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For propulsion, Fulton proposed a screw as Bushnell had already done, a principle that was not to be adopted in general practice until nearly half a century later in spite of its many and great advantages over side wheels. The screw was placed at the stern and directly ahead of the rudder and was operated by a hand crank and gearing turning a shaft passing through a stuffing box. The crank was to be turned by man power only. Plunging was to be secured by pumping water into the keel, while submersion at a given depth, provided the boat was in motion, was to be attempted by means of two inclined planes attached to the sides of the steering rudder. The angle of these planes could be altered from within, thus giving an upward or downward direction to the boat. Motion on the surface he thought to obtain by a fan-shaped sail which, with the supporting mast, could be folded down to the deck and then, preparatory to submersion, covered with envelopes like the wings of a fly. Fulton estimated that he could work the boat with a crew of three men.