First World War submarines depended heavily on surface running for speed, cruising, and battery charging. They could remain submerged only for limited periods and moved under greater constraints below the surface. An aircraft that forced a U-boat to dive could therefore disrupt its patrol without sinking it. Bombs offered a direct attack, while repeated aerial patrols denied submarines easy use of the surface.
Chapter · 7:12-8:49
How aircraft constrained First World War submarines
Aircraft could reduce a submarine's effectiveness simply by denying it time on the surface to charge batteries and cruise.