I have a 15 hp OMC sail drive motor. When I'm under sail I can hear a whirring noise from the motor compartment. I am assuming that this is the prop turning. If the prop turns while under sail, does this cause any sort of drag on the boat? If so, should the prop stop turning if the motor is in gear? I leave it in gear, but I still hear the noise. Can you tell me what the problem is?