Lürssen effect

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The Lürssen effect describes the use of a hydrodynamic peculiarity in boat designs by the German shipyard Lürssen from the 1930s onwards, through which the sailing characteristics were improved. The facilities for this were installed on almost all German speedboats during World War II.

functionality

Boats that exploited this effect were driven by three propellers and were equipped with three rudder blades in the flow of the propellers, with a wedge at the stern deflecting the water flow downwards. The actual rudder was in the middle, the two smaller rudders were inclined outwards at a slight angle to the vertical in the stream of the side propellers.

From around 25  knots (kn), the two rudders were turned outwards by around 30 ° using a handwheel and special tiller handle. This caused a stall behind the rudders and an air-filled space formed in the stern shaft. This changed the flow structure of the stern shaft and on the propellers. After the onset of the Lürssen effect, the angle of attack and thus the flow resistance of the flow rudder could be reduced to around 17-22 °. Due to the asymmetrical flow generated by the three propellers, two of which turned in the same direction, the starboard angle of attack had to be set larger. The effect collapsed again at speeds below 20 knots.

The effect occurred suddenly and manifested itself noticeably in three main effects:

  1. Raising the stern by up to 75 cm, which improves sea behavior and maneuverability due to the more horizontal position of the hull in the water.
  2. Speed ​​increase of up to more than 2 kn without the use of additional machinery, on the one hand because the efficiency of the propellers increased due to the changed flow and on the other hand because the more horizontal position of the hull reduced the flow resistance.
  3. Flattening of the stern wave, which only threw up about 27 m behind the stern of the boats, whereby the maximum achievable speed was increased, because the distance between the bow and stern wave (usually corresponding to the hull length) limits the maximum speed (see hull speed ).

development

From the development of a cheaper hull shape and construction by the Lürssen shipyard, which enabled high speeds of over 30 knots, the S 1 speed boat emerged in 1929 , the basis for the speed boat weapon of the Navy in World War II . Even this boat was able to achieve speeds beyond the classic limit for the hull length due to the favorable flow course and a flattening of the stern wave with the help of a storage wedge in front of the spoiler edge at the stern.

During test drives with the first series-production boat S 2 , it was discovered by chance that the boat no longer reacted to the steering angle at high speed and hard rudder angle, but showed the effects described. In the search for the causes, the foundations of the Lürssen effect were found. The boats from S 2 were then equipped with the two small oars next to the actual oar.

literature