84 70hp. prop

[mwither2]

I am thinking of changing my prop for more top end. I found this ad for a stainless prop.
13 1/2 x 18 pitch raker stainless prop, has a couple of small nicks but nothing that will affect performance, will fit 60 - 140 hp johnson or evinrude outboards with a 13 spline propshaft and a 4" gearcase, i had it on a 16' skeeter bass boat with a 70 hp johnson and i had a gain of 12 mph over stock aluminum prop.

My application is a 17' crestliner jon boat with 10 deg dead rise with an 84 70hp. evinrude. prop is 13 1/4" 17p. Does 12mph seem like alot? Not sure of my wot. no tach. How do I know if I have a 4" gear case?

 

[steelespike]

Re: 84 70hp. prop

Measure across your gear case where it meets the prop.The increase in pitch alone could improve speed a little if the motor can turn up to the motors rated rpm.
If the old prop was poor you might gain signifigant speed.I think the skeeter would run faster than your jon with a similar setup.
Without a tach there is no way to know how your present setup is working.
We really need your max speed and rpm.Check out the tinytach they are reasonable and easy to install.Its important that your motor operates within it rpm range. If wot(wide open throttle)rpm is too low it can cause premature motor problems poor performance and economy.If it over revs this could also cause problems.Its possible for you to gain a couple of mph for free by being sure your setup is right.A stainless prop is not a cureall depending on the application the speed change could be a few tenths to several mph.
Check out the Turning point Hustler; an agressive aluminum prop that delivers
close to stainless performance at aliminum prices.

 

[mwither2]

Re: 84 70hp. prop

My max speed is 37mph, & I seem to gain about 1.5mph when I turn? Thanks for the info.

 

[steelespike]

Re: 84 70hp. prop

Are you using a speedometer or gps? The speedo can be inaccurate up to 5 mph
+ or -.I believe prop torque will tend to slow you in a left turn and speed up in a right turn.
If you don't have any vetilation(cavitation) issues you may be able to raise your motor up.Raise it to a point just before venting becomes a problem.

 

[mwither2]

Re: 84 70hp. prop

I used my GPS for for a speed reading. What do you think of a manual transom jack? Thanks
Mark

 

[steelespike]

Re: 84 70hp. prop

A jack plate can do 2 things.Get the motor into quieter water and make it easy to get a precise height adjustment.I have no personal experience with jack plates but
have been considering one for my boat.