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Propshaft coming away from coupling

Propshaft coming away from coupling

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Jonny Machon24/05/2020 16:27:38
1 forum posts

Hi guys

Took my boat out today for a test and all fine no vibrations or leaks and runs nicely however under high forward throttle the propshaft is unscrewing itself from the coupling and forcing motor backwards

Any suggestions why this maybe and how I can fix it

Motor is a mtroniks m600

Prop 40mm

Propshaft is raboesch waterproofshaft

Raboesch coupling with inserts

Running 3s lipos


Colin Bishop24/05/2020 16:59:06
4968 forum posts
6089 photos
410 articles

You need a locknut screwed up against the coupling.


ashley needham24/05/2020 17:06:00
7323 forum posts
156 photos

Jonny. Welcome to the forum.

As Colin says, a locknut is needed at both the coupling end and prop end ideally.

Some shaft inners are plain though and in this case a grub screw is used. A flat filed on the shaft would then be of assistance.


Malcolm Frary26/05/2020 09:43:54
1029 forum posts

An unlocked coupler has a motor at one end turning it, and a prop that doesn't want to turn at the other. If it is just sitting on the thread unlocked, it will move on the thread. A correctly applied locknut and/or thread locking compound will prevent this, as will following Ashley's suggestion of locking it in place using the grub screw on a flat filed on the shaft. The flat is a must - grubscrewing on to a thread might well render the thing unmovable later.

Dave Cooper 608/06/2020 22:45:26
305 forum posts
32 photos

Hi Jonny

The good news is that the flat doesn't need to be all that big - just enough for the end of the grub-screw to bite on is good enough. As they say above, best to use some 'Loctite' (or, similar thread compound) as well on the grub's thread.

I was a bit tight for space on my little RAF launch, so I took the rudder and prop shaft out to do it. Just a light touch on the bench grinder will do it, or, use a small hand file on the coupling end of the shaft.

If you have a fairly large hull you may be able to do it 'in-situ'. Lock nuts also seem like a good idea, although I didn't find I needed any...

Good luck with the mod'

Dave C

captainslog12/06/2020 20:07:34
226 forum posts
103 photos

This makes me wonder why the shaft threads arnt reverse threaded,as in an angle grinder ( and other things) is there any specific reason why this isnt so? I agree as well, i always use lock nuts but at the motor shaft end only. seems to work ok. dont ask why but it was a long wait on the bank of windless lake on my own and an eventual swim to retrieve.

ashley needham12/06/2020 20:40:01
7323 forum posts
156 photos

I suspect that as the forces are quite minor, the cost of faffing about with reverse threads is not worth the extra cost. A locknut should be more than enough.


Colin Bishop12/06/2020 22:05:35
4968 forum posts
6089 photos
410 articles

And if you have twin screws counter rotating then one will inevitably be affected.

As Ashley says, a locknut is just good engineering practice.


Malcolm Frary13/06/2020 10:59:00
1029 forum posts

Even with a reverse thread, locknuts will still be needed. On many boats that have reverse, reverse is the boats brakes, useful on occasion for avoiding expensive damage to concrete banks.

An unlocked connection between shaft and either coupler or prop will allow the inertia of one or the other to move whenever force is applied at one end with resistance at the other. Turning one way, something will try to unwind, turning the other way, things will tighten up. Both are undesirable, and it doesn't matter whether cheap standard threads are involved, or expensive special ones. The expensive ones just cost more to replace when they fall off.

As far as I know, angle grinders only turn one way, and I'm fairly sure that they turn to cause a standard thread to self tighten. Tooling to produce left hand threads is more expensive than for normal threads, the captains of industry are likely to go for the lower cost option.

I wonder if the OP got sorted, or whether it was a drive-by posting.

Edited By Malcolm Frary on 13/06/2020 11:05:41

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