Anti-submarine warfare is mission denial

Anti-submarine warfare succeeds when protected shipping completes its mission, regardless of how many submarines are sunk. Killing or suppressing the attacker can both protect a convoy if the submarine is denied a firing opportunity. The same logic applies to defensive economics: an interceptor should be compared with the damage it prevents rather than with the purchase price of the incoming weapon. A cheaper escort can therefore be worthwhile when it preserves a merchant ship without destroying the enemy boat.