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World War One

 

On the eve of World War I, all major navies had submarines in their fleets, but these submarines were relatively small, considered of questionable military value, and generally intended for coastal operations. The most important exception to the coastal operation concept was the German Deutschland-class merchant submarines, each 315 feet long with two large cargo holds. These submarines could carry 700 tons of cargo at a speed of 12-13 knots on the surface and 7 knots submerged. The Deutschland itself was fitted with torpedo tubes and deck guns to become U-155, and along with seven similar submarines, saw combat duty later in the war. By comparison, a "standard" World War I submarine was just over 200 feet long and displaced less than 1,000 tons at surface level.

 

Pre-war submarines were usually equipped with self-propelled torpedoes to attack enemy ships. During wartime, submarines were also equipped with deck guns. This allowed them to approach enemy merchant ships on the surface and signal them to stop for a search (an early warfare strategy), and then sink smaller or unarmed ships where the use of torpedoes was not justified. Most submarines built for war were equipped with one, and sometimes two, main guns of about 3-4 inches caliber. However, some later German submarines were equipped with 150-millimeter guns (including the military-configured Deutschland-class).