prop question for 17' with 115 4 stroke

[Matt221]

Just wondering what prop you guys are running who have a 17' with a 115 4 stroke? i think that a pretty common combination now. This is what I posted to Ken at Prop gods:

I repowered my 1977 Mako 17 this summer with Suzuki 115 hp 4 stroke. it came from dealer with 14 x 19 aluminum prop. while i luv the quiet of the motor i am not as excited with performance. the boat previously had a 1996 2 stroke Suzuki 115 which ran great (until it didnt) with top speed of approx 45 and great hole shot. it was running a 13 1/4 x 17 stainless prop. (i think it was even mounted a hole low). the new motor has top end speed of about 35 ( i have lost about 10 mph). i get 6000 rpm but just barely ( it feels like it is lumbering to get there). hole shot is not good. it planes at about 3K rpm and by 3500 rpm i am going about 25 mph. by 4500 rpm it goes approx 32-ish. then, from 4500 to 6000 i gain maybe 3-4 mph. motor is at correct height. i have spring-loaded bennett trim tabs to prevent porpoising. any help you can provide will be greatly appreciated. Thanks!

[JerEazy]

you'll lose the lower end with a 4 stroke compared to the 2. It just wont be as "exciting" when you hit the throttle.

That being said - assuming you've got the 2.59 gear ratio i found online - you've got a 16% slip with the 19 pitch prop. A good slip number is around 10%-ish. So it is a little high.

IF you were getting 45 MPH with the 17 pitch prop, that would mean a either a damn near perfect 3% or a negative slip number (using the 2.08 gear ratio). But you did say "about 10 mph" so maybe a speed of 40-42 was happening. That would put the numbers about on par.

So i would say this:

You probably didn't lose as much speed as you think you did. You just lost the *umph* of the 2-stroke. And, your butt speedo don't like it. Going back and forth between the Mako and the Pontoon boat, the 4 mph difference between 25 (top speed of the toon) and 29 (cruising speed of the Mako) is VERY noticeable at even.

You might just need to get a prop that has some cupping, or even slap a 4 blade on there and step it down to 17 pitch. Either would reduce your slip and either give you an extra mph (or 2) and make the ride up to that speed feel better.

People say you lose top end with the 4 blade. But, considering how much slip you are getting, going to the 4 blade and hitting the same 35 MPH will be optimum efficiency with a slip below 10%. It'll feel faster because of the better hole-shot too.

Hell, when i put a 4 blade aluminum in place of the 3 blade stainless on my 20c with the 150, it was like a whole new boat.

Ken is the man when it comes to props. He'll probably have a great recommendation on brand/style for you.

[cgmiller]

I had a 115 on my 115 on my 17 before I put another 90 on it. I could touch 40 with the 115 spinning a 13x19 prop. My 90 4 stroke with a 13x17 will run 34-35. If you are only getting that speed with the 115 your prop is a little too small. You will get slightly lower wot rpm, but a few more miles per hour. It will also be a little more efficient in the sweet spot.