your 330's. 4900-5000 I would do no problem. As for you props, I would go into the Scarab section on this website and post the same question. There you will recieve answers from people with like boats that can give you good advice. Also you may want to check with the experts in the propeller section for some advice. My thinking is with todays technology you most likely do much better than the clevers.

Re: 5200 Rpm 50 Mph??
 

I had a Scarab Panther with TRS drives for 6 years, the best props were Mirage Plus. I needed a prop that lifted the bow and would accomodate a deep X Dimension.