Steve,
An alternative to sawing off the threaded part of the shaft at the coupling end.
Buy the spiral coupling with a 3mm I/D at the output end.
Drill it out with a 4mm tap drill, = 3.4mm and tap a 4mm thread 0.75 (normal) or 0.76 pitch to match your shaft end. (🙄)
Bingo. Bob's yer Uncle and Fanny's yer Aunt. Job done👍😊
Cheers, Doug 😎
https://www.boltdepot.com/fastener-information/metric-tap-drill-size.aspx

Thanks Rowen👍
Forgot that older shafts were like that.😔
Since I got my mini lathe and milling machine I've got used to making my own!
Cheers, Doug😎

Not unusual. Some shafts fit both screwed propeller and a screwed coupling, thus threaded at both ends.
If you want to use a coupling with flats and grub screws, just cut the thread off. May have to shorten the outer tube to compensate.

Thanks again. Strangely enough the prop shaft is threaded on both ends. The same thread on each end.

Cheers.

Hi Steve,
The inserts of the spiral coupling are not threaded.
They are secured with grub screws.
That's why I milled depressions in the shaft to prevent any slippage.
See my 2nd pic below with the shaft and coupling on the mini milling machine.
Use a flat ended mill bit to prevent it skidding off the shaft - like spiral drills would😠
I also fitted a collet in the same way, visible to the right of the pic.
I then fitted spacer washers between the collet and the end of the shaft tube.
Prevents the shaft and prop exiting stage backwards if the coupling should come loose.😭
Which mine never has 🤞🤞

3.94mm I would say is well within the tolerance of a 4mm drawn rod, especially those produced for the model market, and not for a NASA rocket 😉
I assume that your prop shaft is only threaded at one end.
That's where you screw the prop on. DON'T forget a lock nut and a thrust washer👍
The plain end goes into the coupling. If you can't mill a depression into the shaft for the grub screw just file a flat. Secure the grub screw with Loctite or similar.
Hope this helps.
Any doubts just YELL.
Cheers, Doug 😎
PS Just added two pics re milling the prop shaft👍

"The centre of the sleeve can as also be filled with grease or oil to maintain lubrication "
That's a neat idea Rowen👍
Could give my old couplings a new lease of life.
Cheers, Doug 😎

Many thanks gents (I'm assuming gents, sorry if not), all very useful information for the novice!
Next question if I may - my prop shaft was bought about 20 years ago and when I use my digital calipers the plain bit of the shaft is 3.94mm dia. How do I know what the thread is to order the correct threaded insert for the coupling? Can I somehow convert the number of threads per mm? 13 revolutions of the nut on the prop shaft moves the nut 10mm so i'm assuming that 10/13=0.76 means that it should be an M4. Is this correct?

Thanks again in anticipation.

Agree with the clackety clack noise. I slide a length of plastic tube over the coupling once all is lined up.
This sleeve reduces backlash in the coupling and quietens it down.
The centre of the sleeve can as also be filled with grease or oil to maintain lubrication

Hi again Joe,
It was Krick!
You can order them with a variety of input / output diameters.
Cheers, Doug 😎
https://www.krickshop.de/e-vendo.php?shop=krick_e&SessionId=&a=search&SearchStr=searchfield_technic_0%3Astegkupplungen

Hi Joe,
I like the spiral coupling a lot. Takes the full power of my 1000kV motor running on a 3S LiPo without a whimper.😊 You have to be careful with the initial alignment though as it won't flex as far as UJs. personally I didn't consider that a drawback😉 Being compressible it also absorbs any forward thrust on the shaft👍
This was my first BL setup and I had read / heard of them ripping the inserts out of the conventional plastic / brass UJ couplings. Had used those for years on my brushed motors.
But I learned to avoid the ones made of metal only. Too much clackity clack!
Sometimes the metal to metal contact seemed to create RF interference🤔

I think my coupling was from Krick, was a few years ago now, I'll check.
Maybe Cornwall Model Boats can help you with 3/16" stuff?
https://www.cornwallmodelboats.co.uk/

My Sea Scout originally had a 3/16" shaft. I swapped it for a 4mm one. The original was worn anyway.
If all else fails you could always re-thread the old props!?
Cheers, Doug 😎

Doug,
Regarding the flexible spiral coupling, do you like them, how well do they work?

To date, most of my couplings have been the universal joint type for 3/16" shaft and I am having trouble finding the non-metric sizes now. I have a lot of older brass props hence the use of 3/16" shaft. I will take a look at Krick.
Thanks, Joe

in the second picture the motor holder it's what i call L shaped motor holder

Hi Steve,
First; you can chuck the 'alloy thingy' in your bits box👍
You don't need it, unless you want to build an Everglades Airboat😉
That's for attaching the prop (airscrew) in a model aircraft.
Second; being an outrunner the only fixed part visible from the outside is the chunk of metal that the connecting wires disappear into: Logical cos you don wanna rip 'em off with every start do you! 😁
So; you attach your shaft coupling to the plain 4mm shaft👍
Pic shows my 28mm outrunner mounted in a Sea Scout on a standard 400/540 type motor mount from Krick. Be sure to use the screws provided with the motor. Anything longer and you risk fouling the stator and wrecking it😮😭
Second pic shows the motor on it's mount and the propshaft and coupling just prior to installation. The Coupling I use is a semi-rigid spiral coupling, from Krick I think it was.
Whatever coupling type you finally fit use a rigid coupling to ensure the best alignment when installing the motor mount. I made mine up from a chunk of brass rod, but most online model shops will have one. In your case a chunk of 4mm I/D tube would also do.
I made mine on my mini lathe cos I needed a 3mm to 4mm adaptor.
Hope this helps🤞
Don't forget to post some pics of your build please👍
When she's finished put her in your Harbour😊
Cheers, Doug 😎

From my point of view, you can screw the motor as usual, using an aluminium L shaped support, if the motor is an outrunner one, simply turn the "L" 180 degrees looking at it from the top side, and screw the back side of the motor, the screws to fit to the hull, must be below the motor. Just to cool it, being outrunner the autocooling is more efficient, read the motor's datasheet, to know the working temperature and the max one, supposing you need to cool it down, you can use a fan, you can mount it over the motor but not blowing the motor, just blowing in the opposite direction, you will exhaust hot air, and the cool air will replace it coming from below, and cooling the motor at same time, if i can help you some more, simply let me know it