Are you asking what torque it will produce as a motor as implied in the question or are you wanting to know what torque is needed to turn it as an alternator?
If you want the required torque to turn it as an alternator you can work it out from the expected power out, rpm and efficiency.
Choose a reasonable power curve for your prop, assume the power into it will be about twice what you expect out and calculate torque from power and rotational speed, remembering to increase the prop input torque in relation to the speed increase. Cogging torque will be multiplied buy gear ratio so bear that in mind at starting.
Flux
