Seehund Midget Submarine Development

Realized Midget Submarines
Designs for small and midget U-boats had been rejected by the German Navy, while available boats,  especially Type VIIC, proved successful and so long as the main tactical objective was the attacking of convoys. The antipathy of the Navy towards very small U-boats was of very long standing, going back to the origins of German submarine construction. There had been no lack of suggestions as to their employment, and it had been proposed that small or midget submarines be used as single units and for hunting in packs from 'mother' ship. A noted champion of the small U-boat concept was Dr. Heinrich Dräager, the owner of Dräager-Werke, Lübeck. 
In a memorandum dated 1st October 1941, he presented a series of designs for small U-boats of between 70 and 120 tons, with diesel-electric and diesel closed cycle drive :
1) To build the required number of units, present warship construction methods must be abandoned. Instead of building a complete boat, from keel laying to launch, at one particular building slip on the coast, we must adopt the production methods of aircraft, of tanks and of locomotives. The sizes of boats and designs must be adopted for mass production of these methods.
2) The construction of larger boats must be sub-divided (Cell construction). It is necessary that the cells lend themselves to transportation by rail, by road, by waterway, etc. so that pressure hull construction and time consuming interior assembly can be carried out in those place where, at any time, spare industrial capacity, both in terms of personnel and construction installations are available in safe as possible from air attack.
3) One estimates the following production times :  
   Pressure hull construction and cell - 14 to 20 working days 
   Interior assembly of cell - 30 working days
   Welding together of cell and the remaining completion tasks - 30 working days
   Including an additional period for transportation, Sundays etc. we arrive at a period of 6 months from heat forming to slipway launch, given uninterrupted working. After things have been in progress for some time, some reduction of this time will probably be possible.
4) It is essential to try and achieve as accurate a uniformity of various components in manufacture as possible, so that repair and yard times can be a half of the usual times, through the quick and simple interchangeability of components.
Thus, in 1941, he anticipated Merker's ideas on the section construction of U-boats. Dr.Dräager went on to suggest the construction of Midget U-boats of only 23 to 25 tons, which could carried to their operational zone by surface vessels, or be used by auxiliary cruisers  or aircraft carriers for their own protection. Finally for the two most important aspects of operation, he regarded the development of special 100-ton U-boats as feasible and for them to built in very large numbers for the following purposes.
1) A U-boat for surface night attack, with a good surface shape and a submerged propulsion as in general present use.
2) A U-boat for submerged attack, shaped like a torpedo and, if possible a new type submerged propulsion to provide a high submerged speed.
In fact, test vessels for both types did exist at that time, but Dr.Dräager knew nothing about them because of the secrecy surrounding them. These were : 1) The so-called Engelmann High Speed Boat of 256 tons which in its originally planned from, could not dive, and which was unstable at high speeds, but which, with its low surface silhouette and its strong diesel concentration, came very near to Dr.Dräager's conception, and 2. The Walter test boat V80, which did, in fact, measure up to requirements with regard to a high submerged speed and great manoeuvrability, but which would not be suitable for operation of this kind, because of its very noticeable wake of bubbles. However, Dräager's ideas did not elicit any support from the Navy, or other ship construction cycles and, on 22 January1942, he received a final rejection for his U-boat suggestions from Counsellor of State Rudolf Blohm : 
Even if the small U-boat can be brought to the point of fulfilling technical requirements, we cannot regard it as adequate for operational purposes because, carrying only 2 torpedoes, it has a minimal armament and because, in adverse weather conditions, heavy seas do not allow small vessels to be used adequately in operations. Furthermore , the radius of operation, in the light of the increasing distances over which we are having to wage war, is insufficient, concerning an increase in launching of Types being currently constructed, capacity is totally dependent on personnel and materials. As regards materials in providing power and fuel supplies, bottlenecks in such areas as non-ferrous metal are highly significant.
A small U-boat design was worked out in 'K' Office at the beginning of 1942, with a length of 25.33m and a displacement of 97.95 tons, but it did not get beyond the project stage. One assumes that it had its origins in the impression caused by the exaggerated reports of Japanese midget U-boat successes during the attack on Pearl Harbor on 7 December 1941, and would have had, as they did, a torpedo shaped hull. 

ype XXVII Midget U-Boat Hecht and Seehund
Not until the Autumn of 1943 was there a change of opinion regarding the possible successful application of midget submarines. On 23 September 1943, the British 30 ton boat X6 and X7, succeeded in placing detonation charges under the battleship Tirpitz, damaging her and putting her out of action for at least 6 months.
Subsequently , 'K' office worked out a design for a German two man U-boat designated Type XXVIIA Hecht, which, like the British prototype, would attach limpet mines to ships lying in otherwise safe anchorages. It was to be considerably small, however, and in contrast to the X-class would reach its target purely by submerged travel. Naturally the radius of action was very restricted, and the boat would have to be carried to the vicinity of the target by surface transport. In order to trim the floating boat, and as it was anticipated that the boat would have to break thru nets, adjustable weights on spindles were provided in place of hydroplanes. This method, which harks back to Bauer's Brandtaucher, proved to be completely unsuitable because conditions of considerably inclination these weights could not be moved by hand quickly enough for them to be effective. Hecht therefore, was given forward hydroplanes, with stabilizing fin aft, an improvised measure that did not make for particularly good depth keeping. With only submerged travel in mind, no diving tanks were provided. The residual buoyancy of approximately 200 liters necessary before the crew entered the boat was, in submerged travel, destroyed by the flooding of the compensating tanks. The propulsion unit consisted of a 12 hp AEG torpedo engine, whose revolutions were reduced by a V-belt drive. The battery consisted of five 17T torpedo troughs, the plates of which were replaced by stronger ones, so that the battery would have a greater capacity and a longer life in conditions of low discharges. The designation of this altered battery was 8 MAL 210. The largest installation was a gyroscopic compass with a transformer, for it was believe that this was indispensable for pure submerged travel.
When design work had been completed, the Chief of Naval War Staff made an additional demand that this midget U-boat be able to carry torpedoes, so that it could be used against moving targets in the immediate coastal vicinity. As the volume of the boat was only 9.47 tons, only torpedoes without negative buoyancy could be used, and these, lacking a battery trough, had a restricted range. Hecht therefore had the choice of a mine or a torpedo under its keel as armament. If she were being used for torpedo carrying, a battery head with a further three battery troughs could be installed : in practice, however her range of 69 nautical miles at 4 knots was too small for this type of operation.
On 18 January 1944, Dönitz discussed with Hitler the plan for building 50 midget U-boats of this type. Hitler regarded it as good judgment to develop this type for both mine and torpedo use and, on 9MArch 1944, GW was given a contract for a prototype and, on 28 March, a contract for a series of 52 boats. In April, it was decided to carry on building the XXVII midget U-boats in large numbers.
Meanwhile "K" Office had produced numerous design under the Type XXVIIB, which would have an increased range (a true U-boat propulsion installation with diesel and electric motors) and a heavier armament (two torpedoes). An initial design for which the HSVA carried out shape research in June 1944, strongly resembled Hecht. However, a fore-ship for surface travel had been added to the torpedo shaped hull, and the keel had been enlarged to received battery troughs. Saddle tanks were arranged close to the midships section. All this meant that the surface displacement, without torpedoes, rose to 12.95 tons. A small 22 hp diesel was provided for surface travel, and it was estimated that this would provide a speed of 5.5 knots on the surface and 6.9 knots submerged.
At about this time, the Supreme Naval Command's representative for close cycle development at GW, Chief Naval Construction Adviser Kurzak, Stimulated by the successful closed cycle testing at the FKFS of the two OM59/1 vehicle engines (each of 55 hp maximum performance), set about designing a midget U-boat that could use this propulsion unit. It was evident that the closed cycle drive in these small boats would have to be much more efficient., a sentiment that Dr. Dräager had uttered as early as 1941.

Kurzak's first Design - Small U-Boat K

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