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do I need a new prop?

donnyjay's Avatar

Join Date: Sep 2000
Posts: 52

04-12-03, 04:19 AM   #1  
do I need a new prop?

My 40 hp Mercury, about 10 years old, will sometime go to real high rpm as if the prop quit turning. If I keep a low power setting of around 1500 rpm it seems to work ok.
Is a prop make in such a way as it will spin on the bushing that fits onto the spline of the out put shaft?
High rpm's and what seems like cavitation is my problem I need help with.

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fewalt's Avatar

Join Date: Feb 2001
Posts: 3,100

04-12-03, 04:48 AM   #2  

I doubt you need a new prop. I can't remember for sure on my Dad's old Merc40, but you just may need a new clutch.

The BoatCop will be here shortly for the final word.


BoatCop's Avatar
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04-19-03, 08:22 PM   #3  
Most props are made with a neopreme (rubber) bushing between the hub and the prop itself. After years of exposure to water, heat and exhaust gasses the neopreme breaks down, and the spline will spin, just as you describe.

The neopreme "bushing" is only a few milimeters thick, and is designed to reduce harmonic vibration in that prop, and also act as "weak link" in case of the prop striking something it shouldn't (like a rock). The neopreme will absorb the majority of the impact shock, and reduce it before it's transmitted to the drive gears. Often that's what destroys the bushing.

There are 2 remedies.

1. Buy a new prop
2. Have yours re-conditioned

If it's an "old" 40, reconditioning may be the best bet. It isn't that expensive, and they can also fix any dings or flaws in the blades, and even true up the pitch.

I'm not sure what they get for prop jobs in your neck of the woods, but I pay about 75 bucks when I need mine done. I use high performance stainless props, 21 X 19. The one for your 40 is probably aluminum and much smaller, so it shouldnt be that expensive.

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