German U-Boat

The periscope is the only means for a submerged U-boat to peer out into the sky and scan the horizon for surface activity. The majority of boats were fitted with two periscopes – an attack periscope for spotting and targeting surface vessels, and an observation periscope for scanning the sky and taking navigational bearings. Before a U-boat breaks surface, the skipper would scan the horizon at periscope depth, searching for hostile units which could pose a threat. Only when its clear, the U-boat would surface, led by the watch officer, the crew would rush into positions on deck.

The periscope consisted of a long steel tube which extended out to about five meters from the housing. It had prisms and lenses at both ends, for which there was a switch that allowed the commander to toggle the magnification level – either 1x or 6x. Periscopes suffered from two main problems, the most important was vibration. When fully extended, the long unsupported tube created turbulence on a moving U-boat. At 6 knots, it caused excessive vibration which rendered it almost impossible to use. This was dampened by using an extension bracket to reduce the unsupported length and the pointed end was redesigned to minimize the forward hydrodynamic resistance. Nevertheless, vibrations still occurred, but to a much lesser degree. The other problem was fogging of the lenses. Since the damp atmosphere of the U-boat caused fogging, it was especially important that the tube was not only watertight, but was airtight as well. Any fracture on the airtight casing caused by a depth charge attack would result in fogging of the tubes.

The attack and observation periscope were somewhat different in that the latter had a bicycle like seat and paddles, which were raised and lowered together with the periscope. The observation periscope also had better night vision capabilities.