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. Another advantage was that the entire bow could be used to house a sonar for the submarine's own hunting of opponents; the forward hydroplanes are located aft of the sonar array, or on the conning tower/sail so as not to interfere with the sonar.
The design was initially introduced in 1944 on the German Delphin-class midget submarine, but is more often referred to as an "Albacore hull", because of its use in the more well-known USS Albacore. Designed to be used as a cheap, high-speed coastal submarine, the Delphin was a 2,5 ton vessel built to carry a single torpedo under its belly, and reached submerged speeds of seventeen knots during tests. Although development of the midget submarine was discontinued after one of the prototypes collided with a surface vessel on 18 Jan 1945, some form of the design has been present in the operational submarine fleets of almost every navy since that time.
There are several variations of the basic hull design, each developed for specific purposes. The Delphin itself relied on a, to today's observers, conventional set of control surfaces with a cross-type rudder and aft hydroplane setup, as well as two small forward hydroplanes mounted along the centerline, just forward of the one-person "cockpit". American designers moved the fore hydroplanes to the sail, where they could be tilted upward to aid in piercing thin ice in the Arctic Ocean; the British often located the foreplanes forward, at the level of the deck and enabled them to fold upward so as to not foul harbor structures; those of other countries often retract the foreplanes in ways conventional or novel. The hulls of Western submarines of this type end in a single propeller, so as to minimize drag; the Soviet equivalents often still had two propellers, to provide for a second motor either for greater power or safety. The Barbel hull had a long taper abaft the sail, again to minimize drag, but the British Upholder class had the propeller axis depressed, with the extreme aft of the hull having a shorter taper, so as to provide greater strength to the hull. The German Type VII submarine pictured on this page has the aft of her hull tapering abruptly for this purpose, though its propeller axis follows that of the rest of the hull. The Albacore studied several positions of the afterplanes. American designers settled on a modified version of the Delphin's cruciform arrangement (a Greek cross viewed from behind); they rejected the alternative of an x-arrangement for its complexity, but it was accepted and used by the Dutch, Swedish, Australian and German navies among others, for its ability to snuggle closer to a shallow seabed without striking the rudder on the sea floor. The Soviets often repeated a conventional arrangement, similar to that of the Type XXI U-boat.