China's improved Yu-7 "shrouded propulsor" torpedo | Jane's

[Martian2]

A "shrouded propulsor" is a ducted propeller (aka "shrouded propeller configuration" or pump-jet). The advantage of a pump-jet is that it minimizes acoustic waves in all directions except directly rearward. The disadvantage is a higher acoustic signature directly behind the torpedo's or submarine's shrouded propulsor.

PLAN holds 'largest-ever' firepower display in SCS drills | IHS Jane's 360

"The new torpedo appears to be longer than the PLAN's standard Yu-7 lightweight torpedo, possibly with an extended homing and data processing section. Another notable difference is that the weapon seems to have a shrouded propulsor in place of the contra-rotating propellers on the Yu-7, which may significantly reduce the noise radiated by the torpedo, making detection more difficult."

[(See picture above) Yu-7 lightweight torpedo with a "shrouded propulsor" at the rear end of the torpedo.]
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Old technology. "The contra-rotating propeller section of a Yu-1" torpedo.
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New technology. Shrouded/ducted propeller.

 

[Martian2]

Torpedo counter-rotating propellers obey the "form follows function" design rule

To my knowledge, all modern propeller-driven torpedoes use a contra-rotating (or counter-rotating) propellers-design. Why is that?

When you look at a submarine, it only has one propeller.

A Chinese Song submarine with a 7-bladed asymmetric propeller to minimize cavitation.

Newton's Third Law of Motion states: "For every action, there is an equal and opposite reaction."

When a submarine's propeller turns in a clockwise direction, there is an equal force twisting the submarine in the counter-clockwise direction. However, the submarine is very large in comparison to the propeller. Thus, a submarine can compensate for the torque by changing the dive plane angle on one side of the sub.

To maintain a sleek fluid flow, most torpedoes don't have frontal dive plane control surfaces. Also, the propeller is large in comparison to the torpedo. Thus, the most efficient torpedo design to counter the torque is to use counter-rotating propellers.