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What are the advantages of using X-form rudders in comparison to cross-form rudders?

Why do some submarines, torpedos and underwater autonomous vehicles use X-stern control?

X-stern:

X-stern picture

Cross-form:

Cross-form

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×-form surfaces result in a shallower draft. ×-form surfaces are more convenient for landing on the floor. With +-form the bottom fin which stick down below the bottom of the vehicle.

From what I've noticed, the littoral submarines have ×-form surfaces, because they are intended for shallow waters.

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People, people people (by which I mean the unreferenced other answers at this time) ! look it up! Here is info from a Chinese research paper:

. Compared with the cross rudder, X-form rudder has the following advantages: Firstly, X-form rudder has the higher rudder efficiency and its manipulate surface area is smaller about 10% than cross rudder, so that submarine could avoid the collision problem when it is landing. Secondly, X-form rudder reduces the serious consequences by rudder-blocked and improves the security and the underwater dynamic unsinkability. Besides, X-form rudder reduces the heeling moment, weight and stern heaviness when cross-rudder submarine turning, and having an advantage 13 2017 3rd International Conference on Computational Systems and Communications (ICCSC 2017) Published by CSP © 2017 the Authors to sail steadily when it is attacked. Finally, X-form rudder reduces interaction between propeller and rudder, and reduces the noise[4].

And a page I found quotes the following from a Google Book reference,

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.

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Effectively none, the result is you can control up, down, left, right but with the x-form a combination of fins is necessary while with cross (second image) only a pair of fins is needed.

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    $\begingroup$ In terms of control of the submarine I agree with you, but I'm wondering about the flow of fluid over the different types of rudders & whether the X type might have a more complicated flow which might affect the amount of noise &/or the frequency of noise produced by the rear end of the submarine. $\endgroup$ – Fred Mar 25 '18 at 11:34
  • $\begingroup$ Then you need to update your original question with that information as people can’t guess what you want to ask... $\endgroup$ – Solar Mike Mar 25 '18 at 11:53
  • $\begingroup$ First thought is the prop probably creates so much noise cf the fins... $\endgroup$ – Solar Mike Mar 25 '18 at 11:53
  • $\begingroup$ X fits in the box better. upload.wikimedia.org/wikipedia/commons/thumb/f/f8/… $\endgroup$ – Phil Sweet Mar 25 '18 at 21:44
  • $\begingroup$ I disagree with your claim about "one pair." A single pair out of X-form is the same as a single pair of Cross-form, with the entire body rotated by $\frac{\pi}{4}$ . $\endgroup$ – Carl Witthoft Mar 26 '18 at 17:53

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