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HansB

Spun hub, or what?

8 posts in this topic

Couple years ago I caught a submerged branch and knocked a blade off my prop (original equipment, OMC 14" x 19). Had the prop rebuilt, but in the meantime bought another prop, so this one became my spare.
Last week thought I'd put it back on the motor (the new one getting a little dinged).

Hole shot at WOT was great, and throttled back to about 4,000 RPM for cruising.

I decided to tweak up the trim a little. As I trimmed upward suddenly the RPM's jumped from 4,000 to about 4,300 and the boat got sluggish and didn't want to stay on plane. This was a sudden jump in RPM, not a gradual increase.

Increased throttle just increased engine speed but the boat didn't respond. Normally WOT RPMs on this rig (90HP Johnson on 16.5' boat) are about 5200-5300, but full thottle they went up around 5800 before I backed off the throttle.

Trimmed the motor back down, but the boat remained sluggish and wouldn't stay on plane.

Stopped the boat, and idled for a couple of minutes, then hit it again. Boat came up on plane just fine but as I trimmed out for cruising, the same effect again.

The hub seems tight (I can't rotate it by hand) but I have no experience with a spun hub, so it may be that it only spins under higher power? If so, instinct tells me it would spin under the full power thrust getting up on plane.

Any other ideas?

Walk softly and carry a big fish.

Hans

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I've had a prop on a 9.9 Johnson do that, but I lost total thrust. The compressed rubber inside the prop went and a machine shop had to re-compress the hub of the prop. If that is what you call "spun hub" then that's what it might be.
When you hit objects like that it is best to ruin a prop and spin the hub than destroying the whole lower unit of the motor. Maybe that's why they are designed that way?

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HansB,
Your motor is probably set too high on the transom. When you trim out, your prop is ventilating which will cause a jump in RPM and a loss in speed. I would try setting the motor lower by 1/4" incremates. Try that and you should see a difference. smile.gif

------------------
Adam Johnson
www.adamjohnsonfishing.com

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I don't think it has anything to do with motor height.

This prop worked fine before it was damaged. A new replacement prop has worked fine since then. The motor height has not been changed.

The OEM prop only displayed these symptoms when recently installed after repair. The second prop has been put back on the boat, and it works fine.

Hans/MN

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It's definitely your prop. The prop needs to be re-compressed with a new clutch or whatever the heck it is called. If you see that compressed rubber inside the prop between where the prop shaft fits inside and the outer shell of the prop where the blades are attached....thats spinning with the pressure at high throttle. At least that is what happened to my outboard motor. It can be repaired fairly easily by a machine shop that has the proper tools.

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I have a prop for my 140 Johnson that is kind of similar. It has been rebuilt a couple times before the problem was noticed. My explanation (guess) is that the blades are not properly set. My motor will cavitate at high speeds but works fine at lower speeds and on take off. I bought a new prop and things are fine. I now have a marginal back up. I didn't try to take it back because, like you, after it was repaired I didn't put it on the boat for over a year.

I don't know if there is a way to check the pitch/shape of the blade to know if it's set correctly.

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I had a guy check the pitch (he has some tooling which checks pitch) and it is OK, but I'm suspicious that the blade shape was not done right in rebuild.

As one final check, I'm going to paint a white index mark across the inner hub, rubber donut, and outside hub. Then run it again and see if the index line has shifted.

Perhaps I just have to settle on this being a spare prop.

Walk softly and carry a big fish.

Hans/MN


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Hans, did you try it out with the white marks on the prop? I'm curious of your predicament.

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