A submarine sail is a prominent dorsal appendage of a submarine's hull. Sails typically provide access into the submarines pressure hull (the elevated sail providing protection from downflooding, as surfaced submarines have limited freeboard). They also provide an elevated platform for surface observations or navigation, housing for sensor arrays, additional length for periscopes and snorkels, and/or increased interior space.
Modern sails tend to adopt a teardrop shape to reduce drag, with minimal cross-sections. A number of nuclear submarines have reinforced sails, suitable for breaking through sea ice. Increasingly, the sail is being used as a location for diving planes.
A conning tower is an elevated control station. Traditionally, a submarines conning position was placed in an elevated dorsal structure (a conning tower), and a fairwater referring to the surrounding fairing. On modern submarines, the conning position is in the control room. Sails particularly refer to this dorsal appendage without a conning position; however in modern parlance, all three terms tend to be used interchangeably.