6 Knots max with periscope raised:
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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.
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http://www.uboataces.com/periscopes.shtml
The same restriction was encountered with the snorkrl:
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Another problem with running the diesels submerged was in the engine noise. As diesel engines were much noisier than their electrical counterpart, this meant that the U-boat’s hydrophones could not be used. The U-boat still had to rely on the limited view provided by the periscope, but to add to the problem, the periscope could not be used at speeds of over six knots.
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http://www.uboataces.com/snorkel.shtml
Some more info but without any reference to speeds:
http://www.uboatarchive.net/BDUOrder2.htm