Fun project - need prop help

Our robotics team is having a carboard boat regatta. Our subteam is building a 40" wide, 84" long pontoon boat. The pontoons are square, 12"x12"x84" with points on the end, center of boat is 18"x18"x84". We anticipate the boat will displace 8". Two people will use kayak paddles, but we want to add an additional engineering element which will be a hand cranked prop. The third occupant will sit in the center and hand crank a propeller that will be mounted towards the rear of the boat between the two pontoons. Shaft will be a converted old weed wacker, crank will be made out of 1/2" and 3/4" pvc.

We have available:

1) 11.13" dia. X 11" pitch 4 blade prop from a 40 hp merc
2) a small prop from an old electric trolling motor
3) 11x7 APC gas airplance prop

Does anyone have any thoughts on what the best option is?

 

You will need to get a much higher RPM than you can achieve by hand. I suspect that you can probably crank 60-80 RPM, but you need to turn the prop at 300 or more to make it work with a small prop. So you need a reverse reduction gear or it probably is not worth the effort (probably not worth it anyway)