Why merely forcing a U-boat to dive mattered

Interwar and early wartime submarines spent much of their passage on the surface to gain speed and recharge batteries. An aircraft did not need to sink one outright: detection, attack, or even the threat of attack could force it down, mark its position, and let ASDIC-equipped escorts close. Once submerged, the boat became slow enough for current and escort geometry to work against it. Even superficial damage mattered because a submarine unable to dive faced a dangerous surface journey home.