Same Pitch/Size, Higher RPMs

[Razbre]

Hi all....

Don't know what happened, but I had pretty much beat up my 14x19 3 blade aluminum prop on my 20' Sea Ray, 5L Mercruiser and decided it was time to buy a new one.

Now, with the beat up one at WOT I was averaging right around 4600 RPM, midrange for the recommended RPM's for the engine.

Bought an exact replacement and RPMs jumped to 4900-5000 RPM @ WOT!

Can anyone maybe enlighten me to what/why THAT happened?

Another question, I had a chance to buy a SS prop, 15 1/4x 19, same pitch, larger diameter, very nice shape with no dings or anything for $100.00 and I bought it, anybody have any idea what this will do on my boat? Drop RPM's? Drop top end?

 

[WillyBWright]

Re: Same Pitch/Size, Higher RPMs

Aluminums same dimensions or same part numbers? Different brands perform differently. The thin blades of stainless props often allow them to turn more Rs than their aluminum equivalents. $100 is cheap. For that, you could have it re-pitched and still make out well.

 

[Razbre]

Re: Same Pitch/Size, Higher RPMs

Thanks for the reply!!!!
The aluminum is same dimensions. I believe the orig was a merc prop. The one I bought was Michigan Match.
With the SS one, even with the thinner blades, will the increase in dia bring the RPMs down?

 

[flybuddy]

Re: Same Pitch/Size, Higher RPMs

You'll drop rpms and acceleration with the larger diameter. but should increase top end. It's not optimal for your setup but you could repitch it to 15 or 17 to recover. Other option is buy it and put it on ebay for a profit.

 

[Razbre]

Re: Same Pitch/Size, Higher RPMs

Thanks......!

 

[QC]

Re: Same Pitch/Size, Higher RPMs

Quick note about Stainless vs. Aluminum, although stainless allows for thinner blades, sometimes RPM actually goes down with the same pitch. Aluminum flexes more so they often have better hole shot and higher RPM, but slower top speed . . . Prop stuff is not an exact science that's for sure. As Willy noted, same part number should be same performance. Anything else, all bets are off . . .

 

[Razbre]

Re: Same Pitch/Size, Higher RPMs

Thanks for the replys. I went and bought the SS one, figured I can't go wrong for $100. Now just have to wait foir the ice to melt before I can try it out and see what I get for results.