William Watt says:

no moreso than paddling kills your arms.

Not so - paddling kayak-wise uses a full movement of the arms, ending with the
paddle being withdrawn and the other arm starting to work. No jarring.
Having a blade come out of the water while still under full power, then (say)
half a revolution until the other blade hits is going to be..... hmmmm -
looks for marine analogy.... like NOT having a throttle-man on an offshore
powerboat. The prop will come back into the water after a jump with the engine
spinning maddly, suddenly get loaded and break the shaft/blade whichever. Not
good. ;-(

the only problem I can see is the weak mechanical advantage of the small
diameter rotation of the kid's toy crank. like having the peddal crank on
a bike smaller than the cog wheel on the axel.

THat situation is directly analogous to the kayak paddle, where the lever of
the arm to the fulcrum (other arm) is less than the fulcrum to the blade.
Makes the most use of the minimal power available frm a human body. Lance
Armstrong at peak produces 600Watts when pedalling.

the paddle shaft would have
to be short as feasible. hadn't considered that.

See above.

Steve
Stephen C. Baker - Yacht Designer
http://members.aol.com/SailDesign/pr...cbweb/home.htm