Design for "Tauchschiff Projekt 50", 1918
This plan is quite rare.. perhaps the only one depicting this monstrous U-boat that was never actually built. The Projekt 50 design was for a massive U-Kreuzer sporting four 15cm guns, armor, a fixed smokestack to exhaust steam from a powerful complement of boilers. Equally unusual was her awesome torpedo armament -- 10 torpedo tubes in an era when 6 was considered a lot to put on a submarine.
I find use of the word Tauchschiff ("Dive ship") and interesting upscaling from Unterseeboot ("Undersea boat")... betraying a sense that this design was breaking beyond a diminuative size.
If there is one feature that is truly surprising in this design concept, it is the pair of swivel-mounted torpedo tubes near the bow which appear capable of mating to either of 2 pairs of diagonal hatches on either side in order to discharge their torpedoes.
Michael Meyer (mikemike through cityweb.de) writes to add:
I find the plan you have of the Projekt 50 steam sub intriguing. I had read about the type in Eberhard Rössler's "History of German Submarine construction" and re-read the pertinent passage. One of the type was actually ordered in 1918 from Kiel Navy Yard. It would have been named "UD 1". It was never actually laid down, although building preparations had begun. All existing parts and most of the documentation were destroyed after the German capitulation. Now Rössler writes that the class was originally designed for three 150 mm guns and two 88 mm AA guns, but that the U-Boat Inspectorate (the type command for submarines, the original owners of your plan) demanded four 150 mm guns. According to your plan it seems that their demand was met. Rössler also credits the type with four bow and two stern tubes, while sideways-firing tubes are mentioned nowhere. Now your plan shows two forward-firing tubes with two torpedo storage tubes above, so maybe someone misread a plan. The whole design has its bizarre aspects, especially the torpedo installation - the stern tubes would probably have been quite awkward to reload - and the forward broadside installation reminds me of a tale I've read about a Gatling-type device intended for broadside installation on battleships which was hideously large and heavy; apparently the Torpedo Inspectorate's designs tended to go over the top in later years.