Interwar submarines had to spend much of a patrol on the surface to travel quickly, charge batteries, and preserve their operational radius. Even a slow Fairey IIIF with light bombs could force a boat to dive and delay its movement without achieving a sinking. Those effects could leave the submarine unable to reach a convoy even if it survived. The operational measure of success was merchant ships delivered, not submarines counted, so a temporary mission kill could be as valuable as a sinking. A convoy-commodore comparison makes the priority concrete: saving fifty merchant ships without sinking a submarine is success, while losing the convoy in exchange for enemy boats is failure.
Notable Point · 10:33-15:04
Convoy survival is the measure of anti-submarine success
Aircraft could protect a convoy by forcing submarines underwater and out of position; sinking boats was useful, but delivering merchant ships was the operational objective.