Various Western media have highlighted the new features of China’s latest non-nuclear submarine, the Type-039C of the Yuan class, the new vessel presents a characteristic angled sail. This new technology is believed to be part of the latest developments that the Chinese navy has designed to introduce new anti-sonar stealth. Other countries are also trying to apply this general technique, however, China is the first to put it into practice.
China is building submarines at an impressive rate and has the ability to build them at levels unmatched by any other country. The Yuan class, its latest non-nuclear attack type, is the largest class of AIP (air-independent power) submarines in the world. And now they have been seen with new and unusually shaped sails, hinting at the application of advanced technologies.
It is now almost certain that the distinctive sail shape of the latest Type-039C Yuan class submarines is to increase survivability. The angled sides are a stealthy defense that reduces the submarine’s “signal strength” against active enemy sonar.
This was already the main explanation, but evidence has come to light that reinforces this assessment. Chinese academics published an analysis in the Polish journal Archives of Acoustics. They measured the impact of the designs on sonar stealth. In their study, they used strikingly similar sail designs.
The Stealth Trend
The angular shape of the stealth is an emerging trend in submarine design. Similar principles will be found in the Swedish A-26 and the upcoming German submarine, the Type-212CD. The A-26’s focus is on sailing in the same way as the Chinese submarine.
The 212CD goes further but envelops the entire submarine in an angular outer hull. The additional outer hull will increase resistance, so the Germans are comfortable making concessions. Other submarines are expected to have a similar conception, but currently, the Chinese ship is the only one in the water.
The trend reflects the growing importance of active sonar in underwater warfare. It is much more desirable to detect an enemy passively, simply by listening, than to emit a sonar signal. Active sonar does this by bouncing sound off the target and measuring the bounces. This means that the target can hear you long before you are detected, usually at twice the distance.
During the Cold War, which is the era in which most popular knowledge about submarines is rooted, passive sonar was king. However, passive detection depends on the enemy submarine being noisy. As submarines become quieter, passive detection is becoming less useful.
So active sonar is expected to play an increasingly important role in underwater warfare, even between submarines. At the same time, advances in unmanned underwater vehicles (UUVs) offer navies a way to move active sonar emitters away from the main submarine and onto an expendable drone.
Tactics and technologies are still in development, but the direction is clear. So it’s no surprise that submarine designers are willing to sacrifice hydrodynamics to incorporate these new sail shapes.
It’s not an invisibility cloak
The size of the Chinese submarine’s stealth features suggests it is intended to work against medium-frequency sonar. It will be less effective against low-frequency sonars that have long wavelengths. But those only tell the enemy that there is something there, not what it is. They will also be less effective against short-wavelength sonars, such as those from torpedoes. But that’s where the submarine’s other stealthy feature comes into play, a rubberized anechoic liner.
So we can deduce that the new stealth configuration is mainly intended to complicate classification. Medium-frequency active sonar can be used to classify or identify the target. With the new configuration, the enemy will have difficulty determining the true nature of the submarine. This can cause delays or miscalculations that the submarine can use to its advantage.
The new conformation will only reduce the signal intensity by a few decibels. Still, combined with the sound-absorbing rubber coating covering the side of the ship, it may be enough to save the submarine.