Prop for 1977 70 HP Johnson on 16' Starcraft Supersport

[adamd1357]

Hi all,

I am looking to get a new prop for my 1976 16' Supersport running a 1977 70 HP Johnson. Currently running a 13.25x17. Top end speed is around 28-30. Any one have any experience running at 13x19 on this boat/engine combo??? I think the PO was primarily using the boat for skiing so he may have been running a 17 for better hole shoe. Does my thinking make sense? This is my first time looking into props... Unfortunately I don't have a tach. Any help is appreciated!!!

 

[sutor623]

I have basically the same setup as you. I have an '86 16' starcraft fishmaster (the supersport with no windshield) and a 1984 Evinrude 75hp and a stock 15 pitch is a good match. My boat would hit 34MPH with the stock 15p prop and would be right at the upper end of my rev range which is 5200-5800. With my turning point hustler the 17 really lugs the motor down. The prop is a LOT more aggressive than the stock. You may even want to invest in a tiny tach just to check on things. My guess would be that you are not hitting the upper range of your revs and that a 15 would milk a little more out of her. A higher pitched prop is better for top speed, until you get too high. 19 is a no go, even with a light load. Once again, these are my experiences, and take them for what they are worth

 

[steelespike]

Guessing some of your numbers (not good) the calculator indicates at 30 mph and 13% slip 17" prop and a 2.42 ratio. Your estimated rpm is at
5200. The only sure numbers we have is the pitch and gear ratio.
We need the speed by gps and an accurate tach to be sure of your setup and what prop chage if any makes sense.
A 17" prop was standartd in 77. But the final prop may have been different if the motor was originally on a really heavy or light boat.
​ Rpm range for your motor is 4500-5500 and best if closer to 5500. 5200(if accurate) is pretty good.
sutor623 numbers don't add up with a 2.42 gear ratio guessing 13% slip @ 34 calculates at 7600 rpm.
If we knew the diameter of the 15" prop it may tell use if he has a different ratio.According to the original brochure the ratio is 2.42:1.

 

[sutor623]

Sorry, 32MPH not 34.

 

[Mi duckdown]

YOU really need accurate rpms and gps speed . So you don't throw money at a prop that is wrong.

 

[adamd1357]

Thanks for the input, I will look into getting a tach.

 

[Herr Mustang]

I have a 1970 starcraft super sport with a 1988 88 hp Johnson ptt. Turns a 13.5x18 raker. Maxed out it turns 5,300 rpm and making 43mph by gps.

 

[walleyehed]

My supersport 18 with a 115 done well with a 18P raker but best with a 17p turbo1 41mph, 5900. This 70 is 2.42-1 ratio though, 2 different birds...
A Tach reading would be a must to set it up right but I'd guess a 17P in stiletto with the engine mounted up in the 3rd hole from top will be your best app on that boat.
5700-5800 would be a good number for that engine with a normal load.