No photo or drawing,sorry, but in Eberhard Rössler's "Geschichte des deutschen U-Bootbaus", volume 2, the project "Tümmler" is described as a small coastal sub, which mainly relied on a fixed snorkel, mounted in a streamlined fairing, together with the periscope and "snorkel lookout", a high set place for the commander of the 6 men crew. Main propulsion came from a 2,000 hp MAN Diesel (as used in the type XXI), which allowed a speed of 17 knots submerged. For more silent running (5 to 6 knots) a 60 hp Büssing engine (as in the Seehund) was fitted, that additionally charged a small battery, that allowed for a short distance being really submerged.
The hull was completely streamlined, without deck or sail, apart from the snorkel, the boat would have had a small positive buoyancy for nearly all the time, and would have dived dynamically. Armament would have been four torpedo tubes in the bow without reloads, with the same fire control system, as in the type XXIII.
The idea was this type of sub was mainly triggered by the three points:
- The first experiences with the snorkel were quite positive, and the assumption was, that the head of a snorkel would have been difficult to locate, even for radar. But the modified U-boats couldn't use their full Diesel power when snorkeling.
- Such a type would have consumed less materials, than a full sized U-boat.
- The loss of such a boat would have reduced the headcount of the German Navy by 6 men, and not by about 60, as usually does a Type VII boat.
