propper prop size for 70 horse evinrude

[willy10]

i have a 1978 70hp evinrude and i am running it at about 6000 feet above sea level and am wondering what size of prop i should be running on it. i currently am running a 13 3/4 x 15 and it doesnt seem to fire out of the hole or gain to many rpms.i guess i should ad that it is on a 1973 starcrft 16ft tri hull any help would be greatly apreciated

 

[steelespike]

Re: propper prop size for 70 horse evinrude

Really can't make an accurate reccomendation without detailed rpm and speed.
At 6000 ft. you lose about 18% of power.
Did you use the 15 at or about sea level ?
Its important that the motor operate within its rpm rating both for performance and the health of the motor.

 

[Texasmark]

Re: propper prop size for 70 horse evinrude

Like spike said, details help, but down here at 800' a 15 on a 70 would push a heavy 16'. I ran a 19 on a 15' fiberglass Kingfisher which is a pretty flat bottom, not very heavy, and easy to push. Gonna bet the prop is pretty close to what you need....maybe not, I just ran the numbers on a 16p down here and using spike's 20% that would put you down at 13....2" out of 15 is enough to make a big difference.....so maybe you "are" over propped.

Mark

 

[willy10]

Re: propper prop size for 70 horse evinrude

thanks for the info. i am going to see if i can try one out to see if it goes better