In my experience if you hit the ship in the bow area then they tend to go down within 15-30 minutes. When doing this they tend to maintain a good speed right up until they are about to take the final plunge.
Nemo
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"I'm afraid there is no disguising the fact that King's obsession with the Pacific and the Battle of Washington cost us dear in the Battle of the Atlantic".
Sir John Slessor GCB, DSO, MC, DL
AOC-in-C Coastal Command RAF
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