[hpv-boats] converting pedal power to prop power right angles

[Rick Willoughby]

shaft is olid but pretty thin (just 3 mm !); nonetheless he says it was
builit and worked very well.
Do your calculations confirm this?

If they could have achieved the design performance in terms of drag  
then the 3mm shaft would be OK spinning at higher cadence than I am  
capable of now.

The biomechanical efficiency drops off above 300W with a 300mm pitch  
prop at 6m/s.  It is a slippery slope when drag is higher than design  
and would require a trained "spinner" to overcome the inherent losses  
of a compliant shaft.

I expect if they had taken the design further they would have  
increased the shaft diameter once they understood the problem.  From  
the videos I have seen of Skeeter in operation the pilot seems to be  
delivering a great deal more than the target 250W to get it to fly.   
Hence I would question the comment "worked very well".  My  
understanding is that it performed well short of design objective.

There are other ways to improve the efficiency such as more rotating  
inertia on the driving end of the shaft to provide carry through the  
dead spot.

The biomechanical efficiency is not an illusion.  I can drop my heart  
rate from say 140bpm to 137bpm for the same boat speed by  
concentrating on spinning rather than just pushing the pedals.  The  
problem for me  is it involves untrained muscles that tire quicker  
than the better trained ones. And this is using drive system with  
little compliance.  The benefit of spinning is even greater with a  
compliant shaft.  Spinning is about 10% more efficient than push-push  
on any crank drive.


Rick Willoughby