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

[Giuseppe Carignani]

Thanks Rick
your report is highly informative and seems very accurate, as usual.
I'm just wondering about Mark Drela's flex shaft propulsion system, the one
he donated to The Open Waterbike Project

http://www.openwaterbike.com/components/drive-units/mark-drela-drive-system

<http://www.openwaterbike.com/components/drive-units/mark-drela-drive-system>The
shaft is olid but pretty thin (just 3 mm !); nonetheless he says it was
builit and worked very well.
Do your calculations confirm this?

As you know I believe the availability of a commercial flex-shaft propulsion
system could be the turning point in marketable high performance pedal
boats, so I'm really looking foward to this!

Best
Giuseppe

Giuseppe Carignani
Founder, The Open Waterbike Project

On Mon, Mar 14, 2011 at 11:23 PM, Rick Willoughby
<rickwill at bigpond.net.au>wrote:

> 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
>
>
>
>
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