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[Dreadstar]

How do I prevent propshaft oscillation,I've fitted a balanced prop to the shaft,but at just over 50% revs,the whole shaft vibrates like mad.
 Would fitting a longer skeg and a shaft bearing help?

[andyn]

Sounds as if the motor's out of line with the propshaft. Either that or the coupling is out of line.

[CGAux26]

-Do you have bushings at both ends of your shaft log (stuffing tube)?  Is the clearance proper (maybe .002 to .006 inch)?
-Is the shaft worn where it goes through the bushings? 
-Is the shaft bent?  Check by rolling it across a very flat surface, like plate glass or a piece of granite.  If you hear bumps, it's bent.
-Did you say the prop is balanced?  How about a bent blade?
-And like someone else said, it the motor to shaft alignment right?  A U-joint coupling is forgiving, but get the alignment at least as close as the eyeball allows.

Let us know how it turns out.

[DickyD]

Is your prop shaft properly supported or does it need supporting at the motor end to stop movement ?

[Martin (Admin)]

Yes as CGAux26 says....
Also just try a different prop first and also nip up the nuts as much as possible without the shaft binding.
Also try a few drops ot thick motor oil on the bearings.

[Proteus]

did I read in a previous post you where renewing the motor  .? if so is the coupling the correct one with the grub screws loose is it a snug fit or does it waggle a bit if it does you have the wrong size and that is the reason. there are so many sizes and some are close but not close enough..

Proteus

[Dreadstar]

The coupling is a tight fit on both the motor end,and the shaft end. I've changed the 35mm prop for a 45mm one,(both 3-blade brass), and the vibration has diminished quite significantly.It no longer sounds like a bear with it's nose stuck in the honey pot,there is still some vibration,but it's no longer as severe as it was.Once it's in the water,I doubt that I'll be able to tell that there's any real vibration left.

[CGAux26]

One other thing you can try:  Position the 2 set screws in the coupling so they are equally spaced around the shaft.  That is, when one is on top of the shaft, the other should be on the bottom.  It all goes to optimizing the balance of the rotating assembly.

[DieselDo]

Is it just vibrating in air?

Bob

[Proteus]

does it vibrate without the prop on ?
Proteus

[kenthompson]

Hi Dreadstar

All shafts have eccentricities???

 this means===They will all bounce or vibrate at a certain speed. (Revs per minute)  This is called the critical speed.
Some shafts may just have one critical speed, some may have more. Its a matter of findind this speed and going through it.

 Does your shaft vibrate at all speeds , or when it gets to a certain speed??
If it only vibrates at a certain speed , go faster and the vibrations might cancell out.
 If it does not cancell out, then I'm sorry, I can only suggest, what has allready been suggested??
 Good luck
 Ken.

[Dreadstar]

Yes,you can get to the other side of the vibration node,I swapped out the 540 for a 550 motor,and got through the node,and the vibration wasn't anything near as bad as before. Took her out for a trial run today with the 35mm prop( unfortunately it's a LH prop),and although she's getting partially up onto the plane,the motor is getting quite hot,and my runtime isn't great. I'm using a 6V 4A sealed cell,which is losing power after about 10 mins run time. Surely it shouldn't flatten that fast,or is it a case of she's underpropped,or that I'm using an incorrectly handed prop?

[malcolmfrary]

As long as its turning the right way to send the boat forward, handing is unimportant.  Someone will mention motor timing very soon.  If it was not sold as a super high performance motor, it will have neutral timing, so rotation direction will be immaterial as regards efficiency.
If the motor is getting hot , it is working hard.  This could be either too large a prop or one with too many blades or the blades pitched too steeply, assuming all sensible steps have been taken to keep drag in the drive train low.  The faster spinning prop will shove the water backwards in a faster stream, resulting in potentially a higher top speed.
If the motor is pulling about, say 10A, then in 10 minutes it will have pulled about 2 AH worth.  This would be about half the power, and would give a noticeable drop in performance.  Add to this the fact that running a SLA at a high rate of discharge further reduces its efficiency, and 10 minutes flat out becomes reasonable.
You will likely get a longer run and better planing off a 6 cell 2200mAH pack.  Less power stored, but stored in a much lighter pack, so less to push around.

[DickyD]

What boat is this ? It sounds as though you are using the wrong prop, though being left handed wont matter.
Also if you want the boat to plane a 6V SLA is not a good idea [too heavy]

[Brian Roberts]

Tesred the motors on my 1/12th Severn recently before going to the pond and was horrified to have awful vibration on one of them.
It was fine just ticking over but when it was opened up a bit, what a racket.!
Everything was tight and in line so in desparation I took out the prop shaft, oiled it and replaced it. Bingo, problem solved, runs as smooth as ever!
Not saying this is your problem but may be worth looking at.
Brian
 

[Dreadstar]

I eventually changed the prop from a 3-blade to a 35mm 2-blade racing prop(nylon),lengthened the skeg to the prop end of the shaft,and this has solved my problems,combined this with the use of 7.2V stickpacks,and she planes quite nicely. I ran her yesterday with 2x3700MAh and 2x2000MAh stickpacks,and I managed to sail her pretty much constantly for just under 2 hours. I'm getting about 20 mins from the 2000 packs,and about 30-35 mins from the 3700 packs. I'm going to try using my matched 7.2V 4300 MAh race packs on Sunday,and see just how long a runtime I can get on them.

  Thanks for the help lads. I've posted up a couple of short vids of her in the general section,under Dread's RTTL.