Observations on choosing a prop for small outboard

I wanted to share a couple things I learned from selecting a new prop for my 1984 15 hp Johnson. First, the 200 rpm per pitch size was not even close in my case. And it makes sense if you do the math. The standard prop being a 10. One size up or down is +or- 10%. Also in case of a two stroke, if your dropping below the power band, you can expect gains from getting the rpm's back into the power band.

I went from a 10 to an 8 pitch prop. This changed the rpm's from 4900 to 6400 a gain of 750 rpm's per pitch size!

Thanks (Crazyboat) for hooking me up with a prop guy in another forum. He got me headed in the right direction.




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ktoelke54 said:

First, the 200 rpm per pitch size was not even close in my case.

Click to expand...

Only time I've ever seen it close was on boats 20' or more and V6 motors ...

DaleH said:

ktoelke54 said:

First, the 200 rpm per pitch size was not even close in my case.

Click to expand...

Only time I've ever seen it close was on boats 20' or more and V6 motors ...

Click to expand...

It was right on for my 15hp Johnson. When I bought it, it had an 8 pitch prop and turned 6900 rpms with just me and a typical fishing load. I put on a 10 pitch on it and it dropped 400 rpms to 6500. The difference was with two guys....with two the 8" pitch ran 6800 rpms and the 10" ran 6250 rpms.

The 200 rpm per inch generalization is just that, a generalization.

My recent experience.
2012 Mercury 60hp 4 stroke
14p 3blade Mercury prop = 5400 rpm
12p 3 blade Mercury prop = 6200 rpm
13p 4 blade Powertech prop = 4000 rpm

Not all props are created equal. Way to much going on in prop shape and blade count to generalize.



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ktoelke54 said:

First, the 200 rpm per pitch size was not even close in my case.

Click to expand...

Same experiences I've had on almost every single outboard that was sub 50 hp. But everyone wants to argue with the 200RPM per inch of pitch generalization. It is "close" on motors about 50hp and up, and especially close on 150hp+. But in my experience, not so much under about 50hp.

Both 25hp Yamaha's I have messed with were about 450-500 RPM per inch. 15hp yamaha 4 stroke was 700 RPM per inch. 40hp Yamaha was 425 RPM per inch. 25hp Mercury about 500 RPM. 25hp Tohatsu 4 stroke (3 cyl) right at 500. Yamaha F150 inline 220 RPM per inch. F200 inline 220 RPM. F115LA was 220 RPM.

Those are just the ones I can think of right off the top of my head, and will have some feedback on a brand new VF115 next week. Customer wants more speed, currently 6000 RPM even with a 17", he says a 19" should go faster, and it might see 1-2 mph at the most. $562 prop ordered today, we will see next week. I'd be mad if I was the owner and spend $562 for 1 mph and slower holeshot, but it's not my money that was spent-and I also need the feedback to put in the knowledge bank.

As Ebug said it usually works if it's the same style and type prop only thing change is the pitch. On three different 14 pitch props for my 60 4stroke.
14p aluminum 3 blade 5950 rpms
14p aluminum 4 blade 5800 rpms
14p ss 3 blade powertech 5500 rpms
But the best prop I have found for a PT 175 TX-TXW with good amount of tackle and 3 batteries is a 13p Trophy Sport 5950 to 5850 rpms depending on motor height.


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