The Shipyard ( Dry Dock ): Builds & Questions > Pleasure boats, Sports, Race, Power and Leisure Boats:

Isolating the Prop Tube from the Hull...

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DJW:
Hi All

I was wondering whether anyone has tried to isolate the prop tube from the hull..?


Aim would be to reduce vibration / noise transmitting to the hull. I was talking to UKMike earlier about the bobbin mounts he uses to mount outrunner motors, I think that's a really good approach to mechanically isolate the motors. I'm thinking that the prop tube could be embedded in a Sikaflex or similar bedding compound as it exits the hull rather than rigid epoxy which gives a great seal, but also transmits any vibration or mechanical noise from bearings directly to the hull which then amplifies them...

I'm interested particularly in a solution for an exposed shaft with P brackets. So the P bracket would need the same treatment. A side effect would be that the P bracket could be inserted from inside as the cutout wod be larger, this would allow a more robust top end of the P bracket to be fabricated. This I think would be helpful.

The result would be the whole drive train mechanically separated from the hull. I guess a solid coupling could also be used between motor and shaft.

I think this could reduce the mechanical sounds of a pair of electric motors running at speed.

Any comments...?
Best regards to all.
David.

Colin Bishop:
If you have your driveline properly set up then there should not be any vibration issues.

Colin

derekwarner:
Hullo David.......always a few things to consider with such an issue


1. Minimise any form of out of balance component elements from rotating driveline



2. Harnessing a vibration of the drive train will transfer any out of balance resonance to the mass of the structure [hull] which could amplify the sound, however minimise the amplitude of the cause


3. Adding any form of resilience to the drive train could minimise the amplitude of the sound, but increase the actual vibration


4. This is why automobile manufacturers attempt to minimise vibration, then, if necessary add sound [resonance] adsorbing panels in their builds  O0


I still think 1. + 2. are still preferable to 3. for our hobby builds, ..........1. is the task [and solution] of the manufacturers of rotating machinery


[Just imagine a Supervisor in Rolls Royce or GE aircraft turbine engines quality Department saying......"if the noise of the vibration is too high, just add a few dollops of Silastic to the holding down bolts"  %)  ]

Derek

npomeroy:
I would have thought there was a case for vibration damping for noise reduction.  All motors and certainly propellers make some noise.  It depends on the geometry w.r.t. resonance whether any flexible sheath absorbs or amplifies oscillations.  In my experience with scale helicopters, while the main driving components are rigidly supported, we often use silicone buffers between the chassis and the fuselage shell.  And the servos are generally rubber boot mounted.

DJW:
Hi All


Thanks very much for the input.  Always a good idea to get something like this 'sanity checked'.


Maybe 'vibration' was the wrong word for me to use. This is absolutely not an alternative to good alignment of rotating parts, and I agree that if there is misalignment this could make the problem (a lot) worse. I did mention a solid coupling, this would help to keep the rotating mass aligned.


Some additional information.  I'm currently building an Aquarama, and am contemplating a second, larger scale.  So that's two powerful brushless outrunners, the a ball race at the top of each shaft plus a thrust bearing.  Its the bearings that even when perfectly aligned do make a noise, maybe not a vibration.  Then two of these systems would spun at 10k plus and attached to a large hollow wooden case...  Sounds like a speaker system to me.  So I think its how to set and maintain shaft alignment while stopping the hull amplifying the bearing whine.  I'd have thought some experimenting with the right compound and amount of it would be important.


At the end of the day, its trying to stop the model sounding like a model...


Does that make more sense..?


Best regards to all.
David.

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