The Seehund (German: "seal"), also known as Type XXVII, was a midget submarine manufactured by Nazi Germany during World War II. Designed in 1944 and operated by two-man crews, it was used by the Kriegsmarine (German Navy) during the closing months of the war, sinking nine merchant vessels and damaging an additional three, while losing 35 boats, mostly attributed to bad weather. The French Navy used four captured boats after the war until 1953.The origin of the Seehund began with the salvage of the two British X class submarines HMS X6 and X7 which had been lost by the Royal Navy during Operation Source, an attempt to sink the German battleship Tirpitz.
As the orders were being placed, Hecht variants were under construction. The first was the Type XXVIIB, which had a greater range, could carry two G7e torpedoes, and had diesel/electric propulsion. The design was completed at the end of June 1944 and resembled Hecht but had a better boat-shaped external casing for improved seakeeping while surfaced, and saddle tanks. Additional room had been made inside the pressure hull by moving the batteries to the keel, while the two torpedoes were slung externally in recesses in the lower hull. A 22-brake-horsepower (16 kW) diesel engine was fitted for surface use and was estimated to give a surfaced speed of 5.5 knots, with a 25-brake-horsepower (19 kW) electric motor providing a submerged speed of 6.9 knots. The final variant of the Type XXVII was the Type XXVIIB5, better known as the Seehund ("Seal") or Type 127. Seehund had a small raised platform midships with the air intake mast, magnetic compass, periscope, and a clear dome which could survive depths of 45 meters. The submarine's fixed 3-meter periscope incorporated lenses which let the commander check the sky above for aircraft before surfacing.