Re: dyno test
Your best bet is a fuel flow gauge. In combination with a GPS it's the best way determine your most efficient cruise. You'll find that drag has a MUCH greater effect on your fuel economy than the location of the max point in the torque curve. Since you're cruising at part throttle your engine is putting out no where near peak torque so it doesn't care what the peak torque could be. In the vast majority of cases you'll find that peak MPG is somewhere between 25-30 MPH, irregardless of the torque curve of the motor.
If you prop your boat so that the max AVAILABLE torque point (it's max available torque cause you're going to be cruising at part throttle, using nowhere near all the available torque) is your most effecient cruise, you're going to be propping your boat to move the power available curve way to the left. It would be the equivalent of locking your car in second gear where the max available torque point and max efficiency point happen to coincide. Although you would be cruising at the max available torque point and getting max efficiency in that gear, you will be giving away lots of potential MPG and speed.
For example, the torque curve on my boat is essentially flat with max torque between 2900 and 3600 RPM (3 ft-lb of torque difference across the spread). Best cruise is at 29 MPH at 2100 RPM, where max available torque is about 10% below peak. But since the engine is not being required to deliver all available torque (unless I hammer the throttle open because I want to accelerate as fast as I can) the location of the max available torque RPM is irrelevent.
Boattest.com has lots of tests with tables of speed and RPM vs MPG. They don't have the dyno charts, but you can see how consistently a few MPH makes a big difference in MPG.
For example, a 2010 Four Winns 21' bowrider with a 300 HP 5.7L MPI (the one in the GM power curves I referenced in my earlier post) gets max MPG at 2500 RPM. Max torque is about 3200. If you run it at max torque RPM you'll be giving away about 10% in fuel mileage. You'll see the same thing on carbureted engines.
