The Enemy Below. Should have stuck with the original ending where the Americans and Germans all end up in the sea together, brawling.
"Das Boot" explains the destroyer/U-boat tradeoff better:
- the U-boat can hide, but can't move fast underwater
- the destroyer can use ASDIC to find the U-boat but not while directly overhead or when depth charges are exploding, which gives the U-boat a small opportunity to change course
- the destroyer, for obvious reasons, can only depth-charge when steaming fast
- depth charges have to get very close to a U-boat to sink it, and the deeper uit goes the closer they have to be.
- the destroyer can ram the U-boat but not vice-versa.
Generally the destroyer has the upper hand as long as it doesn't lose the U-boat.