The crankshaft turns the main driveshaft running down the long spine of the motor. A small gearbox at the bottom of the driveshaft converts vertical spinning motion into horizontal spinning motion. The propeller powered by horizontally spinning gears powers the boat through the water. Boat and airplane propellors spin in water or air. ... On all but the smallest propellor planes, and on some boats , the propellors feather, that is, the pitch of the blades can be adjusted to be efficient at different speeds. This is very much like a transmission with multiple gears