There are a number of ways to calculate this torque but all theories need to be validated in the field. I have a patented design on a completely submerged water wheel and I have setup a torque testing platform. Sorry if this is a little late, I noticed that this was posted in 2010. If you're interested I send you pictures of the torque platform. It's very difficult to get flow rates of greater than 4 ft/s in local areas. Let me know if I can help. Thx Paul