Lürssen Effect

The Lurssen effect, used in the design of high-speed boats, is a reduction in wave-making resistance provided by two small rudders mounted on each side of the main rudder and turned outboard. These rudders force the water under the hull outward, lifting the stern, thus reducing drag, and lowering the wake height, which “requires less energy, allowing the vessel to go faster.” The effect was discovered by the German shipbuilding company Lürssen Werft based in Bremen-Vegesack. The Lürssen effect is best remembered for its use during the Second World War in the various classes German "Schnellboot," or fast torpedo attack boats.

Development of a more efficient hull shape and design by the Lürssen shipyard which made possible high speeds exceeding 30 kn resulted in the motor torpedo boat S-1 of 1929. It formed the basis of the E-boat arm of the German Navy in World War II. Even this boat was already able to attain speeds beyond the traditional hull speed barrier due to an advantageous water flow as well as a flattening of the stern wave, achieved by a ram wedge in front of the rear break-away edge.

During test runs with S-2, the first boat from series production, it was then accidentally discovered that the boat stopped responding to the rudder at high speed and full rudder positions but started to display the effects described above. When investigating what caused this behaviour, the basic principles of the Lürssen effect were discovered. Starting with S-2, the following boats were then equipped with the two small rudders on either side of the main rudder.

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Wikipedia