Best prop for speed on 78 johnson 85hp

[Lk40hp]

Title pretty much says it, I have a 78 johnson 85hp with a 13.5x17 prop. Turns 4550 rpm and goes 36mph, want to be doing at least 45mph. What stainless steel prop pitch/style/brand/size should I get????

 

[Texasmark]

Re: Best prop for speed on 78 johnson 85hp

You need another engine with more whoopie and then should do better. Sounds to me like you are about as far out as those ponies can push you. I had a '72 125 and 39 is all I could get with the OMC SST SS prop. Your engine is brake hp rated, not prop rated. So by today's standards you really only have around a 75 hp and 36 out of a 75 with an average boat isn't all that bad.

Mark

 

[steelespike]

Re: Best prop for speed on 78 johnson 85hp

Not to put you off but you should read the stickys at the top of the prop forum.
get us as much info as possible.
Basically the motor needs to operate within the upper end of its rpm range 4500-5500.
One way to increase rpm is to reduce pitch, Each inch reduction adds about 150 to 200 rpm.
At this point it isn't practical to make up the rpm with a prop.
Unless your boat is a real tub your 17 should be very close to what you need.
I think you should check out the motor.A compression check before spending any money.

 

[Lk40hp]

Re: Best prop for speed on 78 johnson 85hp

Ok, I will read rhe stickys, my boat is a 1977 glastron GT150, rated for a 90hp. I had just been looking at like the raker props that supposedly add 10mph. Thanks

 

[steelespike]

Re: Best prop for speed on 78 johnson 85hp

I'm no Raker expert but a simple prop change isn't likely to produce a 10 mph increase unless it going on a poorly setup boat in the first place. A boat setup for pure speed will sacrifice other performance criteria.Hole shot,water sports,added weight.
Your boat should do well if its still in good shape.Its about 600 lbs and probably has a fast bottom.
I think a healthy 85 could come close to 5500 rpm with the 17, speed would be almost 40.
Do you have power trim? Does the boat respond to up trim?