Have you considered the idea of using foot-operated paddlewheels
instead? It would seem to me that they would give you better propulsion
for the same amount of effort. Also, I would think that they would
actually be easier to build and maintain. You could have 2 wheels (1
each side) and have them set up to mount on top of the gunwales. They'd
be nice and quiet too, for those nature pix that you might want to
capture with your camera.

Hope this helps,
Paul