Yamaha 115 prop selection

[MattPark]

A buddy just put a 115 Yamaha on a 20' sled that was previously a 175 Sport Jet. The oiling system on the old Merc stopped functioning correctly, and it lost a couple of cylinders. For about the same price, an outboard was the best choice, it wasn't ever really used for shallow water fishing anyways.

It's a fairly flat hull, I don't know degree. Either way, it's got a 17 inch pitch prop, and can hit 6000 rpm easily with a full load and a full tank of gas, and it will only do 38 or so mph. It needs a bit more prop.

What would be an ideal prop? The owners manual says full throttle RPM range is 5000-6000rpm, but it also says you want the RPM in the middle to high portion of that range (why don't they just make the range 5400-6000 then?). Would a 21" be too much?

[ICHTHYDEMON]

IMO you're right where you need to be. Going to a 19 will lower the rpms by about 400 (5600),going to 21 will reduce the rpms by 400 more (5200).

A 21 is gonna be way to much,my F150 can't even turn a 21.

You might get away with the 19 but you'll lose some hole shot.

[Streetwalker]

If your buddy can go over 6000 then then you need to try a 19. I run twin 115's & started w/ 17 pitch & would turn 6200. Switched to 19's & wide open trimmed all the way up get 5900 now. Yamaha Mech said thats perfect. With a flatter hull on a 20' ride, I would think a 19 would be good. 38 mph is plenty fast tho!

[ICHTHYDEMON]

You might also try a cupped 17. Cupping the prop will drop rpms by about 200. It'll also give you more efficiency and a more top end.

That engine running those numbers has a theoretical speed of 44.9 mph,thats with no prop slip.

The actual speed of 38 means you have about 15% prop slip,not bad but could be better.

With a cupped prop you'll probably gain a couple mpg and lower the prop slip by a few %.

[trap50]

Y Marine worked with me when they so;d me my 115. started with a 17 went to a 19 then a 21. 19 wasn't great. 21 was too much. Ended up playing with my jack plate and back to the 19. That works real nice now.

[Starfish]

I'd expect either a cupped stainless 17 or a 19 would be about right. No real way to tell except to try it.

I'd be really surprised if it could turn a 21.

[MattPark]

I just didn't know how much the props on these effect RPM on these engines. Sounds like it's about 200 rpm per inch of pitch.

He wants the RPM on the low end of the range, with mph and mpg being the reasons why. That and the rest of our boats will do 50+, so it seems pretty slow.

On my boat (351 I/O) going from the 17 to 21 drops the rpm from 5000 to 4600. That's about all I was trying to do with this one. If a 19 will do that, then that's perfect. I'll see if I can borrow a prop or two from somewhere to try out.

[trap50]

I was told to keep the RPM's as close toi mav as possible to get the proper proformance. They supposedly won't over wind. A gvvernor of some sort.

[MattPark]

Almost all electronically fuel injected motors have a rev limiter.

We just want the boat to go a bit faster and burn a little less fuel, and I think keeping the rpms at the low end of the range that Yamaha has provided will do that. If it was a ski boat, you would want a smallish prop that revs out to the high end of the range.

It all depends on what your definition of "performance" is.

[ICHTHYDEMON]

We just want the boat to go a bit faster and burn a little less fuel

Going faster and burning less fuel,haha,good one!

[MattPark]

You know what I mean.

[ICHTHYDEMON]

You know what I mean.

I do,I'm just funnin' with ya.