"It was plain to see that due to the Allied tactics which drove the U-boats from the seas last summer, sinking 90 subs in 90 days, something new had to be added... the newer [German] subs have larger conning towers, painted white this time instead of black - packing at least two new guns, and shooting it out in the open instead of from ambush... There is evidence to support the belief that the U-boats now use a new type of torpedo for the first shot, one which follows a ship by some form of magnetic attraction and is detonated by the vibrations of the ship's propellers..."
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