As mentioned, U-128 was a Type IX U-Boat, which was bigger than the more prevalent but smaller Type VII which had a displacement of 769 tons and a range of 8500 km. The workhorse of the U-Bootwaffe was the Type VIIC, 568 of which were built, the third ‘model’ so to speak of the Type VII. The U-Boat depicted in the book and movie, Das Boot, is a Type VIIC.
(To tell you the God’s honest truth, I have never actually been sure of what “displacement” means when giving the measurements of a ship. I think it means that the ship displaces that much water but if someone knows more about this please make a comment.)
What is unusual about the crew complement on this boat is the presence of three midshipmen. While one sees midshipmen deployed early in the war it is surprising to see three of them on board a combat patrol when U-Boat losses were running so high. While aboard the boat, a midshipmen, or Fähnrich zur See, would have been treated as an enlisted man and not an officer. However, I need to make clear that the next rank, Oberfähnrich zur See, was a full fledged officer and would have been treated as such. The equivalent rank in the US Navy would be “Ensign”.