prop help 15r75c outboard

[underpressure]

this is my first boat ,searched the forums could not find an answer,so here we go i have a 1974 14 foot starcraft boat approx weight is 180lbs ,the motor is a 1975 johnson model 15r75c ,the prop that is on the motor is pretty beat up,took boat out for a trial run and it works but seems to have a vibration at wot and boat dont seem to be moving very fast ,i weigh about 225lbs ,the pin is not sheared,the numbers on the prop are 9.5 by 10 ,we want to get a new prop that will push the boat a little better ,i was at 4800rpms roughly with a tach (digital) motor runs strong just dont know if im expecting to much from the motor or if there is a better prop that i can get to move the boat a little faster,thanks again in advance for any help about prop pitch and diameter

 

[hwsiii]

Re: prop help 15r75c outboard

I need the gear ratio for your motor.

H

 

[underpressure]

Re: prop help 15r75c outboard

how do i find that and where is that info located

 

[hwsiii]

Re: prop help 15r75c outboard

In your manual or on the motor.

H

 

[hwsiii]

Re: prop help 15r75c outboard

How fast were you going, and what are the recommended WOT RPM.

H

 

[steelespike]

Re: prop help 15r75c outboard

Rpm rating for the 15 is 5,500 to 7,000 rpm.Standard prop by the brochure is 91/2 X 10 gear ratio is listed 12:29(1.414).
Needless to say you have a long way to go just to get to 5,500.
On that light boat I feel the 10 should do it. You may be able to have your prop repaired then you could test and get new figures (speed would be a big help)It should manage between 20 and 25 pretty easily with you alone.Adjust trim and distribute weight carefully.