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