Page 301 - Practical Ship Design

Design of Lines 259

have been patent types of rudder on which this was done, but possibly because of
the extra complication in the construction few such rudders were built.
Some rudders on single-screw ships are supported on pintles from a closed arch
type of sternframe with in this case the sternframe acting as the nose of the rudder;
on other ships the sternframe is open aft with a heel extending to provide a bearing
at the bottom of the rudder. On ships with a closed arch sternframe the rudder area
can be somewhat less than should be the practice with an open arch sternframe as
the sternframe tends to act at least partially as part of the rudder.
For single-screw ships of above 100 m in length, a K, value (rudder area / L x T)
of 0.0125 for a streamlined aerofoil rudder in association with a streamlined closed
arch sternframe or of 0.0135 for a rudder without a sternframe providing a leading
edge, has given a satisfactory turning circle on a considerable number of ships.
The author has little data on ships below 100 m for which higher K, values are
normally used, with the value of K, increasing as the length decreases, probably
because the smaller the ships are the more confined the waters in which they are
required to manoeuvre. Barnaby gives figures for a range of vessel sizes in his
“Basic Naval Architecture”. Whilst his figures seem high for ships of 100 m,
possibly because his data may date back to single plate rudders, a brief summary of
his figures for smaller ships is given below in default of other information.

Length (m) K,
25 0.024
50 0.021
75 0.0 18
100 0.016
The most common requirement for performance appreciably better than that
obtainable from a conventional rudder applies when the ship has to manoeuvre at
slow speed and there are several ways of providing this capability, viz.: active
rudders of which there are several very effective types; a bow thruster; both bow
and stern thrusters.
Twin-screw ships used to be built with a single rudder on the centreline, but it
was found that the performance of a rudder operating clear of the propeller race
was often poor and virtually all twin-screw ships built today have twin rudders
positioned in the propeller wake. Because on twin-screw ships stern shaft with-
drawal is almost invariably “out” from the ship twin rudders must be sufficiently
displaced either inboard or preferably outboard, because this position gives a bigger
turning lever and there is less wake effect, of the lines of shafting to permit this.
Twin rudders are helpful in recovery from a broach as one of them should be
well immersed even if the other comes out of the water but the suggestion made in
the paragraph on broaching that the rudder should be as deep as conveniently
possible is still worth following particularly on shallow draft ships.