The rudder is part of the manoeu-vring equipment serving to support course keeping and course changing abilities of the ship. Rudders are usually placed behind the propeller. They increase the propulsive efficiency by using part of the rotational energy contained in the propeller slip stream. Rudder forces and moments, being functions of the effective angle of attack, characterise rudder performance. They are required to determine the rudder gear size, to evaluate the manoeuvrability of ships, and to design the structure of the rudder. Empirical formulas and potential theory are used in practice to determine rudder loads. There are numerous references listing experimental and theoretical compilations of forees acting on rudders in open-water conditions. Whicker and Fehlner (1958) investigated trapezoidal rudders with NACA profiles and various aspect ratios in wind tunnel tests. Thieme (1962) investigated rectangular model rudders with NACA and IFS profiles in wind tunnel model tests. Abott and von Doenhoff (1959) compiled force coefficients for NACA profiles, again based on wind tunnel tests. In reality, the rudder operates in the propeller slipstream, thereby considerably changing the flow past and the forces on the rudder. Baumgarten (1986) experimentally investigated the effect of propeller loading on rudder lift forces for a fully balanced rudder. Kracht (1990) performed experiments on a ship model with appendages in a cavitation tunnel.
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