The fuel oil service system on the type XXI submarine is essentially the same as that on early submarines.  However, stowage arrangements and transfer facilities are for the most part peculiar to this class of vessel.  The basic compensating system arrangement utilizing salt water niches in each normal fuel tank has been retained as have the inboard venting and testing arrangements for fuel carrying tanks.  Otherwise, numerous differences exist both in tankage layouts and in the associated piping.  
          The transfer piping has been simplified in that all manifolds have been eliminated.  Also no transfer pump is installed; stand-by or emergency transfer is accomplished through the use of numerous portable hose connections with the hand circulating water pump, both shallow and deep drain pumps and low pressure air.  The piping and fittings have been designed to withstand a test pressure of 113 psi, a change from previous practice.  
          Another major difference comes from the fact that all fuel is carried in normal fuel tanks and variable ballast tanks fitted for that purpose.  All fuel tanks are outside of the pressure hull and form a part of a complicated interlocking network of external tanks.  The tank locations and shapes resulting from the arrangements have brought about added suction and venting difficulties.  
          A complete fuel oil drainage system has been installed.  The drainage from all storage tank test and vent connections, relief valve discharges, diesel engines, and gravity tanks passes via the main fuel oil drain piping to the fuel oil sump.  
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  REPORT 2G-21
          The fuel oil stowage and piping systems as installed on the IX-C submarine are described in detail in report 2G-9C-S55.  Only the changes in the corresponding facilities on the XXI will be covered in this report.  
          The fuel oil system on the XXI submarine was designed to service two 2000 H.P. MAN diesel engines and two Junkers air compressors.  The stowage capacity will provide a maximum cruising range of approximately 15,500 miles at 10 knots on two-engines.  
          All of the fuel oil is carried in the 15 normal fuel tanks and the two auxiliary ballast tanks that have been fitted for carrying fuel oil as no fuel ballast tanks are installed.  In each case port and starboard tanks are considered as separate tanks.  The fuel tanks have been distributed symmetrically outside of the pressure hull to conform to weight and stability requirements.  This has resulted in tanks of varying shapes and sizes at various levels with respect to the pressure hull.  Each tank has a salt water niche equivalent to 2% of its fuel capacity.  
          The transfer piping has been considerably simplified.  Two transfer lines, one port and one starboard, run inboard the length of the vessel and have a cross-connection in the engine room.  All tanks have leads with hull and back-up valves to this line.  A lead-off runs from the cross-connection to the gravity feed tank via a fuel oil filter and meter that may be by-passed.  The filling line is connected into this lead-off line in such a way that the oil received or discharged can be metered if desired.  The three portable hose connections on this line and the one on the filling line for use with air pressure add numerous pumping and blowing possibilities to the transfer system in case of emergencies.  Both deep and shallow bilge pumps and the hand circulating water pump can be put on the system through these connections.  It is possible to pump oil from any tank to the  
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  REPORT 2G-21
  gravity feed tank by this means.  
          Arrangements are provided for the necessary testing and venting of the fuel tanks as well as for emergency means to blow them individually.  The test and vent lines run from the bottom and top of the tank respectively and have sight gauges on their inboard side.  In addition, bleeder lines take off from between the hull and backup valve on the main transfer piping line to the top of the tank.  These also have sight gauges on them.  The portable air hose fitting lies between two valves on this bleeder line.  To blow the tanks, portable hoses are led to these fittings from the pneumatic tool fittings on the low pressure air line.  
          No measuring lines are provided to determine instantaneous oil levels within the various tanks.  However, it is possible to keep a close check on tank contents by metering the oil when filling or emptying them.  
          The oil is fed to the diesel through the same gravity tank arrangement as on the IX-C.  The only change in the service piping is that the oil for the Junkers compressors comes directly from the main engine service line instead of from an independent service system.  
          The fuel oil drainage arrangement installed forms a comprehensive oil-salvage system.  The tank testing, venting and bleeder lines as well as most of the drain lines from safety valves, the gravity feed tanks and the diesel engine run into the "drainage" mains running the length of the vessel.  The fuel oil sump tank is considerably larger on this vessel than it is on the type IX submarines.  The hand circulating water pump has a direct suction on this tank.  
          A very simple salt water compensating system is used with the fuel oil system.  
          All compensating water piping runs outboard of the pressure hull; no valves are fitted in any of the lines.  The compensating water main runs for the most part in the superstructure the length of the vessel.  Individual leads then run directly to the small niches in the fuel tanks.  
          The simplified compensating arrangement using the outboard compensation water piping and individual expansion niches within each fuel tank has particular merit on this class  
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  REPORT 2G-21
  of vessel.  Inasmuch as this type is to function mainly as a submersible, especially when in an operating area on patrol, the compensating water system and associated fueling arrangements should be designed with this in mind.  When on area (and subject to depth charge) the vessel will normally operate submerged either on the motors, or on the diesel engines utilizing the snorkel.  When operating the diesel engines on the snorkel, the head box on the compensating system is no longer required.  From this standpoint, only an opening to the sea at the bottom of the tank is necessary to bring about the necessary pressure for transfer of oil.  However, to take care of expansion and the danger arising from rupture of lines during depth charge, the salt water niche is necessary.  In order to permit the system to function when the vessel is operating on the surface, the head box and lines leading to it are also necessary.  Furthermore, to insure that oil may be transferred when on the surface, in the event of a casualty to the line leading from the head box to the tanks, emergency pumping means must be provided.  This system does not provide the protection to compensating piping afforded by the U.S. system.  However, it is to be noted that any casualty to the part of the line that has been placed within the pressure hull on U.S. submarines and that is external to the hull on the XXI would not bring about a disruption of service on the individual tank compensating system when the submarine is operating on the snorkel.  In addition, with the individual niches in each tank the possibility that oil slicks will rise by way of the ruptured compensating line is remote.  
          A minor disadvantage arises from the German setup which, however, should not be very difficult to remedy.  Whenever it is desired to remove liquid from the tanks extreme difficulty arises.  The only way at present to get rid of the water is by pumping through the small testing lines or by blowing through the compensating lines.  Pumping takes too long and blowing is nearly impossible as all tanks must be blown simultaneously.  When one tank is blown there is no way to keep the water from siphoning back if the air pressure is released.  This difficulty has been reported by the Engineer Officer from the U-2513.  Several valves (normally locked open) in the compensating line would remedy this condition.  
          The major advantage of a satisfactory outboard compensating system lies in the elimination of hull valves and inner hull piping, both of which form a potential source of danger either through corrosion or depth charge damage.  The number of holes through the pressure hull is considerably reduced and the inboard piping in a crowded inner hull is eliminated.  Recent U.S. depth charge tests have shown that whereas hull valves still form a source of trouble, compensating water piping leading from the head box to the  
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  REPORT 2G-21
  expansion tank as well as that within the tanks has withstood the tests with no damage, as have all other free flooding lines.  
          The disadvantages inherent in the German arrangement are those that would arise from a rupture.  In that it is necessary to run the compensating line through the top of fuel tanks, an undesirable practice.  Also, if necessary to pump a tank with a ruptured compensating line when operating the diesel engines on the surface, a void is made at the tank top that on diving must again be flooded through the ruptured compensating line.  These are not serious disadvantages and can be minimized by proper design.  
          a)  Fuel tank capacities  
Test Pressure
Normal fuel tanks
9340 ft3
17 psi
Auxiliary ballast tanks
934 ft3
355 psi
Gravity feed tank No. 1
20 ft3
8.5 psi
Gravity feed tank No. 2
20 ft3
8.5 psi
Fuel oil sump tank
46.7 ft3
14.2 psi
          b)  Fuel oil piping  
Test Pressures       
Tightness Strength
Between hull valves 455 psi 568 psi
Fuel filling 178 228
Fuel transfer 113 142
Drain, test and measuring 28.4 28.4
Service piping 56.8 56.8
          c)  Fuel oil service pump  
Type Gear pump
RPM 1338
Delivery 14.8 gpm
By-pass relief valve setting 11.4 psi
          The fuel oil stowage and handling arrangements on the XXI have been, in most respects, further simplified over the arrangements on earlier types.  The amount of piping within fuel tanks has been reduced by the elimination of overboard test lines and measuring lines.  The permanent piping within the filling and transfer system has been reduced to a minimum by the elimination of manifolds, the  
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  REPORT 2G-21
  elimination of a transfer pump, and the resort to portable fittings and hoses for handling emergencies.  The service system has also been simplified slightly by the elimination of the separate Junker's service tank and its associated piping.  However, some additional piping is required by the extensive fuel oil drainage system installed for this type.  
          This simplification is more readily acceptable when using gravity feed tanks than it would be with the fuel oil service arrangement on U.S. submarines.  
          Except for the minor defect noted in the text, the simplified compensating system arrangement is considered proper for this type of vessel.  The system provided continued service during submerged (snorkel) operation for any casualty that may occur to the exposed line in the superstructure.  Emergency pumping arrangement furnishes the necessary surface operation.  The extensive elimination of hull fittings and interior piping that results makes the simplification warranted.  
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