Wreakferret,

I hope the below information will help you to answer your question about the hull thickness. I took this out of my design study book that was completed in 1945 at the end of the war. I am sorry about the little yellow faces, I seem to not be able to get rid of them.

C  O  N  F  I  D  E  N  T  I  A  L
  
HULL STRUCTURAL
 
1.  General
 
  The vessel consists of a cylindrical pressure hull with truncated conical sections at the ends and cast end bulkheads, a conning tower of oval horizontal section with a cast top, a system of exterior ballast and fuel tanks enclosed in a ship-shaped envelope, and a light superstructure with accompanying conning tower fairwater and bridge.  A box keel is fitted below the pressure hull.  The designer's submergence depth is 100 m. (328').
 
2.  Pressure Hull
 
  The cylindrical pressure hull has a diameter of 4400 mm (14.42') and is made of 18 mm (.75") steel with inside bulb tee frames 200 x 11 (7.88" web depth x .43" web thickness) on 700 mm (27.56") centers.  The plating is gradually reduced throughout the tapered sections fore and aft to 16 mm (.63"), and the frames are correspondingly reduced to 130 x 9 (5.12" x .35").  Frame spacing remains unchanged in the tapered sections.
 
 
  The specifications for the pressure hull plating and frames calls for a tensile strength of 74000 psi with a yield point of 51300 psi.  The steel is known as No. 52; the specification for plates is KM 9104, and for frames is KM 9103.
 
 
  Framing is modified in the way of the main motors, to provide clearance, by substituting double frames 160 x 9 (6.30" x .35") for the normal 200 x 11 frames.
 
 
  In addition to the dished end bulkheads of cast steel, four other dished cast bulkheads are fitted, dividing the pressure hull into five pressure compartments.  These bulkheads are 22 mm thick; but material specifications, while unknown, are believed to be German cast steel 45.81 per KM 9106.
 
 
  Further subdivision is provided by two light fabricated bulkheads, one in the battery compartment forward, and the other separating the maneuvering and engine space in the machinery compartment aft.
  
These are designed for a pressure differential of 3 psi only, but in practice serve no pressure purpose because they have so many non-tight openings.
 
 
  Large openings in the pressure hull consist of the two torpedo hatches, one galley hatch, one engine room hatch, and openings in the control room for conning tower hatch and two periscopes.  There is also one removable patch in the way of the engine room and another overhead in the battery compartment.
 
 
        Compensation for these openings is obtained in a number of different ways:
 
 
  (a)  For the torpedo hatches, a doubler (in some vessels a single thicker plate) is fitted, increasing the plate thickness from 17 mm normal to 38 mm (1.49").  The frames are cut in the way of the hatches, and are fitted with bearing pieces at the ends, against which strongbacks are wedged and secured in position by toggle pins.
 
 
 (b)  The galley and engine room hatches have no compensation other than their trunks, which are tubes with 20 mm (.79") walls.
 
 
 (c)  The overhead openings in the control room are compensated for by increasing the hull plating to 22 mm, by trunking each opening and by fitting angles inside and outside the hull on each trunk.  This sounds a little overdone.  Further, as the openings make two frames discontinuous, the adjacent through frames are increased to 200 x 15 (7.87" x .59").
 
 
 (d)  The patch in the engine room has double butt straps, double riveted, on the plating.  Frames are butted, with double butt straps on the webs having 6 rivets on each side.
 
 
 (e)  The battery patch consists merely of a plate riveted to a frame about 1.5" thick which is welded into the hull plating.
 
 
  Within the pressure hull the forward and after trim tanks, and the WRT tanks, are the only structural tanks designed for more than a gravity pressure head.  The remaining structural tanks consist of four fuel tanks, the lubricating oil tanks, the fresh water tanks and the sanitary tanks.
 

  The conning tower is a relatively small oval cylinder mounted vertically on the pressure hull.  Plating thickness is 40 mm (1.18").  Frames are vertical.  The structure is closed at the top by an elaborate steel casting incorporating periscope and hatch rings, and ribs extending to the top of the frames.  The specified plating material os identified as special Wh n/A, not further identifiable with available reference material.  The cover casting is of chrome-molybdenum-vanadium steel.
 
 
  The entire pressure hull is welded except for the patches mentioned above.  Butt joints are employed on the shell, and where heavier plating adjoins lighter plating, the heavier plating is scarfed to the lighter thickness at the weld.  Where the shell is welded to the cast end bulkheads and where the conning tower plating joins the cover casting, however, the outer surfaces are flush, but the inner surfaces are not, and a fillet of weld metal has been built up from the plate to the thickness of the casting.  Intermittent welding is used only on the stiffeners for the two light fabricated bulkheads.  The cast type of pressure bulkhead is welded to a ring on its periphery, which in turn is welded to the pressure hull.  The door frames in these bulkheads are riveted.
 
3.  Outer Shell
 
  The outer shell which includes the end portions of the superstructure, encloses the bow and stern buoyancy tanks, three main ballast tanks, five fuel ballast tanks, one normal fuel oil tank, two variables ("regelzelle" and "regelbunker") and the negative tank.
 
 
  Later vessels had four fuel ballast tanks and two normal fuel tanks.  Of interest is the fact that one of the three main ballast tanks has flood valves, discussed further under the section on drain and trim systems.
 
 
  The outer hull does not extend around below the pressure hull except well forward.  In the way of the stern buoyancy tank and MBT 1 it extends across the top of the pressure hull.  As a result of these characteristics, only the buoyancy tanks and MBT 1 and 8 are single tanks.  All other tanks are twins with interconnection only to the extent permitted, by the piping connections in each case.
 
 
  The ballast and fuel tanks are designed for internal pressures not exceeding 9 psi, although the variables and the negative tank withstand greater pressures.
 
 
  Plating is normally run in horizontal strakes of 6 mm (.24") material.  In the way of the variable (regelzelle) tanks there are three strakes, the upper and lower of which are 15 mm (.59") while the middle one is 20 mm (.79").  In the way of the negative tank there are two strakes, the upper of which of 15 mm (.59") while the lower, extending in to the pressure hull at the bottom of the tank, is 20 mm (.79").  The heavier plate extending all the way down serves to compensate in part for the flooding openings in the negative tank.  Elsewhere in the outer shell, as well, heavy plates are worked in lieu of doublers as partial compensation for openings.  Light 5 mm (.20") plating is used on deck at the ends of the vessel.
 
 
  Framing is 60 x 6 (2.36" x .24") bulb tees except in the way of the stern buoyancy tank where part of the frames are 75 x 50 x 8 (2.95" x 1.97" x .31") bulb tees, and in the way of MBT 8 and the bow buoyancy tank where part of the frames are 60 x 40 x 5 (2.36" x 1.58" x .20") angles.  Frame spacing is 500 mm (19.68") throughout.
 
 
  Frames are discontinuous in the way of flood valves and larger flooding openings, and are terminated on longitudinal headers outboard of such openings.  The headers are in turn terminated on web frames on bulkheads fore and aft of the openings.  Tank plating is normally 12 mm (.47") in the part of the tank where frames are discontinuous.
 
 
  Tank bulkheads are 5 mm (.20") bulkheads with light bulb tee 60 x 6 (2.36" x .24") or smaller radial stiffeners.  Web frames are similarly constructed, but with fewer stiffeners and many lightening holes.  The pressure bulkheads for the variable and negative tanks are 18 mm (.71") and are dished inward, without stiffeners, on earlier vessels of the type, although it appears that stiffened plate bulkheads were used on later vessels.
 
 
  Radial bracing of the outer shell frames is accomplished by means of 50 x 50 x 6 (1.97" x 1.97" x .23") angles bracketed to the pressure hull at one end and to the outer shell frames at the other end.  Structure consists uniformly of a V assembly bracing the outer edge of the tank top and the upper portion of the frame, a single bracing member at the point of greatest beam and an N assembly which carries the compressive load around the turn of the tanks at the bottom.  The lower angle of the N terminates on the longitudinal headers where these are fitted.

 
  Except in the way of the variable and negative tanks, the tanks are built with flat tops.  Tops of the variable and negative tanks, however, are rounded over and brought in normal to the pressure hull.  There are drain pipes through the tanks to drain the waterway formed by the tank margin plate and the pressure hull.
 
 
   Welding is employed except between strakes E and F, which are lapped and riveted, for frames at the ends of the vessel, and for propeller shaft fairwaters, which are riveted to the plating.  On earlier vessels of the type, the riveting extended the entire length of the vessel, but on vessels actually seen a lap weld was made from outer hull frame 1 to 118 and riveting was retained from outer hull frame 118 and aft of 1 (frames are numbered from the stern post forward).  The retained section is double riveted.  The section in which welding has superseded riveting was formerly single riveted.
 
 
  Outer shell plating is steel 42 (60000 psi tensile) specification KM 9104, except in the way of the variable tanks and negative tanks, which are specified to be of steel 52, (74000 psi tensile) specification KM 9104.  Outer shell frames are steel 42 specification KM 9103.
 
4.  Superstructure
 
 
  The superstructure consists of a light frame structure of angles, with a strake of plating riveted along the sides, and plating sections secured by machine screws on deck in the way of the two capstans and the 10 topside torpedo stowage tanks.  The remainder of the deck is of wood slatting.  Note here that the two forward torpedo stowage tanks on the starboard side have been removed in those vessels equipped with snorkels.
 
 
  The superstructure encloses, as well, the ballast tank vent piping and valves, the induction and exhaust air ducts, the mufflers, work boat and the usual other stowages.  It also provides foundations for bitts and fairleads.
 
 
  Extending above the superstructure is the conning tower fairwater and bridge structure, projecting from the front of which, at the base, is the non-magnetic (aluminum-manganese 5.25 per specification KM Norm 9304) housing for the magnetic compass.  As originally designed, the structure extended aft of the conning tower far enough to provide a housing for the outboard ventilation valves, and provided a bridge platform with room for one 20 mm gun.  As redesigned, the structure was widened and raised aft of the bridge proper, to provide space for two 37 mm guns, and was extended aft at a lower level to provide room for a 40 mm gun.  Splinter protection was added at the same time.
 
 
  The fairwater and bridge bulwark plating is 3 mm (.12") and the bulwark is further provided with 14 mm (.55") splinter protection.  Steel 42 (60000 psi tensile) is used for plating and framing except for:
 
 
  (a)  Splinter protection, which is probably special steel known to the Germans as Wsho/Mo.
 
 
  (b)  Plating and framing within 900 mm (35.4" of the magnetic compass, which is of aluminum-magnesium 5.25 per specification KM 9304.
 
 
  Riveting is employed throughout except for removable plates which are provided with screws.
 
 
  The keel section is a box section extending from the bottom of the pressure hull, and faired in to the pressure hull at both ends.  It is made up of a central vertical keel of 8 mm (.31") material and a heavy sole piece.  Vertical transverse stiffeners from the pressure hull to the sole piece, complete the strength members.  The outboard faces of the box are closed by light plating secured with screws, and the space on both sides of the vertical center kiel within the light side plating is used for the stowage of cast iron block ballast.  Scantings of the box keel are not available.  Material is steel 42.
 
 
  Aft of the end of the box keel, and extending aft as far as the rudder, a deadwood is built out below the pressure hull, which provides a housing for the stern planes, struts and rudder skeg.  In section it approximates a V.  Scantlings are not available.  Material is steel 42.
 
 
5.  Internal Decks
 
 
  Walking flats are in general very light.  Further, no reinforcing is provided below hatches, so there is a permanent dent in the deck below each hatch.  Substantial decking is provided only over the battery wells.
 
 
6.  Castings
 
 
  There is no stem casting.  A casting is employed at the after end of the deadwood and another casting extends below the deadwood to serve as a skeg and, when docking, as a means of supporting the after structure of the vessel.  The struts are castings the arms of which are riveted to the pressure hull and to the deadwood.
 
 
7.  Foundations
 
 
  Foundations for main machinery are fabricated structures, the inboard elements of which extend the full length of the machinery compartment including the maneuvering room.  The outboard elements are not interconnected longitudinally.  Vertical members run 12 to 14 mm (.47" to .55") laterally braced at each frame.  Sole pieces for the main engine are 24 mm (.94") and for the motors and thrust bearing are 20 mm (.79").
 
 
  Foundations for auxiliaries present nothing of note except for matter which is discussed under sound isolation.
 
 
  Foundations for guns are undeserving of comment except to mention that the foundation for the 105 mm deck gun, forward of the conning tower, incorporates a heavy cast ring for the mounting bolts.
 

8.  Comments
 
 
   The hull structure presents little of interest.  The employment of castings for bulkheads and for the top of the conning tower is novel, but is an expensive way to obtain the desired strength.  The tremendous masses of weld material employed in welding the castings to the plating are of debatable merit.
 
 
   Further, in transferring the basic design to the completed vessel, it appears that weaknesses have been introduced.  Compensation for hull openings does not appear adequate in all cases, and is particularly bad in way of the machinery patch, where any load on the frames must be transferred through rivets in shear.  The lack of means for adjustment of the strongbacks in the way of the torpedo hatches is also undesirable in this regard.
 
 
  The drawbacks to scupper pipes led through tanks are too well known to require comment.  It is curious that they should have been retained, as the lightness of tankage plating and framing makes them very susceptible to damage.
 
 
  While workmanship was in general good, details were not in all cases satisfactory.  Gross pitting of welds of foundations to hull was noted in certain cases, and pipe and cable hangers were apparently located without regard to surroundings, as witness the welding of hangers to rivets, caulked edges of rivets and plates and to foundation bolts.
 
 
9.  Conclusions
 
 
  The hull is, as a complete entity, believed to be inferior to contemporary U.S. Naval construction.