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prop size and pitch

Wooden1

New member
I have an 2004 90 hp merc outbourd 4 stroke my prop size is 13"x19 pitch can i go larger i have a opportunity to pick up a stainless that is 14.75 x 21 pitch
 
Can you or should you? different questions.

First off, you would have to make sure that you have the clearance to run a 14 3/4" diameter. My service manual shows no listing for props with a diameter greater than 14".

Second, you are looking at pushing a whole lot more water there compared to your current prop.

What is your current rpms at wide open throttle?

Moving up in diameter has a significant effect where 1" of diameter is far tougher to turn than 1" inch of pitch, as far as the motor is concerned.

"rule of thumb" - 1" of diameter = 2.5" of pitch

So moving from the 13D19P to a 14.75D21P would be the equivalent of a 13" diameter prop with 25" of pitch

That would drop your top rpms by about 1200, so unless you are way way underpropped you would only accomplish two things:

your boat would go quite a bit slower (since the motor couldn't properly turn the prop),

and, since you would most likely be significantly below your WOT range for your motor you could cause some significant internal damage....
 
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