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CONFIDENTIAL UNITED STATES FLEET HEADQUARTERS OF THE COMMANDER IN CHIEF NAVY DEPARTMENT, WASHINGTON, D. C.   January 22, 1943.        1.      This publication, "Characteristics of Enemy Submarines" contains the latest information available at the Headquarters, Commander in Chief, United States Fleet, on the capabilities and tactical limitations of enemy submarines. This information has been collected from various sources, but most of the details were furnished by the British Admiralty in reports of trial runs on the GRAPH (Ex U-570). Although little information is available on Japanese submarines, it can be assumed that their modern types have similar characteristics to those described herein. The Anti-Submarine Warfare Unit, U. S. Atlantic Fleet, has collaborated in the preparation of the text and the presentation of the diagrams of this publication.      2.      Upon receipt of this publication, the Commander in Chief, United States Fleet, Information Bulletin No. 9, "Elementary Notes on Submarines" and Chapter II, Commander in Chief, United States Fleet, Information Bulletin No. 14, "Anti-Submarine Warfare," are superseded and shall be destroyed by burning.      3.      This publication is confidential and shall be handled in accordance with the provisions of Article 76, U. S. Navy Regulations. It shall be given a wide circulation among all personnel engaged in anti-submarine warfare. Additional copies may be obtained from the Readiness Division, Headquarters, Commander in Chief, United States Fleet.      4.      When no longer required this publication shall be destroyed by burning. No report of destruction is required.     R. S. EDWARDS Chief of Staff.       III

CONFIDENTIAL   TABLE OF CONTENTS Page Part I.  Characteristics of Enemy Submarines - - - - - - - - - - -  1-1    Section 1. Introduction - - - - - - - - - - - - - - - - - - - -  1-1    Section 2. Personnel - - - - - - - - - - - - - - - - - - - - -  1-1    Section 3. General Outline of Hull Characteristics - - - - - -  1-2      Shape of Hull - - - - - - - - - - - - - - - - - - - - - - - -  1-2      Double Hull Explained - - - - - - - - - - - - - - - - - - - -  1-2      Compartmentation - - - - - - - - - - - - - - - - - - - - - -  1-2      Periscopes - - - - - - - - - - - - - - - - - - - - - - - - -  1-2    Section 4. Diving Capabilities - - - - - - - - - - - - - - - -  1-3    Section 5. Limitations - - - - - - - - - - - - - - - - - - - -  1-4      General - - - - - - - - - - - - - - - - - - - - - - - - - - -  1-4      Batteries - - - - - - - - - - - - - - - - - - - - - - - - - -  1-4      High Pressure Air - - - - - - - - - - - - - - - - - - - - - -  1-5      Breathing Air - - - - - - - - - - - - - - - - - - - - - - - -  1-5    Section 6. Difficulty in Sighting- - - - - - - - - - - - - - -  1-5   Part II.  Operations - - - - - - - - - - - - - - - - - - - - - - -  2-1    Section 1. General Organization of German U-boats - - - - - - -  2-1    Section 2. Early Operations - - - - - - - - - - - - - - - - - -  2-1    Section 3. Crash Dives of a U-boat - - - - - - - - - - - - - - -  2-1    Section 4. Lookouts - - - - - - - - - - - - - - - - - - - - - -  2-3    Section 5. Effectiveness of Anti-Submarine Action - - - - - - -  2-4    Section 6. U-boat Attack Tactics - - - - - - - - - - - - - - - -  2-5      The Pack Attack on Convoys - - - - - - - - - - - - - - - - - -  2-5    Section 7. U-boat Escape Tactics - - - - - - - - - - - - - - - -  2-7    Section 8. Useful Information - - - - - - - - - - - - - - - - - -  2-8      Submarines - - - - - - - - - - - - - - - - - - - - - - - - - -  2-8      Surface Vessels - - - - - - - - - - - - - - - - - - - - - - - -  2-8      Lights - - - - - - - - - - - - - - - - - - - - - - - - - - - -  2-8      Radio - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  2-8   Part III.  Characteristics of German Submarines - - - - - - - - - -  3-1    Section 1. Brief Details - - - - - - - - - - - - - - - - - - - -  3-1      Torpedoes - - - - - - - - - - - - - - - - - - - - - - - - - - -  3-1      Mines - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  3-2      Guns - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  3-2      Optical Equipment - - - - - - - - - - - - - - - - - - - - - - -  3-2      U-boat at Periscope Depth - - - - - - - - - - - - - - - - - - -  3-2      Radio Equipment - - - - - - - - - - - - - - - - - - - - - - - -  3-2      Tactical Data - - - - - - - - - - - - - - - - - - - - - - - - -  3-3      German Underwater Sound Apparatus - - - - - - - - - - - - - - -  3-4      Diving Characteristics - - - - - - - - - - - - - - - - - - - -  3-5      Submerged Cruising Radius - - - - - - - - - - - - - - - - - - -  3-5      Rate of Rise - - - - - - - - - - - - - - - - - - - - - - - - -  3-5      Chemical Tablets - - - - - - - - - - - - - - - - - - - - - - -  3-5      Surface Speed vs. RPM - - - - - - - - - - - - - - - - - - - - -  3-6    Section 2. Procedure for Salvaging a 500-ton U-boat - - - - - - -  3-7    Turning Data for Submerged 500-ton U-boat (U-570) - - - - - - - - 3-13   Part IV. Japanese Submarines - - - - - - - - - - - - - - - - - - - -  4-1    Section 1. Characteristics of Japanese Submarines - - - - - - - -  4-1    Section 2. General Outline - - - - - - - - - - - - - - - - - - - -  4-1    Section 3. General Orgsnization of Japanese Submarine Command - -  4-1   IV

CONFIDENTIAL     LIST OF EFFECTIVE PAGES Title Page- - - - - - - - - - - - - - -   I Promulgating Letter - - - - - - - - - -  III Table of Contents - - - - - - - - - - -  IV List of Effective Pages - - - - - - - -  V Part I - - - - - - - - - - -  1-2 to 1-5 inclusive Part II - - - - - - - - - - - 2-1 to 2-9 inclusive Part III - - - - - - - - - - 3-1 to 3-19 inclusive Part IV - - - - - - - - - - - - - - - - 4-1 Why they decided to put this section AFTER the first three sections I don't know.                                 V

CONFIDENTIAL   PART I  CHARACTERISTICS OF ENEMY SUBMARINES   Section 1  INTRODUCTION 1101.  Submarines most likely to be encountered in the Battle of the Atlantic are the German 500 and 750-tonners. Both types are seaworthy. They are manned by hardy young officers and men who have had a moderate amount of training, and a great deal of German propaganda build-up. They are confident of their ability and make a most determined adversary. It will require a persistent effort and daring on our part to overcome and break down their initiative. It should be noted here that their campaign up to the present time has been directed for the most part against unarmed merchant vessels. Attacks on this type of vessel, it must be remembered, are the easiest a submarine can have.   1102.  Other types that may be encountered are the German 1,060-tonners, Italian large type and captured Dutch and Norwegian submarines.   1103.  When sufficient information on Japanese submarines becomes available such will be published. Section 2  PERSONNEL 1201.  Living conditions on board a submarine are naturally crowded. German submarines carry between 30 and 40 ratings. However, the crew has a chance each day when submerged to get a good rest. Much effort is made to serve the best foods. The personnel all like submarines and take much pride in bagging as much game as they can. Do not be misled in regard to their invincibility. Submarines can be effectively combatted by persistent and well planned attacks.         1-1

CONFIDENTIAL   Section 3  GENERAL OUTLINE OF HULL CHARACTERISTICS SHAPE OF HULL  1301.  The general outline of all submarines is very much the same. German submarines have very clean lines. The long cigar-like pressure hull (over 200 feet) has a superstructure built upon its entire length. This is a platform to accommodate the gun crew and allow passage between hatches. The 750-tonners have two guns, one forward and one aft, while the 500-tonners have only one gun and it is forward. There is usually an AA gun built in with the bridge superstructure. The conning tower is in the center of the ship and usually about 10 feet high with a graceful curving windshield around the forward part for the protection of the small bridge crew when the submarine is on the surface. As seen at night, it is likely that only the conning tower would be seen. This area may be as small as 10 feet by 30 feet, and accounts for the difficulty experienced in sighting submarines at night. DOUBLE HULL EXPLAINED  1302.  The U-boats employed against shipping in the Atlantic are "double-hulled" submarines. That means that for the greater part of their length, a casing encloses the pressure hull. This light shell, some 2 feet 8 inches from the inner hull, extends to within a few feet of the keel and is divided into ballast or fuel tanks. When oil from a submarine is seen, it does not necessarily mean the submarine is destroyed or even seriously damaged, but that one or more of its fuel tanks may be ruptured. When such an oil leak is seen, this should be only an inspiration to carry out attacks more determinedly until evidence of destruction is obtained. COMPARTMENTATION  1303.  The pressure hull of U-boats is cylindrical, built of tough steel of great strength to withstand pressures at extreme depth. Ribs, bulkheads and the sides of tanks all add to its rigidity. All hatches, bulkhead doors, torpedo tube doors, glands for propeller shafts, periscopes, etc., are designed to withstand pressure and underwater blast. Gages, meters, lights, pumps and other instruments are shockproof mounted to preserve their usefulness after depth charge attacks.   1304.  U-boats are.cellular structures. They are divided into a number of compartments, separated by pressure tight bulkheads. Heavy circular doors are fitted for passage through bulkheads. To en sure the destruction of a U-boat, it is necessary to puncture the pressure hull and thus flood at least one of these sections. On the other hand, if the outer hull only is shattered, it is probable that it will suffer inconvenience, which may or may not, in itself, prevent diving. This should be kept in mind when making depth charge attacks.   1-2

CONFIDENTIAL   PERISCOPES  1305.  Submarines carry two or three periscopes for use when making attacks from depths just below the surface. These periscopes are long enough so that when they are extended, the pressure hull is below the bottom of shallow draft vessels. Two types of periscopes may be encountered. One is very thin, used in attacking; the other, a thicker one with better optical arrangement, is used for searching. Section 4  DIVING CAPABILITIES 1401.  A submarine can dive to periscope depth in a little over 30 seconds by taking in ballast water, using her hydroplanes and going ahead at about 8 knots. Submarines on patrol will sometimes run with some of their ballast tanks filled so that their decks will be almost awash. The silhouette will then be reduced to nothing more than the bridge structure and gun. In this condition, submarines can fire their torpedoes and then submerge (in about 35 seconds) to periscope depth thereafter gaining depth at the rate of 1.5 feet per second.   1402.  A submarine can dive faster with her rudder amidship, but may be expected to alter course rapidly after submerging.                 1-3

CONFIDENTIAL   Section 5  LIMITATIONS GENERAL  1501.  A submerged submarine's endurance is ordinarily limited by:  (a)  Capacity of the main storage battery.  (b)  Volume of high pressure air.  (c)  Condition of the breathing air. BATTERIES  1502.  A submarine submerged is propelled by her electric motors, the current of which is provided by storage batteries. These batteries are located under the living quarters forward and aft of the control room. During a depth charge attack, they may be the first to suffer casualties. The energy in storage batteries may last from two hours (at top speed) to 48 hours or longer at slow speed. A submarine stopped or bottomed uses very little energy from the battery. The main storage battery furnishes the U-boat captain with one of his major anxieties. When a battery is nearly exhausted, it takes about six hours to recharge completely.   1503.  The batteries are recharged with Diesel engines. This is usually done at night. It must be done on the surface. It may be done with way on or stopped. It can be assumed that a U-boat on passage will endeavor to keep a "full" battery.   1504.  A U-boat put down with a "low" battery will endeavor to surface and complete a charge as soon as it is considered safe to do so. In view of the above, it is emphasized that aircraft and surface craft should press home and continue attacks and remain in the vicinity of a diving U-boat as long as possible. Be ready to ram or sink with gunfire if the U-boat is forced to surface.   1505.  A U-boat diving with a "full" battery can cover some 75 to 95 miles at economical speed; at full speed the distance would be only about 9 to 15 miles. Crash dives, submerged attacks and evasion tactics all require bursts of high speed, which take a heavy toll from the batteries.   1508.  There are two methods by which a diving submarine can conserve electricity appreciably. One is to rest on the bottom. It is believed that this would not normally be attempted in a depth of more than about 300 feet owing to pressure on the hull (45 pounds per square inch for every 100 feet of water). Another might be to catch a static trim, i.e., to rest in suspension without moving.   1-4

CONFIDENTIAL   HIGH PRESSURE AIR  1507.  U-boats carry enough high pressure air to blow their tanks completely about three times. Some "air flasks" are carried inside the hull and some between the inner and outer hulls. A depth charge explosion may rupture some of these flasks or their fittings between the pressure hull and outer casing and cause huge eruptions. This does not mean that the submarine has been destroyed. Cases have occurred when a shortage of high pressure air has been a contributary cause of surrender, owing to excessive expenditure of air in maintenance of trim during attack by a surface vessel. BREATHING AIR  1508.  The so-called "fresh" air a U-boat takes down when diving would require revitalization at the end of about 20 hours.   1509.  U-boats are able to revitalize the air inside to make it breathable. Though these measures are effective, the air may become "unbreathable" from other causes such as (1) excess pressure caused by damaged air lines and inboard venting, (2) chlorine gas formed by acid from cracked battery cells mixing with sea water. Both causes could be the result of depth charge explosions outside of lethal range. There would be no surface sign of damage in such cases. This provides another good reason for remaining in the vicinity of an attacked U-boat as long as possible. Section 6  DIFFICULTY IN SIGHTING 1601.  Submarines may be expected to be camouflaged. Naturally, a dull black reflects less light than any other color and has been used by most submarines for years. However, in the North Atlantic some have been painted a combination of light green, pale brown and sea green-blue that has proved decidedly effective under conditions of low visibility and disturbed surface conditions. One enemy submarine sighted near ice fields was painted white, others have been painted gray.   1802.  Trimming down reduces the silhouette and makes submarines more difficult to see.   1803.  Submarines can be seen with clear 7x50 binoculars on a dark night with no moon at about 700 yards. Of course, with increasing light from the stars or moon, they may be spotted several thousand yards away. To see them, it is necessary to have good lookouts, well trained.   1-5

Part II | Part III

SOURCE:
National Archives & Records Administration, Seattle Branch
"Ship Files, 1940-50"

Transcribed by RESEARCHER @ LARGE. Formatting & Comments Copyright R@L.

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