This help note is from the Model Airplane News magazine and it may be of some good use. To reduce the current draw and increase a 540 or 550 motor's rpm, closely wind the motor with a layer of 0.025 inch galvanized wire. Place the winding over the field magnets and solder both of the wire's ends to the motor.
Once done, coat the wire with epoxy or like substance. This idea is from Paolo Moro and Belo Horizonte, Brazil. This magazine at times has good articles which can apply to model boating. Check it out, money well spent.