In the scene where they decide to use the tanker "as a hat", they approach the tanker from astern and come in between the screws. While this is technically possible, it is physically impossible for a World War II vintage submarine on battery power to keep up to a supertanker, much less overtake one. A supertanker can easily maintain 10 knots, and a submerged diesel sub can't do more than 8 knots flat out.