rudder, a logical development of the older
steering oar which the rudder began to replace in the West early in the 13th century, but see also
whipstaff.
The earliest known representation of a
sternpost rudder in Europe appears on a font in a Belgian church (Zedelghem), dated around 1180, and, though this is disputed by some, another appears in an English church (Winchester). It is also represented on the seal of Ipswich, which is dated at about 1200, and some of the seals of the
Hanseatic League ports of this period show local
cogs with them.
In China the rudder was a much earlier invention. A model ship in pottery, with a rudder in place, was found in a tomb dating from the Later Han period (1st century). However, the rudder never totally replaced the steering oar as it did in the West and often coexisted with it. Oriental rudders were hung much deeper in the water than in the West and, as
junks lacked keels, the rudder helped, in conjunction with
centreboards or
leeboards, to prevent too much
leeward movement. Unlike in the West, therefore, rudders were hung so that they could be retracted. They were also often bored with holes as this decreased the force of the water on the rudder—easing the task of the helmsman—without affecting its efficiency. As with the
magnetic compass, it is thought that the rudder was transmitted from the Orient to the West.
Rudders in
dinghies and many other small boats are usually hung from the sternpost or
transom by means of
pintles, which engage in
gudgeons and allow lateral movement from side to side as required. In many
yachts they are hung on a rudder stock which is led through the
counter, movement being imparted to the rudder by either a steering wheel or a
tiller. Larger ships also have their rudders hung on a rudder stock, the rudder being turned by the
steering gear attached to it. In order to reduce the steering gear force required to turn the rudder, the rudder on larger ships is balanced or semi-balanced. A fully balanced rudder has the same surface area forward of the rudder stock as
aft, so the force of the water on the rudder, which tends to turn it in a clockwise direction, is balanced by the force of the water tending to turn it in a counter-clockwise direction. For technical reasons of design, most modern rudders are of the semi-balanced type. This means that a higher steering gear force is needed compared with a fully balanced rudder but that force is considerably less than that used to turn an unbalanced rudder.
Rudders on modern ships are of an aerofoil section in order to allow for smooth passage of the water and they are hollow in order to provide for a buoyancy effect. That buoyancy reduces the weight that the rudder carrier bearing, which supports the rudder, has to take. A carrier bearing is necessary, otherwise the weight of the rudder would act upon the steering gear.
Podded propulsors and ducted water jet systems do not have conventional rudders as the direction of the ship is controlled by the steering of the pod or water jet outlet. For a description of these see
jet boats; propeller: modern propeller systems;
tugs.
