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                                                                                                 16 April 1946  
From: Board of Inspection and Survey.
To: Chief of Naval Operations.
Via: (1)  Naval Inspector General.
  (2)  Bureau of Ships.
Subject: Submarine (Ex German) U-2513, U-873 and U-858; Items of basic and detail design recommended for consideration in the development of future submarines for the U.S. Navy.
References: (a) Bd I&S report on trials of U-2513, ser. 472P45 of 3 April 1946.
  (b) Bd I&S report on trials of U-873, ser. 40P45 of 5 January 1946.
  (c) Bd I&S report on trials of U-858, ser. 39P45 of 5 January 1946.
  (d) Navy regulations, Art. 417-(1).
Enclosure: (A) Composite performance curves of USS CUSK and U-2513 from data obtained recently during standardization at Providencetown, Massachusetts.
          1.        In compliance with reference (d) the following supplementary report on basis and detail design recommendations of the subject vessels is submitted herewith.  This report has been withheld from reference (a) to (c) pending completion of trials of all three vessels.  
          2.        In general the performance speed characteristics of the U-873 and U-858 do not depart appreciably from those of the U.S. Fleet type.  They are capable of moderately high surface speeds, 18 knots, with the usual slow submerged speed submerged 7 knots.  On the contrary the latest German type XXI, U-2513, has apparently sacrificed surface performance to obtain better performance submerged.  For example at 6 knots submerged, the U-2513 requires but one half the S.H.P. of other German types and the U.S. Fleet type with the corresponding reduction of fuel consumption when operating on the main engines using the Schnorchel, (35 lbs per mile for the U-2513, 70 lbs per mile for other types).  


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  Enclosure A is submitted as a matter of interest to illustrate the influence of the design features of the latest type high submerged speed type German submarine, (U-2513), on its performance as compared to the performance of a U.S. fleet type submarine, (CUSK), having the same surface displacement.  
                  Comparative characteristic values of the two submarines are listed in the following table:  
  U.S.S. CUSK U-2513
Displacement surface 1809 (normal) 1819
Displacement submerged 2428 2100
Positive buoyancy 34.2% 15.4%
Draft 15' 1-1/8" 20' 8"
Length overall 311' 9" 251' 10"
Beam maximum 27' 3-1/2" 21' 8"
Horse Power eng. surface 5400 2228
Horse Power motor submerged 2700 4500
Horse Power eng. submerged none 1500
Speed surface maximum 20.3 13.7
Speed submerged maximum (motors) 9.9 15.6
Speed submerged maximum (eng.) none 9.0
          3.        The following departures in design from the U.S. Fleet type submarine were noted in the U-2513, this type being used for discussion since it includes all the features of design of interest on other German types and those features which may contribute to the unusual performance of this type noted in enclosure A.  
          a.        Hull  
                     1.  Form - The hull is tapered from a bulbous bow to a fishtail stern.  Six torpedo tubes are located forward and all torpedoes are stored forward in a large torpedo room.  No stern tubes are provided.  
                    Streamlining is obtained by greatly reducing the superstructure and conning tower, by eliminating protuberances such as anchors, deck guns, periscope extended supports, life lines, cleats, stub masts, etc., and by fairing as noted, in rounded corners of superstructure and conning tower and deep fillets, enclosing AA guns in fairwater, covering bridge opening (allowing only shoulder holes), streamlining Schnorchel and cutting narrow horizontal flooding holes in superstructure.  Periscopes, radar and Schnorchel are completely housed in fairwater.  
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                  2.  Buoyancy - No fuel oil ballast tanks are provided, therefore surface operations are always conducted in emergency condition which aids in quickening the dive.  So called "buckets" are added to the pressure hull to accommodate additional main storage battery cells, giving deeper draft in surface condition, thus slowing surface speed.  Propellers are correspondingly deep which should tend to reduce cavitation.  
                  3.  Balance - It was noted that this vessel handled easily submerged with only the use of relatively small stern planes at all speeds from dead slow to flank speed, and without regard to amounts of vertical rudder used.  The drag of the conning tower, periscopes and Schnorchel had no effect on control.  Planing effect of decks had little effect on control on all angles observed.  This may be attributed to proper study of moments imposed by protuberances and also in a lesser degree to the effect of a large stabilizer plane located forward of stern planes.  This excellent control greatly reduced the drag which might otherwise reduce the speed and economy.  
          b.        Engineering  
                     1.  Electrical - The main storage battery has an estimated 40% greater kilowatt capacity than that of a U.S. Fleet type.  The battery is divided into six groups; three groups are permanently connected in series to give a port and starboard battery with a total voltage of 350V and the 1 hour rate.  The battery is reported to be relatively short life, 18 months.  It is believed that life of battery is sacrificed to give higher kilowatt capacity per pound weight.  
                     The shafts are driven by two main motors with positive jaw clutch and by two belt drive auxiliary motors, sound isolated and capable of driving the submarine at creeping speeds from 0 to 6 knots.  The double armatures are paralleled to give flank speeds submerged.  The motors are used as generators for charging batteries, no main generators being provided.  
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                  2.  Engines - Due to poor surface speed characteristics it is possible the engine power (weight) was sacrificed for high battery capacity and additional weight required for Schnorchel installation.  However, it is felt that the power is insufficient for quick battery charge which now requires about 17 hours for full charge.  High power would increase surface speeds up to 15 knots as a limit (4000 SHP), and increase speed of Schnorchel operations which is now limited by exhaust temperatures, (m.e.p.).  
                   The engines are directly coupled to shafts through a Vulcan clutch.  This gives high efficiency and weight saving over electrical control on surface but materially reduces speed of Schnorchel operations by limiting m.e.p. due to drop in r.p.m. per S.H.P. submerged.  For submerged operations on main engines a Schnorchel type induction and valve is used.  Four cycle engines presumably were used to work against high back pressures, both for Schnorchel operations and for blowing the main ballast tanks.  This type of engine is reputed not to smoke on starting or change of loads as is often the case on two cycle types.  
          c.        Other Features of Design  
                     1.  Torpedo storage is provided with power operated transferring and reloading equipment.  
                     2.  Closed forward and after trim system with water measuring gage for trimming without use of pumps during silent running.  
                     3.  Fixed eye level periscope, hydraulic operated with hand vernier control.  
                     4.  Junkers Diesel driven opposed floating piston type high pressure compressors.  Exhausts into main engine intake during Schnorchel operations.  
                     5.  Flat disc type engine exhaust valves, do not stick and are easily maintained.  
                     6.  Simple and positive electric steering gear, control by small hinged handlebar or push down contactors mounted in small box.  
                     7.  Circular concentric doors in transverse bulkheads for strength provided with interrupted thread dogging requiring only 180° turn of locking handle to secure.  
                     8.  Traveling cars over the battery cells to facilitate taking gravity readings and watering.  
                    9.  Large vents and scoops in forward ballast floods quickly and reduce diving time.  
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m.e.p. = Mean Effective Pressure The average (mean) theoretical piston-top pressure that would result in the measured power output of an engine.
S.H.P. = Shaft Horsepower The amount of mechanical power delivered by the engine to a propeller shaft.  Note:  the type XXI engines were designed to deliver 1970 HP (supercharged) or roughly 2000 SHP each, for a total of 4000 SHP.


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                     10.  Engine exhaust used to dispel water from main ballast in lieu of low pressure pump.  
                      11.  Underwater exhaust for main engines, (surface) to reduce noise and smoking.  
          4.        As compared with the standards of the U.S. Fleet type submarine the general arrangement and workmanship was poor.  Conveniences were decidedly lacking particularly berthing, messing, washing and toilet facilities and embellishments such as manifold covers, kick plates, linoleum decking, heavy metal partial bulkheads, etc.  Apparently to save weight and reduce man days in construction the Germans had eliminated many emergency and safety features.  
          5.        In conclusion, the Board desires to point out that in the type XXI submarine as exemplified in the U-2513, the Germans developed a high submerged speed submarine capable of considerably improved attack and evasive tactics.  It presents a weapon of more advanced design, for the medium it works in, than any heretofore developed.  
                    The elimination of gun armament precluded any surface engagement.  It is reported that the 20mm AA battery was installed to repluse air attacks only during the period this submarine was forced to the surface in order to negotiate their own minefields.  Bridge arrangement indicates that few lookouts were used when surfacing, evidently the submarine was not intended to surface where danger of detection was imminent and then radar was depended upon to replace lookouts.  This appears to indicate that cruising submerged on main engines to avoid detection by enemy radar (Schnorchel coated with anti-radar compound) was intended for protracted periods.  This emphasized on enclosure A (in red) which shows the vast improvement in economy of the U-2513 compared with other German types during Schnorchel operations.  
                  It is interesting to note that had the main engine power produced 4400 S.H.P. the propellers would have given the same number of r.p.m. for surface and submerged operations as shown in dotted lines on enclosure A, thus making available full engine power for both surface and submerged operations with a direct connected engine without exceeding the m.e.p.  
                                                                                       LEIGH NOYES  
                                                                                     Rear Admiral, USN  
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