The shaft can be replaced, sometimes with great difficulty. I had to do that with my Wiesel schnellboot. I built it with three shafts (per Graupner's instructions) and then changed that to the correct four shafts. That required a 5 inch by 6 inch rectangular hole in the bottom.

Lew
Florida ⛱️, USA 🇺🇸

Correction (edit): The cutout was a cross shape. Added photos in the order of the repair.

Must say the prop. tube is glued in, can't replace, Spence.

Just tried my set-up again today without the prop. shaft connected and everything worked well, motor remained cool throughout, no problems. Will now try with everything connected up. I've noticed where the prop. shaft comes out of the tube at motor end, it is at a slight lateral angle although it turns smoothly. Correspondingly the motor will have to be too. I haven't strained it in any way, I'm guessing it has been supplied to me like this, bit strange. So far so good. Thanks Lew and Chris, you were right, cheers Guys, Spence

Btw Lew thanks for the illustrations, Spence.

Lew, I was going to use the wooden coupling as a temporary measure to get every thing aligned then swap over to the flexible one as permanent. Sorry to have misled you there 🤣. I've just ordered a solid metal one. Spence.

dog bones are my go-to joint. If not use 2 u joints as Lew points out. Len

Spence, That's a new one! I never heard of anyone making a model boat coupling out of wood. For the coupling's small size and speed you most likely need something stronger, balanced, and precise.

I checked on ebay.co.uk and did see some Dumas couplings there. There ae some hobby shops in the UK that carry Dumas model boats and accessories, try checking with them. (Photo #1)

Alignment issues - difference between one and teo u-joints. (Photo #2)

Another alternative is getting two u-joints with a short shaft between them. (Photo #3)

About photo #3, that is from the drive set I ripped out of the Krick FLB2 that someone gave me (someday I hope to get back to it). I added the short shaft and new couplings. Photo #4 is an exaggerated misalignment but serves as a guide as to why two u-joints are a vast improvement over a single u-joint. (Also, I did not realize I had any MFA Torpedo 500 motors. Ha!)

Lew
Florida 😎, USA ❤️

I tried making a solid coupling out of wood but with the motor being 3.2mm and the prop shaft being 4.0mm couldn't get the hole straight. Don't think the Dumas dog bones are available here in the UK. They have a solid metal coupling to get things aligned where I shop, but it is too long for my design. I was under the impression, rightly or wrongly, that you could have a slight mis-alignment with a flexible coupling. Maybe you could advise, or tell me what else I may use. Spence.

This is why I pretty much exclusively use Dumas dog bones. I swear for them, never at them! Even three in my 47" heavy PT61.

Lew
Florida ⛱️, USA 🇺🇸

If the overheating issues started with using a single UJ that points to an alignment issue with the motor and propshaft.

I use mainly Microflex couplings and you need to ensure that alignment is accurate - to do this I use a solid coupling of the same length and then replace it with the Microflex coupling. If you use a motor mount that is screwed or bolted into place you will need to shim with washers to maintain the alignment.