Page 278 - Introduction to Naval Architecture
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MANOEUVRING                         263

          Solution
          Using the simple formula from above to calculate the rudder
          force and a factor of 1.3 to allow for the screw race:












          This can be equated to qf/r where r is the stock radius, q is the
          allowable stress, and/is the second moment of area about a polar
                         4
          axis equal to Jtr /^. Hence








          In practice it would be necessary to take into account the shear
          force and bending moment on the stock in checking that the
          strength was adequate. The bending moment and shear forces will
          depend upon the way the rudder is supported. If astern speeds are
          high enough the greatest torque can arise then as the rudder is
          less well balanced for movements astern.



        Rudder types

        The rudder is the most common form of manoeuvring device fitted in
        ships. Its action in causing the ship to turn has already been discussed.
        In this section it is proposed to review briefly some of the more
        common types.


        Conventional rudders
        These have a streamlined section to give a good lift to drag ratio and
        are of double-plate construction. They can be categorized according to
        the degree of balance. That is how close the centre of pressure is to the
        rudder axis. A balanced rudder will require less torque to turn it. They
        are termed balanced, semi-balanced or unbalanced. The other method of
        categorization is the arrangement for suspending the rudder from the
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