Spun prop hub??

About inboard or outboard motors.

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vupilot
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Spun prop hub??

Post by vupilot » Sat Nov 03, 2018 3:24 pm

Took advantage of nice fall day to warm up the little 9.9 on the skiff and then change the oil. I hit a stick about 1" dia and 3ft long and the prop cavitated and we stopped. Saw the stick right behind us. Kept on going and a mile later we revved up and slowed again. Thought maybe I hit another stick but none in sight. Reversed to make sure no weeds in prop but this lake doesnt have any weeds anyway. Went to a higher cruise throttle setting to see if I could make it do it again. Sure enough as soon as we got up to a high speed cruise it revved up and slowed again. Was getting cold and slowly made way back to dock. Didnt want to strand myself and 4yr old on a crisp fall day close to sunset without many people around.

I suspect the stick damaged the prob hub and now the prop hub is spinning under higher loads. I've never experienced a spun hub. Does this sound like the culprit? Is there anything else I should check? Can the prop hub be repaired or must the entire prop be replaced? Not sure who the prop manufacturer is, it's the stock prop that came with my 2015 Mercury 9.9

Thanks
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hoodman
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Re: Spun prop hub??

Post by hoodman » Sat Nov 03, 2018 4:25 pm

It's probably just a Mercury brand prop. If it's like mine the hub is rubber and easily slips out of the prop for replacement. Might as well pop it off and check it out. wouldn't think a stick that size would do any damage but sure sounds like you spun the hub. They say to check the shaft for line annually anyway.
Matt

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TomB
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Re: Spun prop hub??

Post by TomB » Sat Nov 03, 2018 4:29 pm

Sounds like it. :( Feels like a feathered clutch, if that make sense, revs but not much happens. Tom

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Bill Edmundson
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Re: Spun prop hub??

Post by Bill Edmundson » Sat Nov 03, 2018 4:33 pm

Chris

I agree with Matt. On an old engine, I would say a shear key.

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vupilot
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Re: Spun prop hub??

Post by vupilot » Sat Nov 03, 2018 6:31 pm

Matt, I took the prop off and the backside you can clearly see the rubber hub material. It doesnt look obviously bad to me. I've watched a youtube on replacing the hubs in larger outboards but this one doesnt seam as easily replaced but I am not sure.

One interesting thing that might indicate a different problem and not a spun hub at all is that in forward gear I can turn the prop by hand a little in the direction of rotation and it gives some resistance but then turns without turning the flywheel, I get a "snap" out of the lower unit and it turns fairly easily with a little resistance after each "snap". If I spin the prop in forward gear in reverse of normal rotation it turns the flywheel. If I select reverse when I spin the prop by hand in either direction it is more difficult and spins the flywheel. So now I am thinking the stick caused a problem in the gearcase.

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kens
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Re: Spun prop hub??

Post by kens » Sat Nov 03, 2018 8:54 pm

I would not rule out a spun hub.
Do you have another prop, be it correct pitch or not, to compare it to?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

TAB
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Re: Spun prop hub??

Post by TAB » Wed Nov 14, 2018 7:43 pm

1st disconnect the power( better still is pull the plugs) put it in gear and try to spin the prop by hand. if it moves, you have spun hub/key. very common and easy to fix.

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Re: Spun prop hub??

Post by PeterG » Thu Nov 15, 2018 3:16 pm

vupilot wrote:
Sat Nov 03, 2018 6:31 pm

One interesting thing that might indicate a different problem and not a spun hub at all is that in forward gear I can turn the prop by hand a little in the direction of rotation and it gives some resistance but then turns without turning the flywheel, I get a "snap" out of the lower unit and it turns fairly easily with a little resistance after each "snap". If I spin the prop in forward gear in reverse of normal rotation it turns the flywheel. If I select reverse when I spin the prop by hand in either direction it is more difficult and spins the flywheel. So now I am thinking the stick caused a problem in the gearcase.
That is completely normal. The way the gears shift in the lower unit allows this to happen. There is a drive dog on the prop shaft between the forward and reverse gears which are always spinning when the motor is running. In neutral the gears are spinning but the dog and prop shaft are not. The dog slides to the forward gear and the prop spins clockwise, or slides to the reverse gear and the prop spins counterclockwise. It behaves like a ratchet if the motor is not running and the shift is forward or reverse.
Your prop hub has spun, you should replace the hub if you can, or the entire prop if you can't. I had an original rubber hub prop on a 71 Merc 500 that started slipping and I had to replace the prop. Good luck!
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vupilot
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Re: Spun prop hub??

Post by vupilot » Thu Nov 15, 2018 5:54 pm

Thanks Peter. That makes sense but I dont follow you on why it does behave like a ratchet in forward but not in reverse? In reverse, engine not running, it turns the crankshaft. In forward, not running, it behaves like a ratchet?

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Re: Spun prop hub??

Post by PeterG » Thu Nov 15, 2018 7:10 pm

It acts like a ratchet because the dog has saw tooth shaped teeth on the ends that grab lugs on the face of the gears. When you turn the prop shaft the dog will grab and turn the gear and the engine. When you turn the prop shaft the other way the dog will ride up and over the lugs on the gear. Here is an example of a drive dog, also called a clutch:
https://www.marineengine.com/parts/imag ... 243217.jpg

I thought it should react the same way but in opposite rotation when in reverse, but my Merc 500 reacted like yours, rotating the engine both ways. That outboard runs and shifts perfectly.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

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