Today’s invention is a simple watermill, the blades on which behave like oars…(-or the arms of a swimmer doing the front crawl).
When in the water, the surface of each blade is at right angles to the direction of motion, providing maximal thrust. As the blade leaves the water, it is smoothly rotated by a cam so that the blade surface is aligned with the direction of motion. This streamlines the blade during its passage forward through the air and minimises the air resistance which would otherwise act as an unintended brake on the system.
This could be applied to a new generation of paddle steamers as well. In addition, now that marine vessels are being equipped with electrical power which allows drive units to be bolted on to the hull anywhere, such paddles could be attached entirely underwater…because the drag on the forward stroke could be minimised.