But you would end up with very thin propshafts and a corresponding reduction in length before you would suffer shaft whip,

Yes. Funny how that never happened during the days of imperial measurements – when 1/8" was the standard propshaft size – which is near as dammit 3mm. I was very happy with that on a Sea Commander and a Merco 35 – worked fine. I suppose metric steel must be whippier…?

As it is, I have to use 2mm shafts for my EeZeBilts, unless I want to have a propshaft which weighs more than the motor. But 2mm props are all too small to get the best out of the little brushless motors I use.

3mm is the obvious and correct size for what I need – amongst other things, that's why the small brushless use 3mm shafts. I have just started ordering props with a dog connector to make up my own 3mm drive train – perhaps I should drill a 2mm and tap at 3mm…? Doing this I either have a choice of a prop boss being very thin or being gigantic…

And, as you can see, I make my own couplings. Those are just knurled cylinders tapped 2mm, 3mm or whatever, with a thick-wall silicone tube. The good point about this design is that all my motors are compatible with all my propshafts…

Indeed they do. I omitted to mention it, but my 'eezebilts' are expected to cost under £10 total. Prop, shaft and rudder should be between £0 to £2. This is what a £0 prop looks like:

**LINK**

And in return I should apologise for my rather ponderous sarcasm – since I fear you are quite right – the decision is almost certainly a purely commercial one.

Typical shafts in the 1960s were 2 BA and 4 BA – around 0.14 and 0.19 in, which comes out to M4 and M5 if you round up each time. And that makes commercial sense, because the small/medium sized boat kits that introduced kids to the hobby are no longer made. We now have bigger boats, and much more powerful motors, so if you're going to drop something it will be the smaller items in the range. And you just keep the really small M2 for plastic conversions.

I suffer because I produce boats squarely in the 12"- 24" range where M3 would be ideal. But all is not completely lost. If you look closely at the well-known 'Radioactive' range of plastic props you will see that they offer a 40mm, for instance, in both M2 and M4, and they do this by moulding the same prop, but pushing a brass insert centre tapped either M2 or M4 into it.

The key word here is 'pushed'. The insert does not appear to be moulded in. It just stays in because it's knurled. So it should be possible to remove it and substitute an M3 one – enabling me to do what I wanted – run a straight-through shaft from the motor to the prop and cut out the cost/complexity/off-centredness of a coupling.

Incidentally, for the VERY small props – 1 cm or less – I suspect that the sheet tinplate technique is going to be the easiest way to get a prop, and more efficient than a commercial cut-down one might be. As an aside, I do need to counter whip on the M2 shafts of 9" or longer – I put a small low-friction nylon bead in the middle of the shaft which does not touch the prop-tube sides at slow speeds, and suppresses whip at high speeds…

Edited By Dodgy Geezer on 29/04/2015 20:36:49

Edited By Dodgy Geezer on 29/04/2015 20:37:46

I have some solder, but might not be the correct stuff, will check it out.

There is a difference between 'what is the right stuff' and 'what you can get away with'.

Ideally, mechanical parts like rudder blades should be 'hard' or 'silver soldered' – using a high-temperature solder and a gas torch. But for small items on an eezebilt, standard electrical solder is fine.

I notice that I haven't specified the metal thickness! An original KK rudder for the eezebilts was around 0.013" – 29 gauge? – tinplate. Tin cans (not aluminium!) are usually a bit thinner. If you're buying brass sheet, get something around 0.35mm in thickness. It will be fairly easy to bend, and to cut with a stout pair of (old!) scissors)

I find that plumbers 'self-cleaning' acid flux makes soldering a doddle. Just smear a bit on and the solder sticks, even to a rather dirty bit of metal…

And once you get the hang of soldering, you'll never have to buy a rudder again!  Though you will probably need a Dremel multi-tool or similar for the bigger rudders, where you have to make a slot in the shaft to fit the blade into…

Edited By Dodgy Geezer on 30/04/2015 10:35:37