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Author Topic:   Prop question
newt posted 08-30-2002 07:52 AM ET (US)   Profile for newt   Send Email to newt  
I just put a brand new '03 merc 90 2s on my '84 Montauk, and first time out had the following problem (after the breakin period)

If the boat were a car, I would say the clutch is slipping. If I accelerate moderately there is no problem. If I hammer down, the prop will break loose and the rpms shoot up. Also, I have not been able to open up to WOT for fear of over revving. I can get up to 5400 rpms, but am only going about 32 mph, and still have more stick to give it.

Sounds like an underpitched prop right? Well the prop is a 12.75" x 21" pitch prop. Exactly what the shop recommended, and I have read on this site that others are using the same size prop.

The prop is a cheapo BoatUS aluminum. Would a better (say Mercury 'Blackmax') prop be the way to go. In the same size? Or should I step up to the 23" Merc prop (as recommended on Merc's prop selector website).

The cavitation plate is level with or slightly above the bottom of the transom.

jimh posted 08-30-2002 08:55 AM ET (US)     Profile for jimh  Send Email to jimh     
Input your data to the propeller calculator

and you'll see that your propeller is running with 31% slip.

Better propellers run with 5% slip at those speeds. At 5% slip you'd be going 44-MPH.

Mercury propellers are expensive but in this case it sounds like it might be money well spent. They make excellent props.

BillD posted 08-30-2002 09:27 AM ET (US)     Profile for BillD  Send Email to BillD     
It almost sounds like the hub is slipping when force is applied. This would account for the over reving when you hammer down and the excessive slippage. There is a thread somewhere that detail how to check if the hub is spun. I think it had something to do with putting a mark on the hub and the prop, taking a run and seeing if the two marks are still lined up.

If it is spinning and there is no other damage to the prop, take it back and make them give you a new one, or your money back.

I bought a new prop from Propco Propellers ( on the recomendation of someone else on this site, and am very happy with it. They also will spend time on the phone with you to make sure you get the right size, and will exchange it for a nominal fee.

newt posted 08-30-2002 09:35 AM ET (US)     Profile for newt  Send Email to newt     
Thanks for the replies.

Jim, I didn't realize that there could be such a performance difference for props given the same material, diameter, and pitch.
The dealer has already set aside a Mercury prop for me to pick up tomorrow.

Bill, Regarding the spun hub, I was under the impression that if the hub were spun 30+ mph would be out of the question. Maybe there is some limited slippage.

Regardless, I am bringing the BoatUS prop back, and installing a Merc Prop.

jimh posted 08-30-2002 09:56 AM ET (US)     Profile for jimh  Send Email to jimh     
Read for some comments about how to verify the prop hub has spun.
Sal DiMercurio posted 08-31-2002 08:30 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
What hole is your engine mounted on.
Sounds like you could come down a hole.
You might also think Stiletto prop, half the cost of a new s/s Merc prop, & they hang on far better then an omc prop.
newt posted 09-01-2002 10:55 PM ET (US)     Profile for newt  Send Email to newt     

I brought the Boat US prop back, and they gave me a full refund (which I promptly spent..and then some).

I put on a Mercury 12.75x21p prop, and no more problems. It hangs on quite nicely, and wot is where it should be.

This boat jumps out of the hole like a rat with a Jack Russel Terrior on his ass.


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