I help people with design of boats and drive systems using mostly commercially available gear.  This is an example of a pedal drive installation on a two man kayak using a curved shaft with my folding prop:

https://www.dropbox.com/s/au59nghvaorhs0i/pedalyak1.wmv?dl=0

The builder did a 10 minute shakedown run after completing the drive then spent a week off the Baja coast covering 179 miles on water.  The drive system operated flawlessly.  This particular drive has the largest lateral offset of any of the drives I have designed.  It did not offer any particular difficulty maintaining course.  The offset increased overall drag by about 1%.  On my narrow hulls, drag increase due to shaft offset it is less than 0.5%.  

This is a photo of the drive mast in the pedalyak:

https://www.dropbox.com/s/ulbg0xozudur7vo/Pedalyak_Drive.jpg?dl=0

I supplied the rotating parts in the drive mast but only because the builder did not have access to machining tools.  The gearbox is a Mitrpak R-102 enclosed on the side of the gunwale.  All recent drives have used standard T10 industrial toothed belts.  These are more efficient and more durable than chains mounted on a bicycle chainring.

If you have not seen our prototype drive mast and gearbox this video shows the rotating parts in operation on one of my boats:

https://www.youtube.com/watch?v=55gVsKW_Zv4

The sound is amplified through the camera mount.  The popping sound are waves hitting the bottom of the stabiliser.  The gearbox is essentially filled with gears.  This box is 40% lighter than the Mitrpak R-102 box and has torque rating 2 to 3 times higher.

The drive systems we are aiming to build commercially are lighter with higher power rating than what can be built using commercial parts but it is not yet on the market.  For most purposes industrial parts work well.  The boat pictured here has two pedal drives I fabricated from standard industrial parts:

https://www.dropbox.com/s/wk2geprcl0stnld/Special_K.jpg?dl=0

 https://www.dropbox.com/s/d211o44i8h7hxam/Special-K_Front.jpg?dl=0

I have made a number of similar drives and a few others like the pedalyak have put their own version together. I provide drilling templates to make these and can supply any parts that you cannot make or buy locally.

I supply the folding props for AUD100 plus mail:

http://www.rickwill.bigpondhosting.com/folding_prop.png

If you are not operating in areas with weed then there are a few low cost model aircraft props that work OK - typically cost less than USD5.  Their efficiency will depend on how well they are matched to your application - I can help with that.  My props are aimed at easily driven hulls up to about 400kg displacement.  Heavier than that then there is a good chance you could find a better model aircraft prop or even a marine prop.

There is considerable detail in getting a curved shaft to give long life.  There is a pedal boat friendly supplier in Texas, Gary Jones, who supplies high strength stainless shafts to North American users.  I have a few of those shafts in Australia and there is a known supplier in Europe.  Gary supplies 8ft long shafts.  The shafts are a good price - currently USD40 I think but transport can be significant due to length.  

I have a complete parts list for a typical drive system with cost estimates of all the components including what I can supply.  If you want to get that detail then email direct:

 rickwill@bigpond.net.au

It is not difficult to make a pedal drive.  My aim is to achieve reliability and durability so I am prescriptive on some of the detail particularly the geometry and mounting of the curved shaft.  Some people have picked up good second hand right angle gearboxes on Ebay.  Providing they are rated suitably they work fine.  Bike chains are OK but bikes constrain their design.  The boat crank mast with central belt drive has the advantage of having the belt pulleys both supported between two bearings so overcomes the cantilevered constraint of a chainring overhanging the crankshaft.  Shaft flexing with a chainring on overhanging shaft increases the drag on the chain.  Shaft flexing with the centre belt does not change alignment.

Rick