By the way, if you scale up the propshaft length to match and use a huco style coupling you could end up with the motor in an awkward position under the wheelhouse window frame.

Tim that actually makes for an excellent coupling as long as the power being transmitted is low.  Looking at the size motor Dave has used and bearing in mind a 23″ model I’m sure you will be OK.  I once did a conversion of the Revel U-Boat plastic kit to RC and originally fitted dog bone type couplings.  The thing sounded awful.  I replaced the couplings with silicone tube and it now runs beautifully smooth and quiet.

Tim,  I think that highlights an interesting aspect of couplings that some modelers don’t always seem to appreciate.  While a coupling does allow a degree of misalignment in a drive train the more that misalignment is the more power will be lost in the coupling.  If you imagine the coupling to be perfectly aligned then it isn’t actually doing anything.  There would be no movement in the separate parts and therefore no frictional losses.  The more that misalignment is the more relative movement is and the greater the frictional losses.

Ideally, if we all built our models perfectly and built a perfectly aligned shaft arrangement we should be able to have a solid connection.  I have seen steam engines in the past with quite steeply angled UV type couplings transmitting the power and failed to convince the builder why he was loosing power in the coupling.  This is particularly the case with the plastic centred huco type couplings that are sometimes quite stiff to flex.

Dave was spot on with the huco brass insert and silicone rubber pushed over the splines, it is an excellent coupling  but, even then, there is less losses the better the alignment.  In that case you are actually stretching and then compressing the silicone as it rotates, which requires power to do.

I have used the aluminium couplings with a fixed “rubber” insert on a couple of models . They have worked well.

I have these in my Faireys. Basically they are semi rigid couplings and one of those I bought initially had a slightly off centre rubber section and vibrated horribly. A replacement cured the problem.

For models with lower RPM and power, silicone tube does work very well. However, with most of my models I have literally sidestepped the issue by using pulley drive with ‘O’ rings. Works perfectly and the only alignment you need to make is to get the tension right by being able to adjust the motor mounting slightly. There is a ‘sweet spot’ which you can easily find by putting a very low voltage through the motor and moving it to get max revs without slipping. The setup is very quiet.

The big bonus is that you can put a small pulley on the motor and a larger one on the shaft. This improves the efficiency of the drive by bringing the RPM difference between motor and shaft closer together and does wonders for current consumption. 2:1 or 2.5:1 is a good reduction. (it also allows a small motor to drive a larger prop more efficiently).

Colin

A more sophisticated version of my suggestion John – and effective.

Colin

I’ve used magnets a few times, they work extremely well but, as Ashley says, you need to be careful that they are not too strong.  It doesn’t take much to simply hold something in place against the wind.

They are easy enough to set into a snug hole with some resin or epoxy.  Just make sure you get the polarities right!

That all looks really good Tim.

Colin

What sort of scale is this one Tim?