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[drover]

or problem with me being stupid?


I am slowly progressing with my first build of a powered model boat. I am at the stage of fitting motors and prop shaft. It seemed as though it was going to be straightforward but I have hit an unexpected problem. The build instructions require slots cut in the hull for the shafts to pass through and the addition of P brackets to support the shafts. After fitting the shafts they are bedded  into the hull by sealing the slot with body filler.


Before bedding in I turned the motor by hand to test alignment. In one direction the shaft turns smoothly but when reversing the direction the shaft unscrews from the motor coupling which surely would have a serious, terminal effect on the motor if the shaft was bedded in. What is causing this problem, am I as a beginner missing something dead obvious?

[dougal99]

Have you fitted a lock nut on the shaft against the coupling? You need one on the prop as well.

[derekwarner]

Evening drover................you don't show your location, however why not consider joining a model boating club or group if one is close by

You will find members of such groups are 99.9% more than happy to offer constructive comment  ....... & help with marine issues.......... Derek

[essex2visuvesi]

I use a few drops of blue threadlock on mine and I haven't had a problem

[TailUK]

Two prop set-up usually calls for a Right handed and a Left Handed prop spinning in opposite directions.  The props screw on to the shaft with a right handed thread.  The inclination of the RH prop will be to tighten itself onto the shaft and the LH prop to unscrew itself when under load (pushing the boat forward)  the situation is reversed when running the motors in reverse.  The most common methods of dealing with this are A) A grubscrew in the side of the prop to grip the prop shaft (This may unbalance bigger faster turning props)  B) Having the threaded prop shaft protrude through the prop boss and a locknut on the end of the threaded shaft.  C)  Thread lock adhesive (Epoxy or Super Glue) which works fine on smaller shafts and props .  D) Any combination of the above!  HTH

[malcolmfrary]

Lots of people think that just holding the shaft and winding the nut up to the object to be locked gives a lock-nut.  It dosn't.  The nut has to be tightened (and I mean tight) against the object that it seeks to lock.  If it is a prop with a domed boss, the locknut can be fitted inboard of the prop, but still needs to be TIGHTENED against the face of the prop so that both are tensioned against both each other and the threads of the shaft.
Use of thread locking compound is good belt and braces.
I would only consider going the grub screw route if I was prepared to strip it down after each session, a steel screw and some other metal prop in salt water is not a happy combination.

[morley bill 1]

I never use a threaded shaft at the motor end always use a plain shaft with a flat filed on It and at the prop end a lock nut and a little superglue  Bill..

[TailUK]

Lots of people think that just holding the shaft and winding the nut up to the object to be locked gives a lock-nut.  It dosn't.  The nut has to be tightened (and I mean tight) against the object that it seeks to lock.  If it is a prop with a domed boss, the locknut can be fitted inboard of the prop, but still needs to be TIGHTENED against the face of the prop so that both are tensioned against both each other and the threads of the shaft.
Use of thread locking compound is good belt and braces.
I would only consider going the grub screw route if I was prepared to strip it down after each session, a steel screw and some other metal prop in salt water is not a happy combination.

I bought a load of stainless steel grub screws on fleabay!

[derekwarner]

Guys...from a simple technical point ...it is the threaded section of the shaft that is stretched [elongated] by torsional deformation during the tightening process......then the female [internal] thread elements in connection take up or secure the induced torque

Liquid jointing such as 'Loctite' will secure a threaded element by gluing, however an adequately torqued mechanical threaded joint will not benefit by the use of such glues

Having said this, in model applications...it is not always practical to apply the required shaft torque/tension and so Loctite is acceptable

Just use a low strength variety to enable dis-assembly without the use of the RED spanner

In my engineering life, I have not been a fan of Loctite type bonding agents nor super glue fluids......

Derek

[TailUK]

Guys...from a simple technical point ...it is the threaded section of the shaft that is stretched [elongated] by torsional deformation during the tightening process......then the female [internal] thread elements in connection take up or secure the induced torque

Liquid jointing such as 'Loctite' will secure a threaded element by gluing, however an adequately torqued mechanical threaded joint will not benefit by the use of such glues

Having said this, in model applications...it is not always practical to apply the required shaft torque/tension and so Loctite is acceptable

Just use a low strength variety to enable dis-assembly without the use of the RED spanner

In my engineering life, I have not been a fan of Loctite type bonding agents nor super glue fluids......

Derek

This presupposes that the the person doing the tightening isn't one of the "twist it til it squeals" brigade.  I learned a long time ago that even using the wrong spanner can be catastrophic.

[jaymac]

The inclination of the RH prop will be to tighten itself onto the shaft and the LH prop to unscrew itself when under load (pushing the boat forward)  the situation is reversed when running the motors in reverse. 

I used to think that but it always seemed to be the RH prop that came off  Thinks if  the RH prop is turning against the pressure of the water this surely tries to  unscrew it as opposed to the LH  which  the pressure is tightening it

[NFMike]

I used to think that but it always seemed to be the RH prop that came off  Thinks if  the RH prop is turning against the pressure of the water this surely tries to  unscrew it as opposed to the LH  which  the pressure is tightening it

It depends whether the props are inward or outward turning. But since there is always reverse involved in practice both need restraint against unscrewing.

[imsinking]

You can't beat a lock-nut & grubscrew for either direction , preferably stainless nut & an M3 w point short grubbie ( plenty on ebay ) the w point is best , doesn't do too much damage to the thread top . . . I grub the drive couplings as well . . .  as above not fond of Loctite or nail varnish .
Bill

[malcolmfrary]

I used to think that but it always seemed to be the RH prop that came off  Thinks if  the RH prop is turning against the pressure of the water this surely tries to  unscrew it as opposed to the LH  which  the pressure is tightening it

That depends on whether they are English or Continental - English speakers tend to view the turn from behind the vessel looking forward, Continentals from inside the vessel looking back.  Both, of course, carry the same direction of thread.