[MarkShot]

I was reading some German notes (formerly classified) on sub ops. It noted interestingly that even in heavy seas where the sub would be difficult to spot, the sub could always get picked up by a swell such that briefly the sub would be easy to spot as it would cease to be hull down.

Also, interesting that the Germans considered a raised scope while on the surface a servere liability as it would make the U-boat easier to spot. Whereas the Americans often patroled with the "high periscope" while surfaced to extend the field of vision. Perhaps this was because the lack of periscope sheers on the U-boat would induce too much vibration into the raised periscope.