World War I and II submarines were submersible torpedo boats rather than vessels equally at home below the surface. Aircraft could attack them with ordinary 100-pound bombs while they travelled or charged batteries above water. Submerged speeds around seven knots made the Strait of Gibraltar especially difficult because its current could erase much of a boat's westward progress and force it to surface. Italian designers studied that constraint and sought boats able to sustain somewhat higher submerged speeds. The German Type XXI pointed toward the later true-submarine form, while nuclear boats and modern diesel-electric attack submarines completed the transition in the late 1950s and 1960s.
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Gibraltar's current exposed the limits of early submarines
World War-era boats remained surface-dependent enough that aircraft and Gibraltar's current could exploit their weak submerged performance.