<b>Propellers</b> The rotational energy generated by the propulsion engine is transmitted via the shaft to the ship's propeller, which converts this energy largely into propulsion. A ship's propeller today has between three and - with special cavitation and thus low-noise propellers - up to seven blades. The wing shape is aerodynamically designed. The propeller theory shows that a ship's propeller sucks in the water on the front and pushes it away on the rear. According to the laws of hydrodynamics (Bernoulli's equation), there is a negative pressure on the front and overpressure on the back. The negative pressure “sucks” the propeller and ship forward, the positive pressure pushes it in the same direction. The suction side does about 60%, the pressure side about 40% of the propulsion work. The thrust depends on the speed. The change in propeller speed by ships with direct propulsion is achieved with a change in engine speed. Switching from forward to astern can only be achieved by reversing the direction of rotation of the engine, provided the propeller shaft is flanged directly to the engine's crankshaft and no reversing gear is used. Because of the large masses that move in the engine, all of this can only be done with a time delay after exertion via the machine telegraph. This disadvantage can be avoided with the controllable pitch propeller. The propeller blades are mounted on the propeller hub in such a way that the pitch of the blades can be changed by servomotor; this changes the driving force. The engine and propeller always maintain the same optimum speed. When traveling astern, the wings are adjusted so that their pitch is reversed.