lost rpm's on evinrude 1993 120hp

[steelhead2]

i have recently changed from a 13x17 aluminum prop to a 13x17 stainless rapture prop.my motor runs great and pushes my 22ft starcraft islander well about 35mph.but with the aluminum prop my motor ran at 5600 to 5700 rpm's at wot with the stainless rapture it is lucky to hit 5200 rpm's.why would the rpm's drop so much even though it is the same pitch prop.any answers or suggestions to help me. thank you

 

[G DANE]

Re: lost rpm's on evinrude 1993 120hp

Hi Steelhead.<br /><br />Most steelprops have what is called "cupped"<br />blades. Cupping is a bent edge on the blades. This makes the prop work more effecient. You often have to go one pitch down to reach the same RPM if you change to stainless with cupped blades. I have seen cupped blades on some aftermarket aluminium merc propellers too.

 

[Dhadley]

Re: lost rpm's on evinrude 1993 120hp

It has to do with prop design. Try raising the motor on the transom (increase the X dimension). This will increase top RPM's and improve the holeshot.<br /><br />Good luck!

 

[steelhead2]

Re: lost rpm's on evinrude 1993 120hp

yes i haved raised the motor to the highest safest point.should i go back to a aluminum prop or a different stainless prop?i am taking the motor in to the shop in a month to get the carbs rebuilt and a full tune up.would anything maybe be wrong with the motor to cause such a drop in rpm's.

 

[almost retired again]

Re: lost rpm's on evinrude 1993 120hp

This is my first time using this board, but I,ll add my two cents in anyway. I doubt that anything is wrong with your engine. I,ll agree with G DANE. If you want to go back to the same RPM go down one number in pitch.

 

[Dhadley]

Re: lost rpm's on evinrude 1993 120hp

What do you mean by the highest safest point? When you raised the motor how many RPM's did you gain?<br /><br />Let us know!

 

[steelhead2]

Re: lost rpm's on evinrude 1993 120hp

i still can raise the motor 2 inches higher on the gil bracket.the safest point meaning the guy at the shop said i might not want to go any higher because of the water intake on the lower unit.how would u tell if ur motor is raised to high by just looking?i just want to get my motor running back to the high end of rpms without buying another prop or should i not worry that i am running at the low end of rpm's.any suggestions on what to do i would appreciate.