I noticed that Cornwall has a 3mm shaft coupling for the Graupner props. Is there any reason this wouldn't work?
I had purchase a bunch of these and they do not fit into the props if that is what your were thinking of doing. I have been looking for some time for a coupler to fit but could never find one cheap enough to do the job.
I did end up using them by buying two different sizes one to match the motor shaft and the other for the prop then used a 4 mm bolt with the head cut off to fit into the adapter then screwed into the prop. I then soldered the two brass fittings together. I was very difficult to get them centered when soldering so no wobble occurred. I used them this past summer on a number of dives. I now gave up on that idea and just ruffed up the shaft with a file and epoxied the shaft directly into the prop which was real easy to do and ended up with a nice clean shaft and prop combo. They are cheap enough so I bought extra motors and props as spares in case I should need them.
Yes, that's what I was planning on doing. Thank you for your response---you saved me from some unneeded spending.