The term light hull is used to describe the outer hull of a submarine

Submarine

A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

, which houses the pressure hull, providing hydrodynamically efficient shape, but not holding pressure difference. The term pressure hull is used to describe the inner hull of a submarine, which holds the difference between outside and inside pressure.
Modern submarines are usually cigar

Cigar

A cigar is a tightly-rolled bundle of dried and fermented tobacco that is ignited so that its smoke may be drawn into the mouth. Cigar tobacco is grown in significant quantities in Brazil, Cameroon, Cuba, the Dominican Republic, Honduras, Indonesia, Mexico, Nicaragua, Philippines, and the Eastern...

-shaped. This design, already visible on very early submarines is called a "teardrop hull

Teardrop hull

A teardrop hull is a submarine hull design which emphasizes hydrodynamic flow above all other factors. Benefits over previous types include increased underwater speed and a smaller acoustic signature, making detection by sonar more difficult...

", and was patterned after the bodies of whale

Whale

Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...

s.