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[Dave_S.]

I have given up trying to sell my (bought used) 46" Crash Tender and am refitting it with a brushless motor and hope to have it on the water in the spring.


I have removed the original brushed motor, a Torpedo 850 with a claimed 157 Watts (unsure whether that's its consumption or output) and am replacing it with a 1350 Watt brushless.


Having removed the prop shaft from the tube and measured it, it is 3/16" diameter (not too surprising for an old British kit) but the thread at each end is 5mm - i have checked it against several 5mm bolts and a 5mm nut is a perfect fit. This obviously means more choice when choosing props, but I suspect that if the bearings in the tube aren't worn out already, they won't last very long with almost ten times the power passing through them, which is likely to mean removing and replacing the whole drive system, which I have done on other boats, but it's never easy, and the Crash Tender has a substantial ply keel to get it out of.


Are mixed metric/imperial shafts common, or has this been bodged together by a previous owner? If it looks like it's going to present me with a problem, I'd rather remove the shaft and replace it with a metric one now, as then at least I'll have peace of mind and wider range of props to try.


Any observations/comments/advice/insults will be gratefully received.

[tonyH]

Interesting! 3/16inch comes in at just over 4.76mm (25.4 times 3 divided by 16) so it's odd that the thread is 5mm.
How long is the tube, and do you need 5mm or could you take out the shaft and bearings and put either a slimline 5mm or normal 4mm unit through the existing tube?

[Dave_S.]

Yes, that's what the unthreaded part of the shaft measured at, I am slightly concerned that although the 5mm nuts and coupling thread on smoothly, they may not be fully engaged. The threaded section is the same diameter as the shaft, but the 5mm nut and fastener fit it well. A 5mm drill bit is too big to even start to get through the bearings.



If I have replace the shaft, I will remove the tube. I don't think a 4mm shaft will be up handling nearly two horsepower.

[JimG]

Could the thread actually be 2BA, the BA threads were based on metric sizes and in many cases could be interchangeable depending on the pitch size of the metric thread. The diameters seem to be correct for 2BA and most shafts of that time were BA threaded not metric.
Jim

[Circlip]

JimG seems to have nailed it, M5 is 0.8 mm pitch and 2BA is 0.81 mm pitch. O/Ds are comparable.


 Regards  Ian

[Dave_S.]

Thank you - that sounds very feasible - hopefully, I'll be OK using my 5mm coupling on it, if not, I'll have to look for a 2 BA.