another thing about counting depth charges:
if you are being attacked by 3 or 4 destroyers it does you little good to count the charges, as you dont know specifically which ship is dropping them at the time.
what if a destroyer is sitting still in the water while you think only 3 are attacking.... you cound enough where you think they have run out and then this 4th guy you didnt know was there jumps into the fight.
another thing:
i believe i red somewhere that sub crews would cound depth charges so they could relate the story back in port as bragging rights "Holy cow man the tommies dropped 250 charges on us!"
sure - you could estimate when they would run out by counting them, but as jimbuna said, there was no exact way to tell when they would run out simply based on counting the charges. however you could estimate approximately when they would be getting low.
finally - i think it would be very foolish for an inbound convoy to spend all of their depth charges in one massive attack when they are still out to sea, i think that for the most part they would save a lot of them for later in case they needed them. use a sort of hit and run tactic to scare the u-boat off so to speak.
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