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

[Rick Willoughby]

A while ago I decided to do a dynamic model of the crank drive and  
curved shaft system for a pedal boat.  This was so I could determine  
how a drive would feel before it was made rather than doing it by  
trial and error.

The model is able to produce time based data on the important  
parameters of a drive system.

This trace shows what my current 8mm curved shaft does with a 1:4  
ratio on V14 hull:
http://www.rickwill.bigpondhosting.com/Boat_Shaft_1to4%20_solid.pdf
 From a feel perspective the black curve is the most important.  The  
closer it is to a straight line the better.  At 3m/s average boat  
speed the crank speed is varying between 60 and 80 rpm.

This is the trace with Mark's flex shaft:
http://www.rickwill.bigpondhosting.com/Boat_Shaft_1to4%20_flex.pdf
For the average boat speed of 3m/s the crank speed is going from  
slight reversal up to 140rpm.  With 170mm cranks 140rpm corresponds  
with my maximum foot velocity so that was my speed limit with the  
flex shaft.  I have video that shows the stop-start nature of the  
pedaling so the model is reasonably close to reality.

I have also modeled what the flex shaft would be like with a 1:8  
ratio driving a 12X12 prop:
http://www.rickwill.bigpondhosting.com/Boat_Shaft_1to8%20_flex.pdf
Here the crank speed variation is from 40rpm to 120rpm.  This would  
feel tolerable and I could get slightly faster than 3m/s before  
hitting my 140rpm limit.

So with higher gear ratio the flex shaft system looks to be an  
economic means of getting a right angle drive and mounting the prop  
within reach beside the hull.

Also with practice at spinning, the crank speed would be smoother.   
Even with the 1:4 ratio I could hold quite steady foot speed with the  
flex shaft up to about 8kph. I do not have enough training to "spin"  
as I load up.

The dynamics depends on the boat and prop as well as the shaft and  
gear ratio.  If anyone wants a shaft system modeled in this way it  
does not take long to do.


Rick Willoughby