[Puster Bill]

From what I understand, they generally spent about 20 hours out of a day on batteries running at 2 to 3 knots, and about 4 hours of snorting at a slightly faster speed, 5 or 6 knots. Above that speed, vibration made using the periscopes impossible, and there was the chance of damaging the schnorkel itself.

You want to minimize the amount of time you have the schnorkel up because it can be spotted by radar (and visually, during the day).

The North Sea is a cast iron female dog late war. I've had many a patrol there ended suddenly by either a surprise attack while I was schnorkeling (especially by planes), or by running into one of the 1,348,234,023.5 mines planted there. The water is shallow, which means if you get detected by a destroyer or frigate you are pretty likely to get pounded into the sea floor. Your only hope is to pop them with a Falke or Zaukonig, or to try and evade. Either way, make sure your will is up to date and that your life insurance is all paid up.