Why did World War I submarines carry deck guns as well as torpedoes?

During World War I, submarines were equipped with deck guns to provide a versatile and cost-effective method for engaging enemy vessels. These guns allowed submarines to surface and signal merchant ships to stop for inspection, which was a common policy during the early stages of the war.1

Furthermore, deck guns provided a practical alternative to torpedoes when dealing with smaller or unarmed merchant ships. Because torpedoes were limited in supply and expensive to produce, using a deck gun allowed the submarine to sink these targets without wasting a valuable torpedo. Most submarines built during the war carried one or two guns, typically of three- or four-inch caliber, though some later German models were fitted with larger 150-millimetre guns.1