There's also a circular-slide-rule used in aviation navigation, commonly called a "whizz" or "prayer" wheel, which is a very powerful tool for speed/time/distance/drift and vector calculations (and a great deal more specific to flying). When I was instructing students on air-navigation I'd start by setting them a problem, and they'd compute it with electronic calculators, and I with the "prayer-wheel" - and they never ever arrived at the solution before I did. Having proved the use of the (batteryless) wheel, I'd then teach them how to use it. Sextant use is no longer taught in aviation, but a form of sextant was used in astro-navigation (position derived from stars) until the 1950's when the ubiquity of radio-aids made astro-navigation redundant.
The accurate use of sextants would be extremely difficult on a U-boat due to the pitching and rolling of the boat, so I would expect it was a method of last-resort for fixing position. DFing known LW radio transmitters would be a much easier method, with sextant use only being of use in calm seas.
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