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Old 17-12-2012, 11:22   #1
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Maxprop issues

Hi all.
I have a 3 blade max prop fitted to my beneteau 47.7 and it is set up to be right turning and 22 degrees of pitch
It motors well and does the correct hull speed at the different revs. The yacht has a 75 hp yanmar engine
Now for the strange part.
When I motor in to large head seas the boat will fall off a wave and I will loose the drive from the propeller. To resolve I have to shift to neutral and then back on with the power. A friend of mine says it could be "cavitation" or something similar. I have contacted the supplier and they have done the calcs but say it is set up correctly or near enough!
Does my propeller require less pitch????
Any help would be great
Happy Xmas
Nick
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Old 17-12-2012, 11:34   #2
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Re: Maxprop issues

Yes it probably is cavitation. Can you hear the engine reving when you "fall off the wave". But you shouldn't have to shift to neutral. Dropping your engine rpm for a second and then back up to speed again should do it.

But what engine rpm can you achieve at WOT in flat water. That will tell you how you are pitched.

David
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Old 17-12-2012, 11:38   #3
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One might guess the wave pushes the prop into feather. Why it does not then re-deploy by itself is the problem. I'd dive under to examine its motion.
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Old 17-12-2012, 11:56   #4
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Thanks djmarchand!
Weirdly the rev counter does not seem to change but there is a weird noise. Cant explain what the noise is but a little like when you are reversing and then you engage forward and the water churns.
I can get to around 3100 rpm
Thanks nick
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Old 17-12-2012, 11:57   #5
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Re: Maxprop issues

Quote:
Originally Posted by daddle View Post
One might guess the wave pushes the prop into feather.
If the transmission is in gear and is turning the prop, there is no chance the energy from the wave is causing the blades to feather.
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Old 17-12-2012, 12:17   #6
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Re: Maxprop issues

I would venture a guess that you have a transmission problem. Next time it happens take a look at the shaft.
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Old 17-12-2012, 12:21   #7
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If the transmission is in gear and is turning the prop, there is no chance the energy from the wave is causing the blades to feather.
They will feather when the water from the wave spins the prop faster than the shaft is spinning. That's the principal of the device. Because the engine speed is constant, governed, I could see this happening. However I have yet to experience it with my Max Prop.

It will then be feathered. If it does not then re-deploy it will indeed make odd sounds. Strange that it does just shift itself into forward.

There may be some way for the internal gearing to fail to cause that strange behavior... But that seems unlikely.

Dive in. Hope you are in the tropics. If should move smoothly under hand pressure from forward to feathered to reverse.
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Old 17-12-2012, 12:31   #8
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Re: Maxprop issues

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They will feather when the water from the wave spins the prop faster than the shaft is spinning. That's the principal of the device.
The blades of a Max Prop move into, and stay, in their forward or reverse positions due to the centrifigal force caused by the spinning prop. If the energy of a wave were to somehow cause the prop to "spin faster than the shaft is spinning" (and I do not see how this could be possible), the blades would only be held in whatever position they were in with greater force. They would not tend to feather.
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Old 17-12-2012, 12:38   #9
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Re: Maxprop issues

seems you are having air at the prop
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Old 17-12-2012, 12:53   #10
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Re: Maxprop issues

I have a Maxprop which I'm very happy with. Do make sure yours is full of grease and has full movement forward and rev ( by hand).

I suspect it is cavitation you are experiencing. When you describe "falling off a wave" I feel that to be extreme conditions so cavitation is to be expected.
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Old 17-12-2012, 13:27   #11
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Re: Maxprop issues

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Originally Posted by fstbttms View Post
The blades of a Max Prop move into, and stay, in their forward or reverse positions due to the centrifigal force caused by the spinning prop.
That sounds like a folding prop, not a feathering prop.

Cheers,

Jim
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Old 17-12-2012, 13:30   #12
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Re: Maxprop issues

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That sounds like a folding prop, not a feathering prop.
Please feel free to describe how a Max Prop works then.
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Old 17-12-2012, 13:41   #13
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Thanks for everyone's replies. Just for reference we were motoring in to big seas which is very uncomfortable as we had to get somewhere. It never happens when motoring over a swell always when slamming! Thanks
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Old 17-12-2012, 14:14   #14
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Re: Maxprop issues

I guess you get your maxprop airborne when motoring against this big waves , it happen to me but with a normal prop, sounds cavitation ....
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Old 17-12-2012, 14:23   #15
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Re: Maxprop issues

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Originally Posted by fstbttms View Post
Please feel free to describe how a Max Prop works then.
A MaxProp works by the turning force of the shaft on the internal gears, working against the resistance in the blades caused by water. If you were to turn the shaft slowly in free air with the blades feathered (as when the boat is hauled), the blades would stay feathered.

It's only the resistance of the water working on the blades while the shaft is turning than causes the blades to deploy.

I've had a 3-blade MaxProp for over 20 years and love it. Never a problem. Great control in reverse. No real benefit in forward thrust, but very good to have the blades feather when under sail.

Bill
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