The drainage system on the XXI submarine is serviced by two rugged reciprocating pumps, one in the pump room and one in the after room, for pumping against heads up to approximately 1000 ft., and one relatively high capacity centrifugal pump for use against moderate discharge heads. Piping layouts were simplified from earlier German designs.  
          The flooding and draining of the auxiliary tanks on the XXI is accomplished by use either of a rapid flooding or fine flooding line.  A two-way reading meter in the fine flooding line permits accurate control of quantity flooded or pumped (or blown) to sea.  
          Pumping arrangements for longitudinal trim control are not provided on the XXI as the system used on earlier designs with divided trim tanks has been employed.  
July, 1946
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  REPORT 2G-21
  1.  Introduction  
          Information on the trim and drain system installed on the XXI submarine is given in NavTechMisEu Technical Report No. 306-45.  This section will further supplement the information contained therein and repeat details only where necessary for clarity.  The trim and drainage systems on type IX and XB vessels are covered in the Reports under the corresponding S48-1 group for these vessels.  
  2.  General Information  
          The drainage system on the XXI submarine has been changed considerably from type IX and XB designs; the trimming system, however, has been taken directly from the altered trim system on the IXD2 submarine. (Described in report 2G-9D2-S48-1.)  
          The drainage system on the XXI submarine has been changed considerably from type IX and XB designs, by combining main and auxiliary drain systems into one system, and by the substitution of individual cut-out and control valves for a manifold arrangement.  The drainage system as set up has a main suction line running the length of the vessel.  All individual bilge, auxiliary tank, & W.R.T. suctions lead to this line.  It is noted, however, that individual bilge sumps have not been built into the pressure hull on this class of vessel so that it is difficult to completely dry out any of the bilges.  The line is divided in the control room so that the "shallow" and "deep" drainage pumps in the pump room can take a suction from either (or both) forward and after sections of this line.  A second "deep" drain pump can take a suction from this line in the after room.  Also, to provide emergency service on the drain line, the standby and two attached cooling water pumps in the maneuvering room have a suction connection to it.  The two sea connections that are used for normal and fine flooding and blowing of the auxiliary ballast tanks have connections to the suction line of the drainage system.  
          The discharge from the two control room pumps is normally passed either directly to sea or to No. 3 starboard main ballast tank.  A separate connection from the common pump discharge leads to one portable hose fitting for use in filling the trim tanks and to  
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  REPORT 2G-21
  several additional fittings for use with deck hoses.  Also, a connection from the "shallow" drain pump discharge leads to the circulating water main.  The discharge from the after room "deep" drain pump leads directly to sea.  
          An additional amount of drainage is provided in the machinery and battery spaces by arrangements independent, to some degree, of the main drainage system.  In the battery spaces drainage piping leads from the upper deck to the upper and lower battery well bilges.  This piping is normally used with the slaked lime solution for flushing the painted battery well bilges.  Special portable hose fittings can be secured to the piping at the upper deck level which fittings, in turn, can be connected to the main drainage suction.  These lines may either be used for removing the lime solution after flushing or for emergency drainage service.  
          The special drainage arrangements in the engine and maneuvering rooms are associated with the pressure tight bilge water tank built into the pressure hull under the maneuvering room.  This tank, of 264 gals. capacity, is fitted for receiving the water from the maneuvering room bilges and for blowing it either directly to sea or to No. 1 port main ballast tank.  No means of pumping it is provided.  A large sluice valve connects the engine and maneuvering room bilges so that the engine room may also be drained by means of this pump.  
          The piping arrangements associated with the divided fore and aft trim tanks provide for normal and silent transfer of water by the use of air pressure and for filling or emptying by use of portable hose connections to the drainage system.  The trim line is of 3.15 ins. nominal diameter and is tested to 107 psi.  
          The "deep" bilge pumps are designed along the lines of the auxiliary drain and trim pumps on the type IX and XB submarines.  The same double-acting type of piston with cross-head arrangement has been retained.  However, all parts of the installation have been considerably strengthened to take the increased discharge heads against which the pump is designed to operate.  A water ejector (with sea pressure suction) is placed in the suction to the pump in order to increase the suction head.  
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  REPORT 2G-21
  This pump was designed to deliver 100 GPM at discharge heads up to 300 meters (985 ft.) but, as pointed out in the Nav. Tech. Report, could only deliver 88 GPM at this head on test.  This was the first class of German submarine with drainage pumps that could discharge against pressures greater than those of designed submergence.  (The designed discharge head on the XXI corresponds to that of designed collapsing depth).  
          The single stage centrifugal "shallow" drain pump selected for the XXI submarine is identical to the standby circulating water pump in the maneuvering room.  The characteristics for this pump are listed under the S48-5 section of the XXI report.  This pump provides the vessel with a high pumping capacity (308-440 GPM) at submergences up to 100 feet.  This capacity is particularly desirable at shallow submergences to take care of drainage into the vessel during snorkel operation.  
          The drain piping is designed to be used up to collapsing depth.  The suction piping is tested at 45.2 psi; the discharge piping is tested at 455 psi for tightness and 570 psi for strength.  The main drain line is 125MM (4.93 ins.) in diameter (nominal size).  
          The arrangements on the XXI for the pumping or blowing and flooding of auxiliary tanks are of particular interest.  The associated systems have been set up with the intention of obtaining normal buoyancy control by pumping and flooding, either with or without use of the hovering gear (schwebegerate).  This flooding is controlled from either of two sea connections, one for fine and the other for rapid flooding.  A flow meter that can measure in either direction is on the fine flooding line.  The pumping is also set up for fine or rapid control.  For rapid pumping, the water is discharged to sea through the main drainage system sea discharge in the control room.  For fine control the water is discharged to sea through the fine flooding line and its sea connection (via the above-mentioned meter).  A cross connection connects the drain pump discharge line with the fine flooding line.  Volume gages are provided for each auxiliary tank so that a ready check on volume pumped or flooded is possible.  
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  REPORT 2G-21
          The German designers intended that buoyancy control during evasive action would be attained by a special blowing - flooding technique.  One of the auxiliary tanks was to be set up ready for blowing to sea during shallow submergence with an air pressure on the tank of 43-57 psi; a second was to be set up for blowing at deeper submergences with 230-256 psi. air pressure; the third was to be set up for flooding while the fourth was to be kept in reserve and used as conditions dictated.  With air pressure already on the tank the noise emitted while blowing the water to sea is considerably reduced and kept below that created during normal pumping operations.  (This arrangement parallels that set up for fore and aft trim control with the divided trim tanks.)  To provide additional safety when blowing tanks with air differential pressure gages are installed on all auxiliary ballast tanks to show the differences in tank and sea pressures.  
          Several features in the design of the XXI submarine makes this technique more feasible on this type than on other German types, and, to some extent, more than on U.S. submarines.  The compression of the hull on the XXI is 680 lbs./100 feet change in depth (Equivalent to 900 lbs./100 feet for U.S. fleet submarines).  This is much less than on earlier German types so that the amount necessary to blow to sea for changes in depth is correspondingly decreased.  Furthermore the auxiliary ballast tanks are for the most part "under" the vessel so that large quantities of water need to be blown out (or flooded) from any one side tank to bring about appreciable change in list.  As a result considerable buoyancy control can be obtained by the above technique without a noticeable change in list.  If necessary, however, by originally putting the air on a port and a starboard tank it is also possible to obtain silent list control simply by transferring water athwartships, although, in this case, it would most always be necessary to vent one tank inboard.  
          The quantity that could be transferred silently by the above method is limited both by the reduction in air pressure and by the quantity of water within one tank.  However, except under unusual circumstances, this "limited" quantity will be more than adequate to meet weight change demands over long periods of silent running.  
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  REPORT 2G-21
  3.  Individual Components  
          Deep drain pump characteristics (designed)  
110 lbs
37x28.4x23.6 ins.
Piston Dia. x Stroke
3.55x3.94 ins.
24.3 KW
101 GPM
Total Head
985 Feet
  4.  Conclusions  
          The drainage system for the XXI submarine is well designed in that it can handle all normal and most emergency drainage requirements for all submergences up to collapsing depths.  Some question arises, however, in whether or not the selection of a piston pump is best for use against the high discharge heads.  It is believed that a multi-staged centrifugal pump, similar to the US Gould pump in design would have given better all-around characteristics both for shallow and deep submergences, although certain features such as ruggedness and attainable suction head favor the reciprocating pump.  
          The installation of a system to obtain fine control of buoyancy in addition to the normal rapid, coarser control, was possibly necessary for creeping speed operations.  The XXI becomes very tender and difficult to control at low submerged speeds.  However, this fine control could have readily been obtained without resort to an additional sea connection.  
          The techniques for silent blowing of the variable ballast tanks has some merit.  However, the added complexity in tank construction and piping layouts as well as the several disadvantages arising from the necessary setup, appear to counterbalance the limited need for the system on heavy hull submarines.  
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