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Correct Prop Size for 1982 140HP

I

imajoker2

New member
I need to replace the prop on

I need to replace the prop on this motor and can't be certain that I am getting the right prop. The manual states a 13 spline. The prop I am taking off reads 12-3/4x21. What is correct? Do I have options? Confused at this point.
 
"You have 3 different numbers

"You have 3 different numbers there that mean 3 different things.

The spline count is the number of "teeth" on the prop shaft, so basically any 13 spline OMC prop will fit on the shaft.

The 12-3/4x21 is the diameter and pitch of the prop - 12 3/4" diameter (so if you drew a circle around the blades, the diameter would measure 12 3/4 inches) and 21 pitch (so in theory for each revolution of the prop it moves 21 inches through the water minus any "slippage").

If that prop has worked well for you in the past, simply stick with those specs.

Props can only be properly fitted with the use of a tachometer. If your motor is getting to it's optimal rpms at wide open throttle (which on this model should be somewhere around 5250 rpms), then you have the diameter and pitch correct for "your" set-up.

Two identical boats with identical motors may run different props because the habits of the owners differ - so just because "Bob" runs prop "x", it may work like crap for you.

Basically, each 1" change in pitch will result in a gain or loss of approx 200 rpms.

So if you are only getting a max 5000 rpms out of a 21 pitch, moving down to a 20 pitch would increase your rpms to 5200 (and vice versa).

Changing the diameter will also have the same effect but it's not a straight rule of thumb number like pitch is, so it's best to maintain a similar Diameter and change the pitch if you want to alter the maximum rpms of the engine.

If you run TOO MUCH pitch (many mistakenly believe that more pitch equals more speed) and you drop the rpms of your motor below 4500 when you are running full throttle you can cause damage to the motor - so it is a bit of a balancing act...

Hope you are not more confused now."
 
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