What thrust does your motor and prop combination produce?
I cannot measure the thrust, but I estimate it at 250 Lbs. This is based on calculations using formulas from the Propeller Handbook, by Dave Gerr.
Do you have any idea if this a meaniful way to compare your motor to some of the competition such as the Minn Kota motors?
Static thrust only has meaning if you are pulling against an immovable object. It is like the drawbar pull of a tractor--In low gear you have a lot of pull, but no speed. The typical trolling motor is very limited in power so a large propeller with low pitch is used so that a heavy boat can be moved, but only low speeds can be achieved.
The only true measurement of the power of a motor is the horsepower it produces. The easiest way to compare motors is to compare the electrical power in watts that each uses. Since the electric motors used on trolling motors are of approximately the same efficiency as the ones used on Reservoir Runners, the horsepower will vary in the same ratio as the electrical power.
Reservoir Runners use about 5500 watts at full power (while the boat is moving at full speed), to achieve 6 horsepower. Multiply the battery voltage by the amp draw at full speed to get the watts of the motor you are comparing. Allowing for the efficiency of about 80% for the electric motors on trolling motors, you can figure on about 940 watts per horsepower. A 12 volt trolling motor that draws 35 amps uses 420 watts of power and will develop about 0.45 horsepower. The maximum current figures that trolling motor manufacturers publish are for static thrust conditions and are less when the boat is moving. A true comparison would require measuring the amp draw with an ammeter while in operation.
Will your Kort thrust nozzle bias the numbers one way or the other?
The Kort nozzle increases the efficiency of the propeller by about 50% over an open propeller of the same size, so that a motor with an open propeller must develop about 50% more horsepower for the same thrust. This uses 50% more power from the batteries.
You mention that the Kort nozzle is about 50% more efficent. Does this change as speed increases?
In the range of 0 to 10 knots it is fairly constant, then decreases.
At what speed will the drag from the nozzle outweight the efficiency gain?
Generally, a Kort Nozzle is a big advantage at speeds of up to 12 to 14 knots. This varies somewhat with the design of the nozzle. The nozzle design I use is a low drag design that enhances operation at up to about 18 knots.
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