A cruiser submarine could not choose to fight only the weakest corvette in a screen. Convoys might also include fleet destroyers, Town- or Crown Colony-class cruisers, and occasionally a battleship close enough to intervene. Those ships offered more stable platforms, many more barrels, and rates of fire that erased the submarine's nominal caliber advantage. An eight-inch gun was excessive for an unarmored merchant ship but still inadequate for a reliable fight with the heavier escorts it might provoke. If a submarine needs a six-inch or eight-inch gun to defeat a target, it probably should not be fighting that target on the surface.
Notable Point · 1:43:27-1:47:25
A convoy screen offered the wrong targets for big-gun submarines
The discussion widens from one escort to the destroyers, cruisers, and occasional capital ships that could protect a convoy.