Those were 6.5 ton one-man vessels which could carry two underslung torpedoes of which 324 were built by Flenderwerke in Lübeck in 1944. They had the range of 130 miles at 6 knots surfaced and 8.6 miles at 5 knots submerged. This boat had the diving depth of 65 feet but could not launch torpedoes submerged due to depth-keeping problems.
The Biber (beaver) was a one-man submarine developed from the capture of Welman W46 captured at Bergen, Norway on 22 November, 1943. Developed by Korvettenkapit�n Hans Bartels. He began negotiations with Flenderwerke of Lubeck for the construction of the craft. The first prototype known as either the Bunte-Boot (after Director Bunte of Flenderwerke) or Adam was completed on 15 March 1944. Much of the testing was done by Bartels on the River Trave and on 29 March 1944 the craft were accepted for service.
24 production models were ordered as a first batch out of an eventual run of 324. Deliveries throughout 1944 comprised three units in May, six in June, nineteen in July, 50 (fifty) in August, 117 (one hundred and seventeen ) in September, 73 (seventy three) in October and 56 (fifty six) in November. Some components destroyed by air raids on Kiel but on the whole Allied bombing failed to disrupt production.
The Biber was buildt in three sections which bolted together. The bow section contained nothing more than the main diving tank. Between the first and second bulkhead was the main compartment where the operator sat. The third compartment contained the after diving tank. Further development of the Biber were planned which included two-man versions (Biber II and Biber III), these were nothing more than drawing board projects.
From January through April 1945 the Molch and Biber midget boats went out on 102 sorties, lost 70 of their own and only sank 7 small ships with a total of 491 tons and damaged 2 for 15,516 tons.
More information on midget submarine operations.