Why the faceting? Sutton speculates that the fin shape is designed to reduce the submarine’s radar signature. Diesel-electric subs require air for their diesel engines to operate, and so they must remain surfaced or at snorkel depth to operate. Unlike nuclear submarines, which can cruise at depth for weeks at a time, non-nuclear subs often spend a great deal of time on or near the surface, only submerging once at their patrol area or when they expect enemy contact.
Many modern anti-submarine aircraft, such as the U.S. Navy’s P-8 Poseidon, utilize long-range radar to detect surfaced submarines or submarine snorkels or periscopes. An airplane with a long-range radar could detect a surfaced submarine from many miles away, before the sub spots it, and then close in for the hunt.
A stealthy sail, however, would allow the Type 39C/D to leave port and travel the hundreds of miles to its destination surfaced with less of a chance of being detected. This would save fuel and allow the submarine to patrol even farther. The sail is reminiscent of the A-26 design that’s currently under construction in Sweden, but Sutton says it’s too early to judge the new submarine a copy of the Swedish boat.