CONFIDENTIAL REPORT 2G-21
S40
     
 
FORMER GERMAN SUBMARINE TYPE XXI
 
 
 
 
MACHINERY PLANT
 
     
 
SUMMARY
 
     
          The hull form, and machinery and battery arrangement on the type XXI have been designed for high sustained underwater speeds.  This has been accomplished at a definite sacrifice in surface operating characteristics, notably in maximum sustained speed and in ship handling.  
          The electrical part of the plant - the main motors, the creeping motors and the battery - has proved most satisfactory.  However, major difficulties arising from the original diesel engine installation have reduced the satisfactory output to approximately 40% of designed performance and resulted in a serious unbalance in the machinery setup.  
     
     
     
     
     
     
 
April, 1946
 
 
 
 
PORTSMOUTH NAVAL SHIPYARD, PORTSMOUTH, N. H.
 
     
     
     
 
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  REPORT 2G-21
S40
     
 
MACHINERY PLANT
 
     
  1.  INTRODUCTION  
          Numerous conflicting statements have been made in various existing documents concerning the propulsion characteristics for this type of vessel.  This report will attempt to correlate the available information and present as accurate a determination of the design and operating characteristics as is presently possible.  German test data taken from actual runs over the measured mile and given in "Tests for Type XXI Submarines" (Typ-Erprobungsplan für U-Boots - Typ XXI) is used to substantiate views expressed in this report.  
     
  2.  GENERAL DESCRIPTION  
          The machinery plant on the type XXI submarine is built around a geared drive propulsion arrangement, on each of two shafts, as shown on plate I.  
          The basic design (on which German trial data is available) incorporated a large battery of 372 cells, two 2470 HP (2500 PS) main motors, two 111 HP creeping motors and two supercharged 1970 HP (2000 PS) diesel engines.  The main motor and diesel engine on each side are separately geared to the main shaft while the creeping motor is connected to it by a V-belt drive.  Adequate clutches are provided to give maximum flexibility in the use of this arrangement.  
          On trial runs the propulsion motors developed their rated power.  When using the main motors, submerged speed in excess of 16 knots were obtained.  It should be possible to maintain this speed during a one hour battery discharge.  Surface and submerged speeds up to 6 knots have been obtained when running on the creeping motors.  The cruising range based on one battery discharge is 365 miles at 5 knots or 110 miles at 10 knots.  
          The diesel engines, on the contrary, have proved a major weakness in the vessel's design.  On actual trials the output was limited to 1700 HP (15 knots) by excessive exhaust gas temperatures.  Furthermore, the exhaust driven supercharger when used during snorkelling operations proved hazardous and was subsequently removed.  The diesel without supercharger is able to operate only with snorkel cams in, with a  
     
 
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  REPORT 2G-21
S40
     
  resultant limiting output of 850 HP (12 knots) before excessive exhaust temperatures are reached.  As it is necessary to furnish approximately 1200 HP for 4 hours to the generators for charging each of the two batteries at the normal charging rate, the time required for a full charge is excessive.  This necessarily places a major handicap on the vessel's operation and decreases the value of the higher submerged speeds.  
          The excessive number of clutches required for this arrangement in order to provide the flexibility desired is a further disadvantage.  Continued upkeep on these and their associated synchronizing gear is necessary and much care in operation is required.  Also, with the installed arrangement of clutches (See Plate I) it is necessary when using a diesel for propulsion alone to rotate in addition to the main shaft the main motor reduction gear and armature. However, it is readily possible with this setup to put additional motor power on the main shaft to augment the diesels.  
          Tests show a favorable surface propulsion characteristic existing between 16 - 18 knots.  The horsepower vs speed curve flattens out appreciably over this range - on actual test an increase in 1126 HP brought about an increase in surface speed from 15.11 to 18.08 knots.  This top speed has been attained with the use of the main motors running at full power, or with the main motor boosting the diesel engine while the latter is running at its highest output.  In this latter instance, the batteries furnished current for the main motors and auxiliaries at a 6-hour discharge rate.  
          The creeping motors furnish a valuable addition to the propulsion setup.  They not only give the desirable quiet operation during evasion tactics, but also furnish valuable surface and submerged propulsion while utilizing the diesels for charging batteries.  The fact that they can be operated on 120 V permits a high propulsion efficiency over their entire speed range with a resultant reduced fuel consumption.  
          Compared with U.S. designs, a very small number of auxiliaries have been installed for use with the main propulsion units.  To take care of special contingencies, several of the pumps provided are set up for use on several different systems.  This is particularly true of the various hand pumps.  Blank couplings exist on all systems permitting use of flexible hose connections in emergencies.  This flexibility in use of pumps can result in contamination of systems and is not recommended.  
     
 
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  REPORT 2G-21
S40
     
  3.  INDIVIDUAL COMPONENTS  
          Designed and actual performance data on the various units are summarized below:  
          a.  Diesel Engine  
 
Designed
Actual
HP
RPM
HP
RPM
Speed
With supercharger
1970
520
1700
490
15
Snorkelling to charge batteries (Exh. - 2.6 psi)
1085
430
930
470*
6
Without supercharger (snorkel cams in)
1185
470
850
350
12
  *  Creeping motors used for propulsion
 
     
          b.  Main Motor  
 
 
Design
Actual
 
HP
RPM
Volts
Amps
HP
RPM
Volts
Amps
Speed
As motor
2470
1675
360
5500
2330
1675
326
5964
18.0
As generator
1840KW
1550
450
4080
  Diesels of insufficient power
  *  Submerged run
 
     
          c.  Silent Motor  
 
 
Design
Actual
 
HP
RPM
Volts
Amps
HP
RPM
Volts
Amps
Speed
 
111
350
360
260
*127
292
365
298
6.1
 
1.02
91
120
78
   
  *  Submerged run
 
     
 
  d. Battery
Design
    No. of cells 6 groups of 62 cells
    Capacity of each cell (1 hour 8 minute disch. rate) 5530 amps  1.60 Volts  86 F
 
     
 
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  REPORT 2G-21
S40
     
 
  e. Reduction Gears
Gear Ratio
Eff.
    Main Motor
5.079:1
97.5%
    Diesel Engine
1.62:1
96.5%
    V-Belt
2.68:1
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                           * Includes Vulcan Coupling  
     
 
  f. Propeller
    Diameter
7.05 ft.
    Pitch
6,96 ft.
 
     
  4.  CONCLUSIONS  
          For a submarine that is to operate as a pure submersible, the designed characteristics of the power units used in conjunction with the large battery on the XXI appear properly balanced.  In the German design primary consideration was given to supplying main motor propulsion using a minimum of weight and capable of taking the maximum battery output.  
          Further but secondary consideration was given to obtaining diesel engines that would give as much power for charging and propulsion as possible from the remaining allotted machinery weights.  Had the rather optimistic design ratings of the diesel engines been obtainable on the completed submarine, a satisfactory plant performance would have resulted.  
          However, because of the failure of the diesel engine to meet its designed rating and further, because of the necessity for the removal of the exhaust driven supercharger, a serious weakness in the vessel's operating characteristics has resulted.  The main motors and batteries, on the contrary, have successfully operated at their designed ratings with full reliability.  This fact, coupled with the low output of the diesels, has created an unfortunate unbalance in the machinery plant.  
     
     
     
     
 
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