Prop Pitch and WOT Question

[X53Gunner]

Ok i have a 1982 Johnson 70hp 3 Cylinder 2 stroke. I'm trying to get it in the proper wide open throttle range for my motor, its 5500. I was running a

13 1/4 x 17 pitch and I was getting around 5000 rpm. Was told to go to a

13 1/2 x 15 and that would put me up around 5400, well today I tested it and I lost rpm's and am now only getting around 4600 to 4700 rpm's. I'm hoping someone will be able to help me out. Thanks for your time!

Scott

[HONDAM]

Same number of blades and same material?

[X53Gunner]

yes same number of blade and same material

Possibley a differn't manufature as It was the stock prop I replaced

Scott

[1mainiac]

So it is a different cup in the prop what did it do to your top speed? It sounds like your motor does not have enough power to go any faster and changing the prop only changes what RPM that is. try this out http://www.go-fast.com/Prop_Slip_Calculator.htm

[X53Gunner]

I believe me speed slowed a touch, I was just under the impression you want you motor running as close to its max rpm's as possible. Maybe I shouldn't worry about it. Just want to make sure everything is right on my boat. Thanks

Scott

[1mainiac]

Actually the opposite is true you want the lowest rpm cruise you can get on plane with. Max Rpm is something you do not want it to be able to go past Max RPM on my motor is 5200 however preferred wide open range is 4200 to 4800 because that is where the torque curve and HP curve meet best. Just like your truck you want to cruise on plane at a low RPM so you prop to max out under 5200 and best trimmed cruise as low as you can get it.

[JWheeler]

My BIL had the same issues with his boat/prop. Once he found the correct prop, his boat gets up on plane in a hurry (as it should). With the incorrect prop on his boat it seemed like it was a slow boat to china to get on plane. Like it had to get momentum going, somewhat doggy deliverance of power, to get up on plane. Now you just hit the gas and go

[CITM]

You should be in the correct RPM range at WOT with an average load. Yes you should get about 200 RPM per inch of pitch, but slight prop design differences can change things drastically, especially on an under-powered boat (in my experience).

I couldn't run a Quicksilver prop on my last boat but the "same" prop in a Mercruiser worked fine. I went through at least 6 different props on that boat before I found the right one. Unfortunately it took me that many tries to discover that it really needed a Merc prop. Not sure where you're located but if you're around Grand Rapids, MI, West Michigan Propeller will let you exchange and I believe return props as long as you bring them back within a few days. Get 2-3 at a time and try them. When you find one that works, buy a spare.

You'll likely see much higher RPM in a 13 pitch prop even with a larger diameter (if that'll fit), and that's if you can even find one. If you're running a stainless, try aluminum in the 17 pitch. The flex in the blades and different design (likely much less cup) might do it.

This is all assuming your boat is running correctly and not over-loaded. There are a lot of things that can cause low RPM and the prop is probably the last thing I'd mess with, especially if it's close to correct (like yours is). I don't know a thing about 2-strokes but if it was an I/O or inboard, I'd check timing, plugs, wires, distributor, fuel filters, and compression first, and maybe even the carb.

[sherman51]

I really don't understand your rpm,s going down with a lower pitch prop. but if the max rpm,s are 5500 and your running 5000 I would do nothing I would stick with the old prop unless your having problems getting on plane. if your having problems getting on plane try a cupped 15 p prop or switch to a 17 4 blade. max rpm,s are not as important as performance.

sherman

[X53Gunner]

Well I ended up sticking with my stock prop I'm close enough with it and my boat gets on plane rather nice. So I figure why mess with it if its close. I guess I was just being OCD about it. Vans here in Grand Rapids took the prop back with not problems. It worked out as my starter went bad and used that money towards a new starter...LOL always something. I hope to make a run out to South Haven this weekend for some perch and salmon fishing. Take care and thanks for the responses!