Recent noise and vibration problem

[JeremyParticipant @jeremy15845]

My Dumas racing runabout, now three and a half years old, used to run at speed very quietly. Recently, it has become very noisy and I haven't been able to solve the problem. Disconnected from the propshaft, the brushless motor is very quiet and its mounting is secure. I have regreased the propshaft; there is no play in it and it rotates smoothly. The prop is not the problem as the noise persists when it is removed. Could the flexible coupling (red thing with inserts from Cornwall model boats) have deteriorated. I could buy another or upgrade to the Cornwall Model Boats heavy duty coupling. The line-up between the propshaft and the motor is good – I could almost use a fixed coupling.

Any thoughts on what to do would be much appreciated.

Jeremy

[JeremyParticipant @jeremy15845]

[peter ward 9Participant @peterward9]

ive found that thered coloured motor/propshaft connectors can be fine on slower motor set ups ive had one on a fishing boat for over 11 years.

but on higher speed set ups not so reliable i have seen them disinterfrate on some high power brushless set upsand suspect yours has perhaps worn itself out over 3 years plus

luckily you would only need a ney plastic bit which are cheap enough

[Charles OatesParticipant @charlesoates31738]

Hi Jeremy, I’ve just ditched a Huco coupling,I think that’s similar to the one in your model. The problem with mine was poor manufacture, which made it noisy. I’ve seen others which wore out quite quickly, more so in a quick boat.
I’ll bet that’s your problem.
Charles.

[Mark Jarvis 2Participant @markjarvis2]

HI Jeremy

If you take the ends of your coupling and gently twist it can you feel any movement??? if so its knackered, also are the brass inserts slightly loose this will cause vibs as well

Mark

[Dave MilbournParticipant @davemilbourn48782]

Jeremy

Obtain a short length of silicon rubber tubing and use it as a substitute for the red plastic bit. The splines on the brass hold it very well and it will survive much higher speeds than the present one.
I get mine from these people **LINK**

Dave M

[JeremyParticipant @jeremy15845]

Hi Dave

That looks a good solution – thanks for the link. The diameter of the brass inserts is 8mm. Would 7mm internal diameter tube fit and provide sufficent grip? The next size down is 6.3mm. There is a choice of wall thickness. Based on your experience, what would you recommend? Thanks as always for your help.

Jeremy

[Dave MilbournParticipant @davemilbourn48782]

Jeremy
The couplings I have upgraded are smaller ones than the Huco type but faced with your set of parameters I'd go for 6.3mm ID x 2.4mm wall thickness. It's very flexible stuff and 6.3mm will easily stretch over 8mm splines. I notice they do a "Request a sample" service so you might try several different ones before you buy a full metre.
Dave M

[JeremyParticipant @jeremy15845]

Dave

I've fitted the silicon tube coupling but the problem is not solved. However, I now think that the motor may be shot. If I hold the coupling still (which is safe enough with the silicon tube) and run the motor, the shaft judders but does not turn. Maybe the magnets may have come loose from the shaft. The electrical connections seem to be OK because under zero load the motor runs smoothly.

Have I finally found the source of the problem?

Jeremy

[ashley needhamParticipant @ashleyneedham69188]

Ah Jeremy you thrasher you, ruining motors with too much devilish high speed running.

What motor do you have fitted at the moment?

I had a similar vibration issue (from the start mind you) on the TBD and this was cured by changing the prop.

Ashley

[JeremyParticipant @jeremy15845]

Hi Ashley

Guilty as charged but it's not been run that much – maybe 40 outings to the lake over two years.

The motor is a BL 2845 3600KV water cooled motor from Giant Cod (since renamed Giant Shark!). It's the one boat motor that they still have on their website at £13.17. I am tempted to get it to see if it solves the problem. I don't think it can be the prop as the vibration occurs even when the prop is not fitted.

On a technical note, should one expect the motor shaft to vibrate if prevented from turning with the motor sounding as if something in there is turning or is this a sign that it is shot?

Jeremy

[Dave MilbournParticipant @davemilbourn48782]

Jeremy
I'd agree that your tests seem to indicate the problem to be with the motor. The last sentence alone would have me lifting the lid on the waste bin! I would raise an eyebrow at the rating of the motor – 3600KV is pretty high for a model boat. What voltage are you using and what size prop?
Dave M

[JeremyParticipant @jeremy15845]

Ashley

7.2 volts, 32.5mm prop. I don't know the pitch but it looks pretty high. Boat ran very well with this motor but I could always go to a lower Kv now it has to be replaced. It would have to be a water cooled inrunner to fit a 600 size mounting. I would appreciate your recommendation.

Jeremy

[Geoff SleathParticipant @geoffsleath41411]

3600 kv is quite high. With a 7.2v motor that's a theoretical 7.2 x 3600 rpm = 25,920 rpm! Probably quite a lot less than that on load but still very high. I usually use motors with a Kv of 1000 or less on model aeroplanes using 12×6 props on 3s LiPo (11.1v) so you would need perhaps a rather higher kv on 7.2 volts.

Is water cooling really necessary? If you could manage without, it opens up a much wider motor choice.  Most inrunners have a high kv.  Without water cooling you could use a lower kv outrunner.  They're pretty cheap so it might be worth trying if you can fit one without too much trouble.

I suspect your problem may well be causes by the magnets coming loose. You could try stripping the motor and having a look. If it's only one or two, you could reglue them with cyano. Worth a try, even as an experiment to prove the motor fault.

Geoff

Edited By Geoff Sleath on 31/01/2015 15:31:54

[JeremyParticipant @jeremy15845]

Thanks Geoff

The boat already has watercooling fitted. To switch now to an outrunner would mean changing the motor mount – not easy because it is glued in place. Simplest is to replace the motor with one of the same dimensions, using the same mount.

I am using a lower kv outrunner (1250kv) on 12v with my nearly completed 1/16th scale Vosper crash tender. If I were starting aagain, I would probably have used a similar motor on the Chris Craft.

I would strip the motor if I could but there is no obvious way of getting into it.

Jeremy

[JeremyParticipant @jeremy15845]

I am stumped.

Switch of motor did nothing. The shaft stopped turning if I held onto it as with the current motor. Maybe that is just how they are?? I also switched the speed controller – that did nothing. I removed and reattached the motor mount, greased and oiled the prop shaft and reassembled. It is still noisy when run fast out of water. although there is no obvious vibration.. Maybe on water with the resistance to the prop, it may quieten down.

Thanks for the advice along the way. I will report back.

[JeremyParticipant @jeremy15845]

Problem solved but I'm not quite sure how. Took out and reinstalled the motor and drive train and regreased the prop shaft. Now as quiet as she was before.

Thanks to all for their advice .

Jeremy

[Mark Jarvis 2Participant @markjarvis2]

Hi Jeremy

Your last post sounds strange, is your prop shaft quite long??? posible that the shaft is whipping insde the prop tube, re-greasing the shaft is helping to hold the shaft true.

Mark

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