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Prop shaft vibration

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Jeremy07/07/2016 12:28:25
181 forum posts
255 photos

I am getting quite serious vibration at certain speeds from the prop shafts of my Amati Titanic. The shafts turn easily by hand without any apparent resistance points. As shown in the photo, the prop shafts are connected to the motor by a double coupling as specified in the RC conversion instructions. The shafts themselves are firmly glued to the three frames though which they pass. Any thoughts or suggestions would be appreciated.


Dave_P07/07/2016 12:35:44
207 forum posts
2 photos
Looking at your photo I would suggest your problem is due to the couplings not in line. This would cause vibration at different speeds.
Jeremy07/07/2016 13:08:38
181 forum posts
255 photos

That, I did not want to hear crying. If the couplings have to be in line, I will somehow have to extricate the prop shafts and level them up. Any thoughts on how to do this, particularly where they run through two frames to which they are firmly epoxied.

Dave_P07/07/2016 13:23:35
207 forum posts
2 photos

Sorry to be the bearer of bad news.

I would leave the shafts alone and move the motor. Could you not elongate the holes for screws and motor shaft and move the motor up to meet the shaft centre line?

I am assuming (dangerous I know) that you have not planked the hull yet and looking at the picture I would think it not be too difficult to do.


Edited By Dave_P on 07/07/2016 13:25:41

Jeremy07/07/2016 14:06:54
181 forum posts
255 photos

Unfortunately, I think it's the shafts which are incorrect. The motors are in the right place and it would anyway be difficult to access the screws fixing them to the frame. I now realise that the shafts should run horizontally and not slope down towards the stern. I should be able to extricate the shafts even at the expense of writing them off. I may need to patch the two frames they go through to make new holes but all will be hidden once planked. Wish me luck! Will I ever learn......?

Dave_P07/07/2016 14:23:10
207 forum posts
2 photos

OK I now see what you are describing. Shafts will hopefully come away without too much damage.

I have everything crossed for you.

Don't worry we've all been their.


Keith Long07/07/2016 14:49:50
66 forum posts

Jeremy - while you've got the shafts out you might try aligning the uj's rotationally so that they are "in -phase" rather than your arrangement which is "out-of-phase" the pictures on the following link should explain it better. In-phase should give a smoother drive if the joints are running angled.


Jeremy07/07/2016 19:01:59
181 forum posts
255 photos

Prop shafts came out easily without damage, thanks to a tip I found on another web site - a hot soldering iron degrades the epoxy allowing, in this case, the shafts to slide out. Keith - thanks for the in-phase suggestion. I will make sure I do this although, if I get it right, the drive should be virtually straight.

I think that the conversion required a double coupling for dimensional reasons.. The prop shaft tube is 9" long and the second coupling adds 1.5". if a 10.5" shaft had been available, I could have done without the second coupling. However, 10" or 11" were the only options. It is a pity, however, because the junction between the coupling and the prop shaft will be inaccessible. The lift-off section of the superstructure does not go back quite far enough, at least according to the plans.


Dave_P07/07/2016 19:59:52
207 forum posts
2 photos

Good tip Jeremy, thanks for sharing.

Dodgy Geezer07/07/2016 23:43:45
818 forum posts
59 photos

Are you testing this layout at full revs with no drag on the propeller?

You will find that vibrations which occur when the prop is running in air are damped down considerably - perhaps non-existent - when the prop runs in water...

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