A teardrop hull is a submarine hull design which emphasizes submerged performance over surfaced performance. It was somewhat commonly used in the early stages of submarine development, but was gradually abandoned in the early 20th century in favour of designs optimized for high performance on the surface as a result of changes in operational doctrine. Although naval doctrine changed, design practices remained until the later parts of World War II when the German Kriegsmarine suffered ever-growing losses of submarines in the Battle of the Atlantic.
This replica of the Ictíneo II of the mid-to-late 1860s may represent one of the earliest attempts at a hull shape optimized for underwater travel.
Less than 10 years after HMS Holland 1 entered service, the British B-class submarine shows the transition in hull form from submarine to diving surface vessel. The bow is slightly raised, as is the stern above the two propellers.
Unrelated to the later German Elektroboot program, the Japanese Submarine no. 71 was launched in 1937 and achieved just over 21 kn (39 km/h) submerged, a feat only exceeded by japan's smaller Ko-hyoteki midget submarines at the time, until the German V-80, powered by hydrogen peroxide, achieved 28 kn (52 km/h) during testing 1940-1941.
Universally optimized for surface cruising, most submarines used in the second world war featured high, somewhat flared bows, as well as propeller and rudder arrangements similar to that of a contemporary cruiser, albeit with an extra set of rudders for depth and pitch control. Here a US Navy Balao-class Fleet Submarine from the early 1940s converted to a museum.
A submarine hull has two major components, the light hull and the pressure hull. The light hull of a submarine is the outer non-watertight hull which provides a hydrodynamically efficient shape. The pressure hull is the inner hull of a submarine that maintains structural integrity with the difference between outside and inside pressure at depth.
U-995, a U-Boat of WWII, showing the typical combination of ship-like non-watertight outer hull with bulky strong hull below
Type XXI U-Boat, late WWII, with pressure hull almost fully enclosed inside the light hull