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Author Topic: prop shaft length  (Read 1255 times)

lozvik

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prop shaft length
« on: July 22, 2009, 08:31:07 am »

Hi guys a newbie here who has built a bait boat and is having problems with motor alignment, prop shaft and vibration.
so ive made a bait boat from scratch in 2mm ply, its approx 600mm long and 320m wide, its catamaran style and has two 540 motors that are attached to 10" long prop shafts by universal joint couplings, now the problem is this, I cannot get it to run quietly no matter what i do, ive spent hours fiddling and trying all different mounting options (on rubber mounts etc) but to no avail its sounds like a chainsaw revving over lol. so theres one thing ive noticed that some of the bait boat manufacturers do and that is to use a short prop shaft of around 4" or 5" and a straight coupler instead of a u.j. so my question is, if i was to remove my long shafts which run most of the length of the boat and replace them with ones half their length would i reduce the amount of vibration? if I put my hand around the shaft at present it quietens it down considerably, the vibration definitly seems to be coming from the shaft section and not so much the motor end. thanks for any advice anyone can give.
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boatmadman

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Re: prop shaft length
« Reply #1 on: July 22, 2009, 09:51:23 am »

hi,

yes, shortening the shaft will help, keep the length of shaft that exits the end of the tube as short as possible. Also, use two uj's on each shaft, this will reduce vibration as well.

If you can use lower revs, that will be a big noise reducer as well.

Ian
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lozvik

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Re: prop shaft length
« Reply #2 on: July 22, 2009, 10:05:00 am »

thanks for reply Ian, what about rubber tube coupling are they quieter?
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malcolmfrary

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Re: prop shaft length
« Reply #3 on: July 22, 2009, 10:20:09 am »

It could well be that the unsupported length of shaft inside the tube is very slightly out of true.  Rotating it at revs will cause it to throw yet further out of true and cause the vibration.  Shorter shafts cut down the unsupported length and reduce the chance of vibration. 
As boatmadman says, lower revs will also help, but to keep the performance this means either bigger props or a heavier pitch, or both, and the motors might not be happy with that.  The conventional answer is a gearbox, but these tend to their own noisiness, or a toothed belt drive.
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