1987 Johnson 70hp - Prop

[tlcboat]

Hey Guys, newbie here, I have what amounts to a fiberglass john boat, C/C, make is fish&ski barge, 16ft, beam just a little over 5ft, with a 1987 Johnson 70hp. Prop is a 13x19. Your first thought would not be performance when checking out the boat. Top end speed is not what I'm looking for. I'm looking for a second prop that will put the boat on a plane and with a good cruising speed and fuel economy. I fish a couple of rivers in central Florida. I was told the 13x19 was more of a speed prop for that motor. I've been looking at the FAQ's - starting to understand diameter & pitch, but, still a little confused about S/S vs aluminum. There is a wide range of pitch - and I am concerned about going to low. Shouldn't I stay in the range - 15 to 17 ?

 

[rickdb1boat]

Re: 1987 Johnson 70hp - Prop

Hi TLC. Without knowing the wide open throttle as verified by a Tach, it's really a guessing game as to the correct pitch for your application, whether it be aluminum or stainless. But a ballpark would be a 17 or 19 inch pitch, depending on how heavy your hull is...

 

[ddaigle]

Re: 1987 Johnson 70hp - Prop

My boat sounds similar to yours, 1989 17ft bayhawk with 70 hp evin. A 17 pitch SS has been a good all around prop on this boat. It will pull skiers and still get decent fuel economy and speed. That beeing said, I currently run a customized 15 pitch but only because hole shot is more important to me than anything else. I think the 19 is too much prop for the 70hp. with any sort of load.

 

[tlcboat]

Re: 1987 Johnson 70hp - Prop

Thanks for the info. I just made a trip to a prop shop. The guy at the shop, loaned me a protable tach and a gps unit. My existing prop is stamped 13x19.
It also has 2 other #'s stamped on it, indicating it has been refurbished.
In comparing my prop with a 13x17 s/s, my prop has lost 1/4 of the blade surface area and the blade thickness looks thing compared to a different prop. The prop guy had me test run the boat with my 13x19. It maxed out at 45 to 4700 rpms and 25mph. At WOT the prop starts ventilating. So, he loaned me the 13x17 s/s. The boat maxed out at 5000rpm's and about 29mph and it did not ventilate at WOT. The prop guy suggested a 15 pitch. From what I have read on here and other sites, my main concern is getting the boat in the correct operating parameters for load, speed and rpms. The prop guy also made the statement that the diameter wasn't that big of an issue.
I'm making an assumption here, that I should stay in the 13 diameter range.
So, anything from 13 to 13 & 3/4 would work.
Thanks for your input.

 

[steelespike]

Re: 1987 Johnson 70hp - Prop

Yes your diameter thoughts are right.It would appear a 15 would put you at
about 5,400 rpm.Terrific hole shot probably easier to keep on plane.MPG should be best.If your really into best mileage you could try something like
smart tabs help you get on plane and stay on plane at lower throttle settings,should also improve handling/ride.
Have you tried fine tuning your trim and perhaps your vertical height.
This could improve both speed and mpg.

 

[tlcboat]

Re: 1987 Johnson 70hp - Prop

The 15 pitch prop for the 70hp johnson seems to be hard to find. I've checked 4 or 5 used boat shops and 2 dealers - no used ones available.
I found one on ebay for 60 bucks - take it to the prop shop for a check out before using it. I also found a stingray hydrofoil that would fit it. As far as the ride - it's okay - rough & wet in choppy water.
I did adjust the trim when trying the 17p S/S prop. I was amazed at the difference. Even for a newbie, there was a noticable difference.
I think it's time to go fishin.
Thanks guys
Tom