The type 21 hull is of great interest in all respects, and while the design has in part been compromised by the attempt to work out the solution partly in terms of preceding practice, the hull is deserving of detailed analysis.  
July, 1946
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  REPORT 2G-21
  1.  General
          The vessel consists of a pressure hull which for part of its length in section like an inverted figure 8 and for the remainder is cylindrical, with truncated conical end section having fabricated stern end bulkhead and cast forward end bulkhead, a conning tower which is oval in horizontal section with a cast top, a system of external ballast and fuel tanks enclosed in a faired envelope, and a fairwater for the conning tower which includes a bridge.  The designer's depth is 135 meters (440') with a safety factor of 2.5.  
  2.  Pressure Hull  
          The upper cylinder of the pressure hull has a diameter of 5300 mm (17.38') with maximum plate thickness of 26 mm (1.02") and external bulb tee frames 240 x 11 (9.44" x .43") on 800 mm (31.5") centers.  The lower section which is not a true circular section except in the way of the two end compartments, and is further discontinued in the way of the machinery compartments, has 18 mm (.71") plating, a flat plate keel 1000 mm x 40 mm (39.37" x 1.58") at the bottom, and 140 x 7 (5.51" x .28") bulb tee internal frames on 800 mm centers.  
          At the point of intersection of the upper and lower segments of the hull, heavy transverse beams extend across from side to side, and the flanges and webs of the external frames are carried down along the lower segment and are tapered off to the plating.  The deck carried on these beams is of 18 mm (.71") material, and the beams themselves are tees made from halves of NP40 (standard 15.76") I-beams on 400 mm (15.76") centers.  At the bottom of the tub there is, in addition, a 10 mm (.39") decal supported on 285 x 100 x 15 (11.2" x 3.94" x .59") flanged floors on 400 mm centers.  
          At the ends of the pressure hull were the fairing of the outer envelope introduces minimum clearances between the pressure hull and exterior tank plating, internal frames are substituted for external frames.  At the same time, in order to conserve space within the hull, the scantlings of the frames are reduced from 220 x 11 to 160 x 7 (5.51" x .28") to offset which frame spacing is reduced from 800 mm to 400 mm (15.75").  
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  REPORT 2G-21
          Plating thickness is likewise reduced, but on a more gradual basis, throughout the length of the tapered sections at the ends of the vessel.  Lightest plating is 12 mm (.47") on a diameter of 1492 mm (4.90').
          Transverse framing is employed except in the way of the variable tanks, where there is an elaborate system of floors extending up to the pump room deck, and longitudinal frames.  The pump room deck is of 26 mm (1.02") material.  
          Pressure hull plating and framing material, and as well, the material for the end bulkheads and their stiffness, is steel 52M.  Material for cold-formed frames is aluminum-normalized steel 52 AM.  
          In addition to the bulkheads at the ends of the pressure hull, five fabricated pressure hull bulkheads are provided, dividing the hull into six pressure compartments.  These bulkheads are not intended to withstand full submergence pressure, and are further not designed with the same factor of safety.  The test pressure corresponds to 50 m (159.0') submergence and the designed safety factor at that pressure is 1.5.  
          One light water-tight bulkhead is fitted in the machinery compartment, between the engine room and the maneuvering room.  
          Pressure hull openings consist of a patch for auxiliary machinery access in the engine room, one battery patch in the after battery, and two patches in the forward battery compartments, access hatches in the maneuvering room and galley, conning tower lower hatch, two periscope openings and one entering opening, and a torpedo hatch in the forward compartment.  Attention is invited to the absence of a patch for the main engines.  
          Compensation for hull openings is of the same character as that on previous types.  
          There are no true pressure tanks within the pressure hull proper, but the WRT tanks and forward trim tanks are in the tub which is below the torpedo room, and the after trim tank is located in the tub below the after compartment.  Further, another pressure  
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  REPORT 2G-21
  tight structure built below the pressure hull in the way of the engine room houses the bilge water tank and two tanks into which oil from the hydraulic clutches is dumped.
          The conning tower is similar to those on earlier vessels in size, in scantlings and in material except that the top casting is identified as special material Wh oMo with carbon .2-.25, silicon .35-.50, manganese 1.0-1.2 and vanadium .1-.15, and possessing physical characteristics as follows:  
55kg/mm2 (78000 psi)
elastic limit
40 kg/mm2 (57000 psi)
elongation with gauge ratio 1 in 5                  15%
reduction in area
impact value
10 mkg/cm2 (140000 psi)
          The entire pressure hull is welded except for the patches mentioned above and door frames in pressure bulkheads.  Intermittent welding is used only on the stiffeners of the one light watertight bulkhead.  
  3.  Outer Shell  
          The outer shell is generally speaking, an envelope enclosing the entire vessel.  There is no true, separate, superstructure as such, for several of the exterior compartments are built as saddle tanks extending across above the top of the pressure hull, and the free flooding space below the superstructure deck is limited to those parts of the structure which enclose duct work, or which are cut away for access purposes.  
          The outer hull encloses the bow and stern buoyancy tanks, 5 main ballast tanks, seven normal fuel oil tanks, two variables ("regelbunker" and "regelzelle") and a pressure proof void space which was originally a negative tank but which has had the flood valves removed and is known to have been used as an uncompensated fuel tank.  It also encloses several ballast compartments, one of which extends all the way to the superstructure deck abreast of the torpedo room, and a large free flooding space aft along the sides of the vessel.  
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  REPORT 2G-21
          The tanks are not distributed horizontally along the hull as had been previous practice, but are superposed one upon another.  Further, the location of the end bulkheads of the tanks is modified as made necessary by considerations of physical shape and stability.  For example, at frame 22.4, MBT 1 is over NFO 3a which again is over a ballast stowage compartment.  At frame 32, there is no free flooding space over NFO 4a, over MBT 2, which in turn is over NFO 3a, inasmuch as this tank extends from the after end of MBT 1 well forward under MBT 2.
          Tank test pressures are 17.5 psi over the base line for fuel oil tanks, and 4.4 psi over the top of the tank under test for main ballast tanks.  
          Exterior tank plating is 5 to 8 mm (.20" to .31") thick.  For variable tank, trim tank and WRT plating, see above under the description of the main pressure hull.  
          Framing is generally bulb tee sections 60 x 5, (2.36" x .20") above and 80 x 5.5 (3.15" x .22") below, on 400 mm (15.75") centers.  The two sizes are butted and welded to complete a single frame from the deck to the bottom of the lowest tank.  
          Frames are discontinuous in the way of flood valves and certain of the flooding openings, and where this occurs the frames are terminated on longitudinal headers.  
          Radial bracing of the shell framing is provided by means of 100 x 25 x 5 (3.94" x .98" x .20") Yoder angles.  
          Two stringers, run for about half the length of the vessel amidships, provide additional support for the light plating and framing.  Scantlings are 120 x 5 (4.75" x .20").  
          Tankage bulkheads run from 5 mm (.20") to 8 mm (.31") thick, the usual thickness being 6 mm (.24"), and are stiffened by bulb tee sections of appropriate size, usually on 400 mm centers.  
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  REPORT 2G-21
          The material for plates and shapes are the same as those on earlier types of vessel.
          The bridge and fairwater structure serves as well as a housing for the induction and exhaust air outboard valves, for two twin 20 mm gun turrets and operating gear, and for related ready service and spare barrel tanks.  
          Plating is generally 4 mm (.16") thick, but splinter protection 17 mm (.67") thick is provided for the bridge and the gun positions across the top as well as along the sides and at the ends.  The material for the splinter protection is identified as Wsho/Mo, but its characteristics are not otherwise known.  
  4.  Keel  
          The vertical keel extends aft from the fabricated forward structure, forming a deep centerline division extending up through the forward trim tanks and WRT tanks, to the forward end of the forward battery, where it is reduced in depth to 285 mm (11.2") and extends aft at that depth to the after end of the after battery, except for the section in the way of the variable tanks, which is carried as a centerline bulkhead up to the pump room deck.  Aft of the after battery it is carried at the full depth from the sole plate to the bottom of the pressure hull as far as the forward end of the after trim tank.  It is carried aft through the after trim tank, but does not extend below it, as the longitudinal member at the bottom of the vessel is a bulb tee 120 x 6.5 (4.72" x .26").  Connection is provided at the after end of the trim tank to the cellular structure which carries aft into the tail of the vessel.  
  5.  Stern  
          The stern frame of a casting (a weldment is permitted by the specifications) with carriers for the rudder and for the stern planes.  
          Also at the after end of the vessel is a pair of fins which serve the dual purpose of stabilizers and struts for the propeller shaft bearings.  As shown on plans, the fins are the widest part of the vessel, but on U-2513 and U-3008, the ends have been omitted.  
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  REPORT 2G-21
  6.  Foundations
          Foundations are similar in design and material to those on earlier types, except as modified by the introduction of reduction gears on this type of vessel.  
          The hull structure is an interesting solution to the problem presented, and is a radical departure from previous German practice.  At the same time, a number of the design details could have been improved.  A number of details characteristic of previous types have been retained.  
          The method of connecting the upper and lower segments in the way of the battery compartments has been criticized by the Germans, who have ascribed the failure of the lower segment under test at less than designed collapse pressure to the manner of connecting the external frames to the internal frames and the cross beams.  
          With specific reference to the external frames, attention is invited to the fact that although they are deeper, their dimensions in other respects do not vary materially from those on the 9C vessels.  Flanges have not been widened to improve the lateral stability of the frame section.  
          Further criticism can be leveled at the design on the score of unsatisfactory bulkheading, as the collapsing pressure for the pressure tight bulkheads is only half the designer's submergence depth for the hull.  
          Again, the retention of bolted plates, and of rivets in shear, as elements of hull opening design, is questionable.  
          The interruption of the circular section at the bottom of the vessel, and the substitution of a heavy sole piece, an additional deck and floors also appears to be unfortunate from a stress distribution standpoint.  
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  REPORT 2G-21
          The tankage design is very light, and repeats the fault of the 9C in this regard.  It is further believed that the intricate compartmentation could have been simplified.
          Regardless of the foregoing, the hull was found satisfactory by the Germans for its designed purpose.  
  8.  Conclusion  
          The hull is of great interest, but is open to question as to certain details of construction and arrangement.  
  9.  Recommendations  
          It is recommended that the hull design be thoroughly exploited.  
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