Agreed!!! The sextant and charts were the primary means of navigation. One German sextant company even developed an "averaging" sextant not unlike 40's and 50's era airplane sextants because the amount of time a Navigator would have topside would be limited especially in later stages of the war. The easiest time to use a sextant is around sunset and sunrise...it is at this time you might have a choice of celestial "targets". A lot of Navigators take these sights and then possible the so called Noon sight to check their work. Depending on how accurate your "dead reckoning" courses were, these sightings could be pretty time dependent. By the way, shooting star sights is pretty tough even without a war going on around you. The noon sight can/could take a half hour. Near the end of the war, they grabbed any sighting they could. Another thing to think about is when the snorkel was introduced, the theory was to stay submerged as long as possible...making any kind of celestial sextant sighting impossible. I think you would have to greatly admire the work these gentlemen did on board.