I use the schnorkel as much as possible, going at pd in calm seas. But any rougher sea then that will have your engine crew running back and forth between the engine rooms as the diesels cuts out when the schnorkel "drowns". And going at pd in rough seas will expose the conning tower for enemy radar.
I read somewhere about U-boats going for many days submerged, I probably remember wrong but up to 50 days, getting all sorts off problems with the systems onboard due to the extreme humidity, not to mention how the crew felt.
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