Stainless steel props

[Ron Perry]

I know this question has probably been asked a hundred times before but here goes. How do you know what size prop to buy when switching from aluminum to stainless steel? My boat dealer says to ust buy the same size as the aluminum, but that seems wrong to me because of the difference in the amount of flex. And what about cup size and shape, how do you know what to buy and what brand to buy?
Ron

 

[Russ Charles]

Ron,

As David said, your best bet is to find a dealer that will let you try different props. You didn't say what rig you have but if you are now turning at or near the max rpm then start with the same pitch in stainless. In determining the right pitch you should be at the upper end of the rated rpm at WOT with trim at optimum (the point just before any increase in trim increases rpm but not speed). If you want a better hole shot go with a 4 blade. If you need more lift go with a 3 blade. Remember a 4 blade of the same pitch will yield about 200 rpm less. Also, subtract 10 rpm's for each degree above 70 degrees temp you are at. That is if your motor is rated at 5600 rpm and the outside temp is 80 degrees the right prop will turn about 5500 rpm. Again the best bet is to try a few, as the same pitch may yield different numbers depending on whose prop it is. Hope this helps.

Russ

 

[Russ Charles]

Forgot to add:
Max theoretical speed is caculated by:
RPM divided by lower unit ratio times pitch divided by 1056.
Example: 5600 divided by 1.87 times 23 divided by 1056 = 65.22 MPH