History
Since the limitations of the propulsion systems of early military submarines forced them to operate most their time on the surface, their hull designs were a compromise with outer hulls allowing for good surfaced abilities, and a relatively streamlined superstructure. Because of the slow submerged speeds of those subs, usually well below 10 knots (19 km/h), the increased drag for underwater travel by the conventional ship type outer hull was considered acceptable. Only late in World War II, when technology enhancements allowed faster and longer submerged operations and increased surveillance by enemy aircraft forced submarines to spend most of their times below the surface, did hull designs become teardrop shaped again, to reduce drag and noise. On modern military submarines the outer hull (and sometimes also the propeller) is covered with a thick layer of special sound-absorbing rubber, or anechoic plating, to make the submarine more difficult to detect by active and passive SONAR.
Read more about this topic: Submarine Hull
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