The ships that durvived I'm sure were carrying some kind of cargo that was... favourable whilst in Convoy. What the ship's carried was for the sailors alot like how the special food rations were handed out to Canadian soliders in Normandy "What am I gonna get? Something that looks like congealed Beef Stew or is it gonna be Candied Peaches?"
Favourable cargoes were things like Lumber that actually increased the boyouncy of the ship so much that you couldn't really sink it.
A bad cargo was of course oil or gasolene but even worse was lead cause that would sink so fast that you'd barely be able to jump ship.
I'm gonna wager these "surviviors" were either carrying lumber or nothing.
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