This is a complex problem. The shape, diameter, width and depth of penetration of the paddle wheel must be determined to obtain the top speed you want to reach. Then the torque and rpm of the wheel should be able to be determined. With the torque and rpm choosing the other components is simple.

I assume you need to abandon the current drive because of weeds wrapping up. This could still be a problem. Paddle wheels may not be efficient either. An horizontal screw on each side may provide self cleaning and better efficienty than the paddle wheel.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

I have built several paddle boats but they were much larger.
They were actually passenger cruise boats, one for a river, one for the ocean.  60ft, and 100 ft craft.

First of all, the paddles were about 90% efficient.
The hull speed was about 90% of the paddle speed up to the hull speed limit of the vessal.  

The fastest my boats went was about 7-8 MPH.

You still need some type of rudder, we had one in front and in back of the paddles.  The one in front was for backing up and manuvering in tight spots. With twin paddles you can turn on a dime, but need a rudder to track straight.

How fast would the 75 HP engine push the craft?   
Use GPS to track speed accurately.
This should give you a nominal hull speed with 75HP of propulsion.

That hull speed can be translated into a paddle speed based on the diameter of your paddles.

You will need about 25% more HP with a hydraulic system so I would plan for a 100HP engine,  preferably a diesel as it will be more fuel efficient, and safer.  But more money.
After you have the engine, you will need a pump mount and drive plate to mount pumps to.

I would use two pumps and two motors, then you can run each paddle seperatly.   Each motor would need to deliver about 35-40 HP and be reduced to the speed of the paddles with a planetary reduction.  You could use chains, but if this is a work vessal you probab;y want to do it right.

All hose must be USCG approved, And I would suggest a biodradable oil as a hydraulic fluid.  USCG rules require working pressures be below 3000 PSI, and no hose longer than 3ft.  Not sure if that would appply to your vessal but it might if it needs a USCG inspection.

Not sure how you would fine tune your paddle height, Suggest 100% of paddle in the water with the ability to fine tune the height with spacers to raise or lower them.

With pumps, motors, planetaries, coolers, filters, tanks, Etc,  You could easily have $25K in this plus the engine.

Good luck!
 

Tom Nelson
Application Engineer (CFPS)
CATCO Parts and Service
St Paul,MN