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I had my out on SF Bay today with a new propeller installed on the 50 HP Johnson outboard. The old prop had some years on it, and some cavitation damage, so I decided to replace it. The new prop is the exact same size and pitch (this has been verified).

I noticed a big problem. The motor will not turn above 4000 rpm, maximum, at WOT. I used to get 5500 rpm on my old prop. My top speed with ballast was limited to 8-9 knots (it used to make 13 kts with ballast). The fuel consumption at WOT seemed about the same as before, but the RPMs were limited, as I said earlier.

When I got back to shore, I raised the motor, and spun the prop by hand. I heard a clicking noise every revolution, and a slight scraping noise and slight resistance to turning.

When I installed the prop, I followed the instructions in the engine manual exactly. I torqued the prop nut to the minimum spec, but I still had to rotate it another half a turn or so to get the cotter pin holes to line up with the nearest openings in the nut. After installing the prop, I spun it, and there was no clicking or scraping.

I'm thinking that something is not seated or engaged properly, or perhaps that I overtorqued the nut. Any help would be appreciated. I'll be out there first thing Monday morning to work on it.

Thanks,
Roger

53 views and nobody has any ideas or comments?

New prop is the same manufacturer?

I doubt it is an installation problem. The clicking is probably normal.
Try putting the old prop back on and see what happens.

Could it be the old prop was repaired and not true to the original size anymore?
What size is the prop?

When I read your post, my first thought was that the prop was most likely re-pitched. Since your engine doesn't hit recommended RPMs. You might need to get a new prop with shorter pitch, change pitch on the current one.
Also, I do not think there should be any clicking noise.

Try putting the engine into gear, (off of course) and turning the prop by hand to see if the noise changes.

My guess is your new prop is a different pitch than your old one and is mislabelled

Thanks all for the responses. It appears that the dealer sold me a 11 3/8" x 14" prop, which is the standard prop for the Johnson 50 HP motor. So, he did nothing wrong, in that respect. However, looking at the old prop, it appears that the PO installed a 12" x 9" (I can barely read the markings, it is so worn). So it seems that for some reason the boat requires a lower pitch prop than the standard. I reinstalled the old prop, and I'll take it for another spin next weekend.

Roger

Roger - there isnt such a thing as a standard prop for a given motor - the boat on which it is going is critical parameter. If someone selling you a prop doesnt ask what motor you have (including final drive ratio), what kind of boat you have, and your intended use, they shouldnt be selling props.

Catigale wrote:Roger - there isnt such a thing as a standard prop for a given motor - the boat on which it is going is critical parameter. If someone selling you a prop doesnt ask what motor you have (including final drive ratio), what kind of boat you have, and your intended use, they shouldnt be selling props.

Catigale,
Thanks for the comments. The Johnson motor manual lists the 11 3/8" x 14" as the recommended prop, so I believe that the dealer might have just went by that. I agree that he didn't do his due diligence. But this is as much my fault for not checking the pitch on the old prop. Pretty dumb Anyway, now I know what I need. I'll test the old 12 x 9 next weekend, and if all goes well, start searching out a replacement with the same dimensions.

Yea, that's a big difference in prop sizes.

At least you know it wasn't your installation. "Standard" is more like for a "typical" power boat which the Mac isn't.

You should see what the max RPMs for that motor are. If 5500 is the top end, then the old prop is probably the correct size. You can have the old prop reconditioned for a fraction of the price of a new prop, but you will be without until it's repaired. I kept one for a spare on my power boat.



--Russ

RussMT,
The max WOT operating range is 5900 - 6500 RPM.
I'm wondering if I could reduce the prop pitch to 8".

If I want 6000 RPM:
(RPM 1 / RPM 2) x Prop Pitch 1 = Prop Pitch 2
(5500 / 6000) x 9 = 8.25"

I'd welcome the extra speed. I could never get much more than 11-12 kts at WOT and full ballast.

Well, I tested the new 12 1/4" x 9" prop today. I'm very happy with the results. 12 kts (13.8 MPH) upcurrent on SF Bay, and 15 kts (17.25 MPH) downcurrent. 6500 RPM on both tests, and full ballast. 50 HP Johnson 4 stroke. The current was only a couple of kts at the time, and the winds were light (5-10 kts). The usual SF Bay heavy winds held off until after 2 pm, so I got some good test time, about 24 nm worth

This is about 3 kts faster than the old 11 3/8 x 9 prop, which had quite a bit of cavitation damage. The RPMs were about 500 RPM faster.

Roger