Same prop, different performance?

delovett

Cadet
Joined
May 10, 2010
Messages
14
Hi everyone,

Have a 1989 Johnson pushing a 1979 17' Apollo boat. Got caught in some big swells the other day and while making our way to a beach, the prop clipped something and cracked one of the blades. That prop was OMC part number 382763, 13 3/4" x 15P. We called around some shops at the lake and someone had one on hand for a good price. We picked it up, put it on and the boat seems to perform differently. It is marked as the same size and pitch, but at WOT the revs are lower (5750 vs 6000), it seems to come out of the hole slower, and feels like it has better top end speed (speedo doesn't work).

My brother still has the boat at the lake and he will be sending me part numbers in the morning, but is this normal?

Any help is appreciated.

Thanks,

Derek
 

45Auto

Commander
Joined
May 31, 2002
Messages
2,842
Re: Same prop, different performance?

Lots of possibilities. Sounds like the newer prop is an inch or so higher in pitch than the old one, or possibly has some cupping that the original didn't have.

Pitch on a prop isn't a constant, it changes across the blade. Different manufacturers measure it slightly different. OMC probably didn't make it's own props, the original one could have been made by one subcontracter in 1989, and the newer one by a totally different subcontracter sometime later.

Even props made by the same manufacturer have a tolerance in the pitch range. One may have measured at 14.5" of pitch and been called a 15", the other may have measured at 15.5" of pitch and also be called a 15".
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Same prop, different performance?

You just learned that just because two props have the same diameter and pitch they may not and usually do not perform identically. It is called propeller "design". Blade shape, rake, cupping, and a host of other design characteristics go into a prop -- all of which affects performance. That is exactly why prop selection is such a crap shoot unless you have a trial and error situation where you can try many props to find one you can live with.
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: Same prop, different performance?

Exactly - not all 15" pitch props are created equal even if the diameter is the same. And we could be comparing a worn prop to one that isn't worn.
 

delovett

Cadet
Joined
May 10, 2010
Messages
14
Re: Same prop, different performance?

Thanks for the input guys, really appreciate it!
 

delovett

Cadet
Joined
May 10, 2010
Messages
14
Re: Same prop, different performance?

Finally got the boat home, referenced the pt numbers. Both are 13 3/4 x 15, but the replacement is a cupped prop which explains the drop in top end rpm.

Thanks again!
 
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