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Old 18-09-2019, 00:30   #1
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Max prop

A couple of times recently we have been motoring along and all of a sudden there is a crunching vibrating screeching sound. We immediately put it out of gear and the noise stops. When we go in to forward again all is back to normal. We've had the gear box off and checked but that doesn't seem to be the problem. I'm wondering if it's possible that one of the blades let's go somehow and it goes back into position when we go into neutral ??? Any ideas ?
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Old 18-09-2019, 01:34   #2
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Re: Max prop

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Originally Posted by Rumdoodle11 View Post
A couple of times recently we have been motoring along and all of a sudden there is a crunching vibrating screeching sound. We immediately put it out of gear and the noise stops. When we go in to forward again all is back to normal. We've had the gear box off and checked but that doesn't seem to be the problem. I'm wondering if it's possible that one of the blades let's go somehow and it goes back into position when we go into neutral ??? Any ideas ?
The "crunching vibrating screeching sound" might be a symptom of broken motor mount(s) causing the shaft to move out of alignment.

It is unlikely that a Max Prop blade could move out of position or out of balance and then move back into position again.

Paul
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Old 18-09-2019, 15:44   #3
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Re: Max prop

Thanks Paul. If it was an engine mount broken, would the shaft realign itself (the noise stop) when we put it into neutral and then be OK for a couple of hours before doing it again ?
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Old 18-09-2019, 16:21   #4
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Re: Max prop

Yes, when the torque on the engine is removed by shifting to neutral, the engine could move back into position enough to stop whatever was rubbing and making the noise.

It might be hard to see the damaged engine mount. The steel part of the mount and bolts might look good, but the rubber is no longer bonded to the steel. Also, more than one mount could have gone bad.

On my boat the engine moved a little to the side, no unusual noise, but I knew there was a problem because the packless seal system started leaking. All four mounts had the rubber delaminated from the steel.

Paul
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Old 18-09-2019, 17:36   #5
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Boat: Beneteau 432, C&C Landfall 42, Roberts Offshore 38
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Re: Max prop

...I have fitted two max-props to my boats.....both two blade.
..in the fitting process, the max-prop has to be disassembled to enable it to go on the prop shaft and also to set the pitch.

..in general, they are massively built and I find it hard to believe a problem could occur here at the prop, because as the prop is engaged both blades turn because of internal torsional gearing and the internals are swathed in grease....or should be swathed in grease...

..once the max-prop has rotated, the blades pretty much stay fixed in their preset position...so I don't see movement here, all the blades rotate simultaneously, so again, if there was a problem here it would affect all the blades, not just one.

...once the engine goes into neutral, the water pressure should " feather " the blades...one way to check is to see if the shaft continues to rotate with the engine out of gear...if the shaft is still rotating and you can't stop the rotation by hand or other means...the blades could be jammed due to lack of internal grease. or even some seaweed or other debris..
I only had the 2 blade, so it was my habit to go below after the engine was shut off to rotate the prop shaft so that the blades aligned vertically behind the keel...I had a little mark on the shaft for this purpose.

You could also try putting the engine in reverse to see if the same sound or vibration results. I could always hear a " clack" sound when changing from forward to reverse as the prop gearing hits its internal stops..

" a crunching vibrating screeching" sound.....does this sound emanate from the water..or from the engine....this would be your first clue...hard to believe you could actually hear this sound from your prop several feet below the surface...so coming from the engine would be my first guess..

..other posters have opined it could be the engine moving on it's mounts..the end result of this movement is shaft misalignment or other torsional shaft issues.

..another possible culprit could be the stuffing box...be it the old type shaft log type or the newer dripless type. A lack of lubrication for what ever reason......or shaft misalignment from engine movement could have the shaft misaligned and rubbing up against something.

...finally...occasionally, I would hear a similar sound from my engine bay, which turned out to be the alternator belts...I had double belts going to a heavy duty alternator...but occasionally the generator got too hot and and the belts would slip and make an awful noise...

...well, that is my 2c for the day...hope you can find something her of use to you....
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Old 19-09-2019, 03:17   #6
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Re: Max prop

Just a guess, could be the woodruff key which locks the prop onto the tapered end of the shaft is broken and has just enough of an edge to hold the prop in position, most of the time? I can't think of any way the maxprop itself would be making that noise. Could be an anode has come loose though?
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