[hpv-boats] Magnetic Transmission

[Rick Willoughby]

I have been playing around with the idea of making a magnetic right  
angle drive unit for a while.  Over the past few months I invested in  
some magnets and built a prototype:
http://www.youtube.com/watch?v=jCuCmzcjpao

The video shows the torque test.  I have a 5kg weight suspended on a  
lever arm connected to the output shaft.  The lever gets out to 230mm  
before the magnets let go. This particular variant has a ratio of  
1:3.3 and gives in at 11Nm on the output shaft.

The torque at this ratio is ample for cruising to speed of up to  
12.5kph but would give out in a sprint.

The unit was not executed to the initial design because I could not  
get the quantity of magnets of the particular size I wanted.  It was  
intended as a 1:5 step up but I could not get the alignment right  
with only 4 magnets on the output so went up to 6 thus reducing the  
ratio.  However I got better than a commensurate increase in torque.

This unit is extremely robust and heavier than I would want in a  
boat.  One feature is that the weight is distributed at a large  
radius so the crank and output both have high rotational inertia -  
flywheels if you like.  This leads to smoother pedaling and can  
compensate a little for a torsionally compliant prop shaft.   Another  
feature is the load limiting on the prop shaft.

If I made it to tighter tolerance I could get higher torque.  In a  
way this would defeat the original idea because I was exploring the  
concept for ease of building without need for precision machining.

I expect with ideal magnet selection and simple method of  
construction this arrangement could be built for under AUD200.

Rick Willoughby