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Most modern military submarines have a hull form that at least approximates an axisymmetric body of revolution. Most of these have four control
surfaces at the stern for steering the vessel, that is, for making it turn left or right--the rudder--or rise or dive--diving plane--or a combination
of both. In turn, in most modern submarines these control surfaces are in cruciform. That is, the rise-dive surfaces are generally in the same plane
as the horizontal plane through the centerline of the vessel, and the turning surfaces are in the same plane as the vertical plane through the
centerline. Thus, the control surfaces are generally in the form of a Greek cross.
In most cases the two rudder planes are yoked together, and the two diving planes are yoked together. Because of this yoking, each pair of control
surfaces is operated by a single actuating rod. Thus, one rod turns the ship, and the other rod causes the ship to rise or dive.
It is known that arranging the control surfaces or planes of a submarine in an X configuration has certain advantages. In this form, the control
surfaces are in the form of an X. Unlike cruciform designs, X-stern designs utilize all four planes as part of any maneuver. Therefore, an X-stern
design enjoys more maneuvering force per unit of control surface area than cruciform designs. X-stern ships can be designed with smaller control
surfaces while maintaining maneuvering envelopes comparable to cruciform ships with larger control surfaces. Smaller control surfaces obviously have
less drag, but may also be quieter--a very important factor today for a submarine.
The submarine USS ALBACORE had an X-stern configuration where the opposite control surfaces were yoked together. Australian submarines of the recent
COLLINS class have X-stern configurations, but the control surfaces are not yoked together and each of the four surfaces has its own actuator. These
are two examples of the current known methods of actuating X-sterns. In both cases, the control system for the operating rods is more complicated than
that aboard a cruciform ship. In a cruciform ship, if the helmsman wants to turn the ship, the control system commands the rudder operating rod to
extend or retract. If a change in depth is required, the control system commands the diving operating rod to extend or retract. In both X-stern
designs, the control system commands every operating rod to move in one direction or the other, for any maneuver. Controlling these coordinated
operating rod movements is a complex task that can be accomplished with a computer. However, manual coordination of the operating rods, in the event
of a computer casualty, is difficult.
the proven X-rudder configuration was chosen for a high manoeuvrability. The rudders are laid out in a way that they press a preswirl on the propeller
inflow which homogenizes the wake flow field and increases the propeller efficiency on the one hand and reduces the noise signature on the other hand.
The propelling power is provided by a Skewback propeller whose extremely low rotation speed prevents cavity and which generates thrust silently even
at high speeds.