Sail.

This is coolbert:

"The fairwater sail or bridge fin is a totally undesirable appendage when viewed from any hydrodynamic or hydrostatic aspect or, indeed, from any other aspect including stealth." - - Javier Silvano Arzola.

Sailless submarine?

From the Internet web site of H I Sutton and thanks to same.

"Chinese Navy’s New Mystery Submarine"

"China continues to surprise the defense world. It is probably the only country on earth who can build a full-size submarine without any details leaking out."

"This submarine is a unique design which has no sail to speak of. All other submarines have this fin-like structure rising up from the middle of the deck where the periscope goes. Until now it has been a defining characteristic of a submarine: this boat is flatter and has a tiny bump where the sail should go."

No sail but a slight bump. Image thanks to H. I. Sutton.

Also from the Internet web site of H. I. Sutton see an entire article as devoted to the sailless concept. As if the idea has never been proposed before. Various nations including the Soviet Union, USA and France have see an obvious advantage to the sailless submarine but never became an accomplished fact.

coolbert.