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[Tug Fanatic]

Does anyone have experience of using a long propeller shaft inner as the motor drive shaft?

Many prop shafts are 3mm 4mm or 5mm as are many brushless motor shafts which are also easy to change. With a simple flat to locate the grub screw it should be possible to replace the motor shaft with the prop shaft giving no coupling, a shorter installation, no alignment issues etc etc. 

I have heard this suggested but not seen it done. There is presumably a problem that I have not considered.

[Martin (Admin)]

I'm sure, HS93 (RIP), did just that for his RAF 'crash tender', from memory it worked very well.
You might also think about using a solid coupling to achieve the same thing.

[Unsinkable 2]

Hi Tug Fanatic, if I understand correctly you wish to remove the shaft from your brushless motor and fit the prop shaft inner straight into the motor?


In theory I suppose it's a good idea, in fact if nothing goes wrong then it should be quite alright. Remember though that if you need to remove the motor you will need access. If this is achieved by releasing and sliding the shaft out of the motor that when you come to put it back in you will have to push it back up through a grease filled outer shaft first. If the motor is fitted close to the outer shaft to save space then you may not have a lot of space to get fingers cloths etc in to degrease it. If it isn't fitted close then why bother in the first place?
 As for alignment, well if you connect the motor to the shaft with a shaft adaptor (the brass tube type with the two grub screws) then this will also create a straight fit and be easier to access the motor.
 Also the length of the inner shaft would have to be longer than the one supplied so you would have to buy a new shaft.
 It doesn't mean it can't be done (I think, maybe I've missed something too!)   See what the others suggest.  U2

[Unsinkable 2]

Martin types quicker than me and replied whilst I was writing, but yes 'solid coupling' is what I meant by adaptor. U2

[Tug Fanatic]

Yes a solid coupling would achieve the same thing and I agree that it would be easier if the motor ever needed changing or the prop shaft needed servicing but ..........

I guess the question becomes what experience do we have of using a motor solidly coupled to the propshaft? I have always used a flex coupling like every other modeller that I have seen but never felt that a single universal type coupling was a very good answer. Alignment feels like more of a problem with a solid coupling that with a one piece shaft  - I can't see why it would be with my logical cap on but my mind works in mysterious ways.

[inertia]

There's a chap who sails with us who has done just this with a brushless "bell" motor, and it works very well.
DM

[Plastic - RIP]

A long motor shaft will whip if it only minutely off centre and cause massive vibrations. The faster it goes, the worse the problem and it will probably get so bad it will break its own mounts. The shaft would have to be perfectly straight - which is unlikely.

Same will happen with a solid coupling - it would need to be amazingly accurately made and balanced. That's the whole point of flexi-joints and couplings - to be able to secure the shaft & motor seperately and couple the drive.

I wouldn't do it.

[Stavros]

 Getting  a 100% straight alignment of shaft and motor is so so easy  especially if using a solid coupling or even putting it through the centre if a brushless motor.........All you need to do is mount the motor first offer shaft to either motor or coupling and only then do you glue the shaft to the hull.

As for breaking its mounts I have 8  boats with solid couplings and don't have a problem and 2 of these rev to 20000 rpm....solid couplings are from Steve Tranter Model Boat bits.



Dave

[Tug Fanatic]

Getting  a 100% straight alignment of shaft and motor is so so easy  especially if using a solid coupling or even putting it through the centre if a brushless motor.........All you need to do is mount the motor first offer shaft to either motor or coupling and only then do you glue the shaft to the hull....................

Dave

That is exactly what I was thinking. With a bell type motor it would also make for an incredibly compact power system ideal for a tight space.

[Colin Bishop]

If you are not careful you may encounter wear problems with the motor bearings. One reason for having  a coupling is that the shaft can have washers and locknuts adjusted to take the thrust on the ends of the stern tube which in turn transmits it to the hull of the boat. The motor bearings are intended only to absorb lateral forces from the driveshaft. So in a well set up conventional installation the motor only has to turn the propshaft and doesn't have to absorb any thrust from the propeller. If you make a solid connection between the motor and propshaft then you need to be very careful to ensure that the motor doesn't take any of the propeller thrust, maybe by giving the motor mount a bit of fore and aft flexibility. You will still need the locknuts and thrust washers on the propshaft of course.

Colin

[Tug Fanatic]

I have often wondered about the fore & aft thrust with the single universal coupling so commonly used. It seems to me that all fore & aft propshaft movement would be sent to the motor bearings just as easily as in a solid coupling. Add to that the problems caused by the horrible allignment that you sometimes see.

I am surprised that nobody this side of the pond seems to use/ sell a metric simple dogbone type coupling as sold by Dumas in the US and used by many modellers. This removed the fore & aft problem completely & effectively isolated the motor fro the rest.

[Colin Bishop]

' It seems to me that all fore & aft propshaft movement would be sent to the motor bearings just as easily as in a solid coupling.'

That is why I said well set up. A lot of modellers don't do this correctly. Glynn Guest covered the subject in one of the Model Boats Special Issues, see drawings below.

Colin

[Plastic - RIP]

I use a thrust washer behind the prop and collet & washer on the inner end to stop excessive end-float of the shaft and to keep the grease better contained to assist in sealing. Works for me. 

The motor and coupling have no push or pull forces applied to them.

[Leaky Bottom]

I've used this method of fixing the motor on my remora and it works very well, you do have to get a flange made to fit to the outer prop tube and fixed to the prop tube to stop the front part of the motor rotating

[essex2visuvesi]

Looks very smart LB

[Liverbudgie2]

 My botty 'aint leaky.

LB

[davidm1945]

My botty 'aint leaky.

LB

Preparation H?


You can buy the direct mounting shaft to motor flanges from Hobbyking or ebay.


Dave.