AFAIK, at least in WWII and for the German submarines of the VII class, the normal mode of operation was surfaced. The submarines would only submerge (go on a "

AFAIK, at least in WWII and for the German submarines of the VII class, the normal mode of operation was surfaced. The submarines would only submerge (go on a "Tauchfahrt") if it was absolutely necessary. Don't forget that these boats still operated with Diesel motors or on batteries when submerged. It was later possible to use the Diesel motors submerged with a snorkel [0], but this would of course only work very close to the surface. [0] https://en.wikipedia.org/wiki/Submarine_snorkel https://en.wikipedia.org/wiki/Submarine_snorkel