In the UK the fin is a structure above the main hull of a
submarine that serves
mainly as a streamlined fairing around the masts and antennas. On top of the fin
is the
bridge which is only used when the submarine is surfaced. If the submarine
has a
conning tower, the structure is known as a
fairwater. In the US the
fin is known as the
sail.
See
sail for an ASCII diagram.
The terms
conning tower,
fairwater, and
sail are often (incorrectly) used
interchangeably.