Chapter V

Section 1
Defensive Measures -- General

5101. Basic Defense -- The basic defense of the submarine lies in submergence. Submarines must be prepared for defense against:

      1. Depth charge, mine, and other forms of underwater explosive attack.

      2. Supersonic and listening screens.

      3. Aircraft attack.

      4. Torpedo fire.

      5. Small surface craft.

      6. Ramming.

Section 2
Basic Defense

5201. The following basic assumptions are doctrine for submarines making contact with other unidentified craft:

      1. That all surface ships and aircraft will consider any submarine as enemy.

      2. That all submarines should consider all surface ships and aircraft as enemies. Therefore, it is incumbent upon the submarine to submerge, if possible, otherwise to initiate identification.

      3. That a submarine normally should not initiate the identification signal unless attacked or unless it is vital to establish her identity in order to carry out her assigned mission.

5202. (a) The basic defense of the submarine lies in submergence. By going deep and running at slow speeds, it is unlikely that a submarine will be picked up by aircraft unless the submarine leaves an oil slick or air bubble.

      (b) Defense against surface craft lies in submergence if the enemy can not be evaded on the surface. Running silently at slow speed will reduce chances of detection by enemy sound listening gear. Observation of action taken by the enemy will lead to the most likely course for evasion.

Section 3
Defense Against Depth Charge, Mine, and Other Forms of Underwater Explosive Attack

5301. In general, defense against underwater explosive attack consists primarily of avoiding discovery, operating in a manner and at such a depth as to avoid striking the weapon involved, or so as to keep the total stresses below the strength of the already hydrostatically stressed hull, or outside the area of disturbance of the explosion.

5302. Generally-known mine fields should be avoided. When necessary to pass through them, they should be passed at a depth and time (dependent on the type of field, depth of water, rise and fall of tide, current, and surface patrols existent) so as to pass over or under the mine case.

5303. Explosive sweeps should be avoided by under-running.

5304. Net obstructions probably will contain contact mines and be protected by listening posts and surface patrols. They should be avoided.

5305. (a) Depth charge attack presents the gravest menace that submarines have to face in the open sea. To evade damage from this cause, operate to escape discovery either by being sighted or heard.

      (b) If conditions are such that the submarine can remain at periscope depth, a well placed torpedo in the enemy ship may prove to be the best defense.

5306. Depth charge attack may be expected at any time when in the vicinity of enemy fleet or detachment, or when in waters patrolled and controlled by the enemy. Such attacks on the submarine may be made by light screening vessels, submarine chasers, patrol vessels or by aircraft. The depth charge attack is the most damaging to which a submarine may be exposed. For a successful attack by depth charge the surface ship must know the approximate position of the submarine. This can be ascertained only by:

      1. Sighting part of the submarine or its wake.

      2. Contact and tracking by means of sound equipment.

      3. Sighting of the submarine by aircraft which communicates the position to surface craft, or conducts the attack itself, if equipped with aircraft depth charges.

5307. It may be expected that the following are the most valuable parts of the submarine susceptible to damage by depth charge attack:

      1. Depth and steering control mechanisms. Contactors of electrical controls are not particularly resistant to shock.

      2. Non-watertight motors offer no protection from incoming water.

      3. Except for later reinforced type, battery jars are susceptible to cracking from shock of depth charge attack. Leaking electrolyte will collect in battery wells and cause grounds in addition to being a potential source of chlorine gas. Its prompt disposal must be provided for.

      4. Leaks may be expected in the packing glands of the stern tubes, sound equipment shafts, etc.

      5. Bow and stern planes, which may become jammed from shock.

      6. Torpedo firing circuits, instruments, etc., may become temporarily disabled.

      Recent experience has demonstrated that loose articles may become missiles creating an additional and unnecessary hazard when depth charges or aircraft bombs detonate close to the submarine. It is of greatest importance that miscellaneous gear, tools, manifold wrenches, etc., be properly secured to reduce the number of loose articles to a minimum.

5308. When subjected to depth charge attack, the following, which are based on wartime experience aboard as well as the opinions of experienced submarine officers, should be accomplished:

      1. All sea chests secured and all sea and stop valves to outside connections closed. Sound gear sea chests may be kept open if equipment is being used for escape tactics, but prompt closing must be provided for, should extensive leaks develop. Watertight doors should be left open for ready communication.

      2. Antenna lead-in tube flappers closed.

      3. Personnel stationed near important electrical contact panels, circuits, and vulnerable installations in order that prompt repairs may be effected. All starting and control panels of auxiliary machinery should be made ready for easy access.

      4. Watertight covers installed for important motors and panels which may become exposed to salt water leakage. In this connection, machinery under soft patches may be subjected to salt water leakage.

      5. Be prepared to quick shift to hand control of bow and stern planes and vertical steering.

      6. Spare flashlights available in each compartment in case normal and emergency lighting circuits go out. All personnel shall be equipped with pocket flashlights.

      7. Facilities for disposing of acid, leaking into battery tanks.

      8. Preparation and organization for rapid "jumping" of leaking cells.

      9. If there is any doubt that marker buoys, deck gratings, etc., may become dislodged during depth charge attack, they should have been previously removed.

      10. Torpedo tube outer doors should be closed to avoid damage to torpedoes unless making an attack and near firing point.

      11. House periscope completely, unless being used at periscope depth.

      12. Keep main ballast tank vents open. Keep safety tank vent closed.

      13. Close depth charge switches.

      14. Only depth gauges which are necessary for control shall be in use; others shall be cut off from sea pressure. This should be normal procedure whenever in water where depth charging may be expected.

      15. Torpedoes in racks should be secured to prevent movement, in case ship takes large angles.

      16. Operate as noiselessly as possible. Commanding officers should know which auxiliaries must be stopped for this purpose. If being hunted by ships equipped with listening equipment only, as contrasted to echo-ranging equipment, it is a cardinal principle that absence of acceleration or deceleration of propellers is essential. A slow steady speed under these circumstances is preferable to alternate stopping and starting. Course should be continuously changed. If being hunted by ships equipped with echo-ranging equipment, formation of knuckles, changes of speed and changes in course will provide best chances for evasion.

      17. All personnel not on watch or specially stationed, get into bunks.

      18. Each compartment should be provided with a tool kit containing a hammer, pliers, wrecking bar, and pipe plugs to fit the main pipes in that compartment.

      19. Wrenches and hammers used should be silenced by taping with friction tape.

      20. Stop battery blowers and seal battery wells. Care should be taken that hydrogen content in battery-well does not reach dangerous limits.

      21. Depth to take in case of depth charge cannot be specified. This depends upon conditions existing at the time and must be left to the discretion of the commanding officer. It may be possible to stay at periscope depth to obtain information for escape or it may be

preferable to take deep submergence. It is always preferable for depth charges to explode above, rather than below, the submarine.
5309. Available data indicate the following in the case of a 300 pound depth charge: if it is detonated within 14 feet of the submarine it will probably rupture the pressure hull; within 28 feet, the submarine will be seriously disabled; within 60 feet, it will have a demoralizing effect on the crew which can be minimized by training, discipline and morale.

5310. Depth control will have considerable bearing on the success or failure of evasive tactics in connection with depth charge attacks. While deep submergence is a recourse available to assume the farthest position away from depth charges, it also enables the depth charge which is set for deep explosion to be effective at greater distances from the pressure hull. On the other hand, with the submarine at shallow depth and with depth charges exploding below, there is tendency to force the submarine up with possible complete broaching and exposure. Since all underwater explosive effects tend upward, if there is a choice, it is always preferable to have depth charges, mines, explosive sweeps, etc., explode above, rather than below, the submarine. A case is on record of a mine being exploded in direct contact with bow planes and the submarine still being able to return to port.

5311. Submarines operating in the vicinity of enemy ports or bases may expect that enemy harbors will be protected with anti-submarine nets and mine fields in conjunction with surface patrols. These areas should only be penetrated when the mission demands. Locations of swept channels and openings through barriers may be determined by observing, at periscope depth, the ingress and egress of enemy surface craft. Areas that are well lighted at night most probably indicate the presence of mine fields. The regular anti-submarine net can be expected to extend to the bottom, where the depth is 200 feet or less. It is probable that the net will be made of ⁵/8 to ³/4 inch steel wire with about 12 feet mesh. Some anti-submarine nets are made up of heavy steel bars designed to block passage. Another consists of chain and dangling wires primarily to foul submarine's propellers. Others are mine nets, in which a submarine struggling to free itself makes its presence known ashore, where shore controlled mines are exploded at proper places along the barrier. Either shore controlled or contact mine fields may be expected in the vicinity of nets. Nets may be expected to be patrolled by craft equipped with depth charges. If it is necessary to attempt penetration of a net or mine area, it should be done when the tide is running the strongest in the direction of advance. Net cutters, mine cable cutters and clearing lines should be in use, if available. The submarine should submerge until near the bottom, or to a depth below 200 feet; if there is a greater depth of water, then proceed at slow speed in an effort to penetrate without contact. It is best to strike nets as near their moorings as practicable, because there is less wire whose elasticity must be overcome, and because less surface disturbance will result. If it is found that penetration can not be effected, back clear before becoming broadside to the net, thereby fouling the propellers. If it is possible to do so, attempt penetration at night to provide for the contingencies that it may be necessary to surface in order to clear, after fouling a net. After passing the net, any unusual noise of cable or chain on the hull may be assumed to be from a mine anchor. Attempt to clear by maneuvering at slow speed, in order to avoid pulling the mine down to contact detonation. Ingenious uses of various types of nets may be expected. For example, the mine net is non-rigid and folds around the submarine, as it moves forward. A mine in the net detonates, when contact is made. Avoid ordinary fishing nets and stake lines of the drifting type. If any portions of them are carried along by the submarine, they reveal the presence of the submarine. Rough weather and heavy storms usually cause considerable damage to anti-submarine nets and mine fields. It is therefore advisable to attempt to penetrate an area suspected of being so protected, immediately after a storm. In penetrating mine fields, great advantage accrues if it is known what type of firing mechanism is employed, i.e., whether contact or antenna type, how old the field is and hence how effective, the state of tide and tidal currents, and whether planted as an anti-submarine or general field. If it is known that the field is for general defense, safer penetration will be accomplished (most likely) by running under, at full speed and good depth. If an anti-submarine field exists and it is known that contact mines are used, better success will be obtained by attempting passage on the surface during high tide with strong tidal currents.

5312. Harbor Defense. There are five fixed defenses used in detecting a submarine attempting to enter a harbor:

      1. The indicator loop.

      2. The controlled mine loop.

      3. The harbor defense listening gear.

      4. The indicator net.

      5. Supersonic set for boom gate vessels.

      The indicator loop is a warning device, but the controlled mine loop provides a lethal weapon as well as a warning device. Both are operated by magnetic influence. They will probably be de-energized while channel is being swept by magnetic sweeps as the magnetic sweeps interfere with the detector instruments. Harbor defense listening gear consists of sensitive elements mounted on the sea bottom and controlled from the shore. Indicator nets offer no definite obstacle, but give visual notice of the presence of submarines. Supersonic sets for the boom gate vessels are for the purpose of preventing an entry into a protected harbor when it is opened for the entry of friendly ships.

5313. Submarines, even though degaussed by permanent degaussing or wiping should at all times avoid charging or discharging in shallow uncertain waters, if it can be done, on account of fields set up which might affect magnetic mines or detecting loops.

      Magnetic Mines

5314. The magnetic mine is a ground mine. Degaussing coils are installed to reduce the effective magnetic force of a ship and thereby protect it against magnetic mines. The current use of moored magnetic mines in depths of 300 fathoms has been reported. Such mines are known to be feasible in depths up to 500 fathoms. It is incumbent on the commanding officer to keep degaussing coils energized whenever there is any possibility of the presence of magnetic mines.

      Acoustic Mine

5315. The acoustic mine has been developed during the present war. It is a ground type of mine carrying a very heavy explosive charge. A period delay mechanism requiring as high as twelve separate actuations, at least two minutes apart, is being used in some foreign acoustic mines. Instances have been reported where the second or third ship in column in a channel actuated the acoustic mine. The firing mechanism of this type of mine is actuated by sound energy radiating from a ship through the surrounding water. Propeller noises or supersonic "pings" may also actuate the acoustic mine.

Section 4
Defense Against Supersonic and Listening Screens

5401. The avoidance, or penetration, of a supersonic screen is dictated by the submarine's mission, the relative position of the submarine with respect to the line of the screen on initial contact, the position of the screen relative to the body screened, and the sound conditions in the area. These factors will determine whether it will be better, or necessary, to avoid the screen by a flank approach or to penetrate it by a direct approach.

5402. Information obtained, while operating with supersonic equipped screens during the past few years, indicates that the following tactics are the best so far evolved for penetrating undetected, or for escaping from supersonic screens:

      1. Endeavor to pass directly under a vessel of the screen on opposite course at slow speed and at depth of about 120 feet.

      2. Keep bow or stern toward nearest screening vessel, thus presenting smallest possible target.

      3. When there is a negative temperature gradient (cooling with depth) of 10 degrees difference between the surface and 100 feet, it may be possible to pass screens at periscope depth at a range of 2,000 yards or more.

      4. When propeller sound of screening vessel indicates the screen has slowed for listening purposes, the submarine should use silent running speed.

      5. Avoid high speed while in vicinity of screen except in attempts to escape, after being detected. Tracking of submarines is made difficult for surface vessels, and confusion to unseasoned personnel may be created, by large changes of course at speeds varying between three and seven knots. High speeds must be made in short bursts, or contacts can be easily maintained by the attacking destroyers. Submarine procedure, when detected, is to stand on at high speed on screen's course to create wake interference, then turn 90° (approximately) with from 10° to 15° rudder to remove submarine from disturbed area. For best results at this critical time, an alert submarine sound operator must gauge the proximity of approaching vessels by the intensity of propeller sound and rate of change of bearing, and keep the commanding officer continuously informed. When a surface ship is considered to be coming close, a sharp turn should be made to put attacking vessel astern followed shortly by a turn inside the enemy turning circle.

5403. If the submarine is detected prior to the time of delivering its attack or while observing the enemy, it shall use every effort to escape attack by the screen or its supports, while still accomplishing its task. Except with most favorable sea conditions, discovery at time of firing torpedoes is almost inevitable, and the submarine must begin escape tactics immediately thereafter.

5404. Tactics to avoid, penetrate, and escape the screen are based on knowledge of its presence, its location, and its movements. This information must be gained by utilization of all the submarine's listening facilities, by recording everything heard and plotting the data obtained and also estimating positions and movements of the screen. Thus a "sound-eye" view of the surface picture is gained, on which the submarine commander must base his moves.

5405. Vessels of the supersonic screens are capable of both echo-ranging and listening, but are most likely to resort to use of the echo-ranging facilities alone, although both will sometimes be employed together. Listening screens alone may be encountered. The submarine must be prepared for action against both types.

5406. The submarine commander must keep himself informed of sound conditions in the area of operations in order that full advantage may be taken thereof, both offensively and defensively. The effective echo range for any depth of projector and target down to 300

feet can be determined from range prediction curves, provided the local velocity gradient is known. For practical purposes, and in the absence of equipment to measure velocity directly, the velocity gradient may be obtained from standard curves using the measured temperature gradient corrected for constant pressure gradient and assuming negligible salinity gradient. (Reference:--Pamphlet entitled "Prediction of Effective Echo ranges", revised September, 1941, issued by Commander Destroyer Division Fifty.)

Section 5
Defense Against Aircraft Attack

5501. The best protection against aircraft is to avoid detection when on the surface or when submerged. Once discovered, the submarine finds the best protection against attack by deep submergence.

5502. When the mission permits, submarines should dive before enemy aircraft are encountered. This applies particularly to twilight periods and when in the vicinity of a large force or an enemy base.

5503. The use of protective paint, low speed at shallow submergence and during periscope exposures, deep submergence between periscope exposures, short duration periscope exposures with minimum periscope exposed, and avoidance of oil and air leaks, all must be employed to prevent discovery.

5504. All planes sighted by a submarine must be considered to be hostile and to have sighted that vessel. Submarine should immediately submerge to a safe depth, which depth will depend upon the clearness of the water, the state of the sea, and the percentage of clouds in the sky. If unable to dive, resort must be had to anti-aircraft fire and rapid high speed, cross wind, zig-zagging.

5505. After being driven down by aircraft, or when their presence can be expected, take precautions to avoid coming up in their vicinity without ability to dive again immediately. Search with radar before surfacing.

5506. When faced with the choice of failing to reach an attack position, because of submergence to avoid unidentified planes, or of proceeding further toward the attack position on the surface, the decision must be reached by weighing the assigned attack mission against the importance of remaining undetected and the risk of damage to the submarine.

5507. Lighter-than-air craft are potentially dangerous, but alert submarine lookouts should provide sufficiently early information to enable the submarine to avoid attack.

5508. Available anti-aircraft gun fire is used by submarines only if unable to dive for physical reasons (or the mission demands acceptance of the air hazard and requires maintenance of surface speed). In these cases the battery should be kept in condition for immediate use. The machine gun is the best defense against strafing attacks while high speed zigzagging is the only means of avoiding surface bombing attacks if unable to dive.

Section 6
Defense Against Small Surface Craft

5601. The offensive weapons of a submarine are torpedoes, mines, and guns. Submergence in time to avoid detection or attack, or both, is the best and therefore the primary means of defense. Evasion by use of high speed on the surface at night or in low visibility is preferable. Submarines will avoid engagement with aircraft when possible to do so.

5602. Unless the submarine is unable to dive, combatant ships should not be engaged by gunfire, except they be of the small patrol type (more dangerous because of their depth charges than because of their guns), definitely preventing the accomplishment of a vital mission. In the latter case, the probable chance and relative advantage of possible destruction of the patrol vessel must be weighed against the disclosure of the submarine's presence or position, and the probable damage that may be received. Gun attacks on important merchant vessels will normally not be made, as the submarine will usually find itself out-gunned and at a disadvantage. When engaging surface targets, endeavor to do so as a surprise and from a position that will be poorly protected by lookouts and where the minimum number of enemy guns can bear. Engage on bearings that will permit the best service to own guns.

5603. Conditions of Readiness. Main Battery. Keep telescopes installed at all times and guns boresighted for long range. Keep ammunition in magazine, but be prepared, when action is imminent, to advance ammunition as near gun(s) as permitted by readiness to dive without delay. In order to be prepared for surprise night gun attack, set deflection scale to compensate for drift and set range scale between 2,000 and 4,000 yards.

5604. In establishing and maintaining hitting range and deflection, use the simplest effective control methods against both aerial and surface targets.

5605. Against surface targets use ranging shots, spread in range, to establish hitting gun range. Maintain hitting gun range by spotting methods that cross and recross the target and that depend only on the determination of whether shots are short or over (in range).

5606. In general, procedure for submarine gunfire is very similar to local control of a single gun, or of local group control in the case of two-gun submarines, as on surface ships. The spotter and control officer are usually combined and communication with the battery should be as simple as possible. Absence of instruments and assistants, plus low spotting heights, complicate this problem.

5607. Considering the type of target against which a submarine may expect to effectively use its gun, ammunition supplied will normally be common and high capacity point detonating in such proportions as is found to be most useful from time to time. The high capacity point detonating projectile and fuse have been found effective against steel plating as follows:

      3" up to 1" thickness
      4" up to 1¹/2" thickness
      5" up to 2" thickness
      6" up to 2¹/4" thickness

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