Is 24% prop slip about normal for 115hp 17ft boat.

[GrapeApe]

I have a 2.59 ratio, 21 SS pitch prop and my boat runs at 6000 rpms wide open but only hits 35mph. That calcs out to 24% slippage.

I do have a couple of nicks in the prop but they aren't really bad at all, would that be causing it to slip so much?

Thanks,..

 

[Faztbullet]

Re: Is 24% prop slip about normal for 115hp 17ft boat.

DSounds like your over trimming it....

 

[steelespike]

Re: Is 24% prop slip about normal for 115hp 17ft boat.

Give us all the info; boat make style and year,motor brand year and hp.All the info requested in "read this first" at the top of the prop forum.6-12% is more normal for a setup like that.I would suspect that the tach is reading high.
All the prop calculations I've done I don't recall a 2.59 ratio.Especially on a 115.

 

[Texasmark]

Re: Is 24% prop slip about normal for 115hp 17ft boat.

I have a 2.59 ratio, 21 SS pitch prop and my boat runs at 6000 rpms wide open but only hits 35mph. That calcs out to 24% slippage.

I do have a couple of nicks in the prop but they aren't really bad at all, would that be causing it to slip so much?

Thanks,..

What engine are you talking about? Most 115 OB's that I am aware of have a 2:1 gear ratio. 90 hp's usually have a 2.33. I usually agree with Faztbullet as he has helped me a lot, but I am going to disagree here. When I trim my bow into the water, I load my engine which drops my speed and my rpm's. Conversely, I can be running in a mid trim position at some rpm and mph, and touching nothing but the trim button, move my rpm's and mph up and feel the bow of the boat floating in the process by trimming out.

Soooooo with the lower gear ratio, your prop should be spinning faster so slip is worse than you calculated.....sure your hub isn't slipping? Mark it, run it, and check it. Was your speed GPS? Is your tach set on the right ratio by the switch on the back of it? Besides, when I ran my 115 Merc on my 17' pad boat at 6000 rpm's I was snuggling up to 50 mph running a 21P SS Quicksilver Laser and it did thoroughly enjoy the rpm's. Engine just sang and you could hear the prop blades smacking the water....had her trimmed out just below blow out started (where mph started dropping).

HTH,
Mark

 

[GrapeApe]

Re: Is 24% prop slip about normal for 115hp 17ft boat.

Give us all the info; boat make style and year,motor brand year and hp.All the info requested in "read this first" at the top of the prop forum.6-12% is more normal for a setup like that.I would suspect that the tach is reading high.
All the prop calculations I've done I don't recall a 2.59 ratio.Especially on a 115.

2004 LUND 17ft fisherman with Johnson 115 EFI fourstroke (Same as Suzuki 115)

My speed is based off of my GPS so I know the speed is correct.

I think you guys are probably right, either the tach is reading high or the prop is slipping. I don't know how to confirm the tach speed but I'll mark the prop and check if it is slipping..

 

[steelespike]

Re: Is 24% prop slip about normal for 115hp 17ft boat.

WOW the ratio is indeed 2.59. And it sure appears the tach is at fault.Maybe borrow a shop tach or have a tech take a ride.
You could get something like a Tinytach,can be used on any motor.
If we run calculator at 10% slip we get about 5000 rpm at 35 mph.