18-1. The vulnerability of a submarine to damage from underwater non-contact explosions depends upon the shock resistance of its vital equipment and fittings at least as much as on the dynamic strength of its pressure hull. From the standpoint of vulnerability to underwater explosive attack, the ideal submarine can be considered as one with a hull having maximum damage-resistance for the weight of material devoted to it, and with all other equipment, appurtenances, systems, etc. of such strength or so well protected from shock that the pressure hull itself will be critically damaged before other vital components sustain serious damage.
18-2. The term "shock" implies a transient or sudden and violent change in the state of motion of the component parts or particles of a body or medium, resulting from the sudden application of a relatively large external force, such as a blow or impact.1 There are two distinguishing characteristics associated with shock motion: (1) the time duration of the motion is limited, and (2) generally the motion is non-periodic, that is, it does not repeat itself at regular intervals of time, as does, for example, a sustained steady-state motion such as a "forced vibration".
18-3. An object will acquire shock motion upon the application of a sufficiently intense force system whose charactistic vary with time. The nature of the motion will be determined by the nature of the applied force system and by the characteristics of the object, principally its mass distribution and elasticity. Shock motions may vary widely in character and type. The initiating force systems which produce shock motion on submarines when subjected to underwater explosive attack are the pressure wave or waves created in the water by the explosives (see Figure 15-1). It is fairly well established that, except for damage caused by the subsequent "whipping" of the hull, shock damage from underwater non-contact explosions is due almost entirely to the initial steep-fronted pressure wave created at the instant of detonation. Contrary to what might be expected, the most serious and extensive cases of shock damage to surface naval vessels have been the result of non-contact underwater explosions rather than contact explosions, probably because water is a more effective transmission agent than the structure of a vessel for distributing the shock throughout the vesssel. In submarines, only the problem of non-contact explosions need be considered since contact explosions would invariably cause lethal damage by rupture of the pressure hull and, in any event, the magnitude of shock damage resulting from non-contact explosions is greater than for contact explosions.
18-4. Upon application of pressure pulse loading to a submarine through underwater explosive attack, a very complex transient motion may be induced in the hull structure. Structural members may acquire a very high velocity which endures
for a sufficiently long time to impart a substantial displacement to, and within, these parts. The motion of these structural members is eventually arrested by other parts of the ship's structure and by their own internal damping and elastic properties. This results in a reversal of the initial motion, followed by a transient roughly periodic damped vibration which characterizes the return of the members to their original position in cases of elastic deformation or to some new position in cases of permanently distorted structure. The sudden initial acquisition of velocity is by definition characterized by a high acceleration acting for a short time. Since objects directly mounted upon these structural members are restrained to move with their supports, they must also reach closely equivalent velocities and as a result be subjected to high accelerations. These high accelerations can produce extremely high inertia forces in certain parts of the equipment. The stress effects produced by these forces may be varied and numerous, e.g., breakage of materials having low impact resistance, rupture of pipe joints, misalignment of machinery, malfunitoning of relatively delicate mechanisms such as circuit breakers, relays, tripping and latch mechanisms, and derangement of relatively delicate equipment such as gauges, meters, radio and sound equipment. The classic equation which determines the magnitude of these forces is F=Ma, where "M" is the mass of the equipment being accelerated and "a" is the acceleration imparted thereto. Instead of considering "a" as a peak value only, it should be regarded as varying with time. The force "F" therefore also varies with time. Multiplying the peak acceleration by the mass of the object being accelerated will give the true peak force applied to the object at that instant but an erroneous impression of the response of the object to this force may result unless the time duration of the acceleration is taken into consideration. For example, certain materials, such as medium steels, can withstand the high forces resulting from application of a high acceleration for a virtually instantaneous period of time but may fall if the time duration is increased, whereas other materials, such as glass, may fail when subjected to the brief but high acceleration only. The force "F" may induce stress in a member of the ship's structure due to accelerating a piece of equipment mounted upon such structure; it may result in a stress component induced in the casing or chassis of a piece of equipment due to a difference in acceleration of the components mounted upon such casing or chassis; or it may induce stresses in an internal component of a piece of equipment due to the relative acceleration of a sub-component. Damage to equipment may be reduced or prevented by interposing a flexible mount between the equipment and the structural member of the ship so that motion can take place between the structural member and equipment mounted thereon. The equipment is thus gradually accelerated to the velocity of the ship's structure or to some lower velocity, if at all; i.e., the acceleration lasts for a longer time at a reduced magnitude.
18-5. Because of the importance of shock resistance to submarines, consideration has been given over a long period of years to this factor in the design of submarines and their components. Research in this
field became more active in 1935 with the explosion testing of riveted and welded full scale caissons and has been continued since by this Bureau and by many naval field activities and private concerns. Included in the shock protection program for submarines were: (1) the further replacement of low impact materials with high impact materials; (2) use of resilient shock mounts, principally rubber, felt and molded phenolic material, for small items such a gages, lighting fixtures, electronics equipment, and resilient mounts plus common foundations for auxiliary machinery vital to submerged operation (in conjunction with sound-isolation); (3) use of fitted foundation holts and/or large diameter dowels plus common foundations to retain alignment and reduce the effect of impact forces for various auxiliaries not used in submerged operations and hence where sound-isolation was not required, and for massive equipment such as reduction gears, main motors, etc., for which resilient mounting was not easily adaptable; (4) shock mounting on resilient rubber mounts of the entire propulsion control cubicle; (5) careful detail design of equipment, where possible, to absorb deflection if necessary without failure, i.e., increasing the energy absorption ability of the equipment; (6) careful detail design of mechanisms to produce characteristics which were inherently shock-proof or which could be made so by the use of simple anti-shock devices; (7) proper location of equipment so that the most favorable supporting structure could be utilized, and arrangement of equipment so that ample external clearance was available for movement of the equipment as a whole, and ample internal clearance so that component parts, such as vacuum tubes, could move relative to each other without causing damage, and (8) numerous other improvements such as laminated main battery cell jars, substitution of silver-brazed and welded piping joints for cone and bolted joints, and such small details as lock washers to prevent nuts from backing off and anti-shock retaining devices for ferrule type fuses, vacuum tubes, etc.
18-6. As the National Emergency was at hand and the research work being conducted with caissons and models was not being completed as rapidly as deemed necessary, it was decided that the task would be completed more rapidly by tests with a full scale submarine. Accordingly, a series of such tests was conducted at Portsmouth, N.H. in 1941 with Tambor (SS198), Trout (SS202), Gar (SS206) for the purpose of determining the vulnerability of all of the features of modern submarines to underwater explosive attack. Each of the three submarines was submerged to periscope depth and subjected to the detonations of 300-pound charges of TNT placed at depths of about 60 feet and at varying ranges and orientation with respect to the vessels. The closest charges in all three of the submarine experiments were 33 yards from the hulls. These tests revealed important deficiencies and the data obtained from them by expert observers formed the basis of practically the entire shock protection program for U.S. submarines. Information from these tests, obtained before our entry into the war, influenced the course of shock-proof design on U.S. submarines more than the combined total of the subsequent voluminous war experience since very few deficiencies
were brought to light by actual war service that had not already been disclosed by the 1941 tests.
18-7. As a measure of the effectiveness of the various shock protection measures incorporated in our submarines, the controlled depth charge tests on Dragonet (SS293) during April, May and June of 1944 off Portsmouth, N.H., provide interesting comparative data.2 Explosive charge weights, number of shots, ranges, and depths of Dragonet and the charges were selected to be identical with the tests conducted on Tambor, Trout and Gar in 1941 except that the ranges were corrected for the increased thickness of hull plating on Dragonet so as to obtain approximately the same severity of shock loading. During all of the shots, Dragonet was submerged to periscope depth. Fifteen charges were fired at a depth of approximately 60 feet, the first containing 600 pounds of TNT and each of the others 300 pounds of TNT, and at various ranges from 1500 to 75 feet as shown in Figure 18-1 below. The depth of water was approximately 150 feet. The observed damage to the vessel and equipment for the various shots is contained in Table 1801 below.
Observed Damage to Dragonet
During 1944 Vulnerability Tests
Shot Location of Charge Damage 1-5 Charges 1500 to 300 feet from the midship section on the port side. No damage resulted. 6 Charge 200 feet from the midship section on the port side. During this shot various running lights and upper deck instruments broke. Within the ship, the water circulating pump of main motor No. 2 was thrown out of alignment. 7 Charge 150 feet from the midship section on the port side. No damage resulted. This was undoubtedly due to the fact that the charge was unintentionally fired close to the surface of the water, rather than at the usual depth, 60 feet. 8 Charge 138 feet from the midship section on the port side. The auxiliary gyro compass spilled mercury, causing the instrument to operate improperly.
A pipe hanger in the pump-room was broken.
9 Charge 100 feet from the midship section on the port side. The outer prism in periscope No. 2 was cracked and the instrument was jarred out of collimation.
Periscope No. 1 became defective on low power.
The selector switch for the conning tower lights was broken.
The vent of main ballast tank No. 1 leaked.
The gyro repeater lens on the bridge was broken, and the instrument was flooded. All gyro repeaters were affected.
10 Charge 75 feet from the after quarter on the port side. Vents on a number of main ballast tanks were jarred open.
Shot Location of Charge Damage 10
Charge 75 feet from the after quarter on the port side. Both forward and after battery cells moved. Some wedges and strongbacks were loosened. Slight fissures appeared in the sealing compound of some cells.
An SJ-1 radar rectifier tube, Type 5U4, in the pump room was broken. This tube was not shock-mounted.
The voltmeter on lighting motor-generator set No. 2 was broken. The voltmeter on motor-generator set No. 2 was sticking. This meter was mounted on the control cubicle.
The ammeter on main generator No. 1 in the engine room was sticking. The gyro repeater system was rendered inoperative.
A non-shockproof lamp in a shock-mounted light fixture in the after engine room was broken.
The rudder steering gear became defective.
Sound heads, both port and starboard, became noisy when training.
The radar training mechanism became noisy.
Contact makers on a number of vents gave wrong indications.
The port shaft transmitter was jarred out of alignment.
Fuse retainers on the radar panel were jarred out of place.
Mercury was splashed from the master gyro compass. In addition there were various air, hydraulic, and water leaks, and some equipment had shifted position.
Shot Location of Charge Damage 11 Charge 75 feet from the midship section on the port side. A rough-service light bulb was broken.
The telephone selector was not operating.
The telephone ringer was broken.
(The preceding three items were in the forward torpedo room.)
12 Charge 75 feet from the forward quarter on the port side. Only minor damage was reported. 13 Charge 75 feet from the midship section on the starboard side. The master gyro follow-up in the control room became inoperative.
A light bulb in the control room was shattered.
Three cells in the forward battery tank showed fissures in the sealing compound.
Threads were stripped from several strong back holding rods in the after battery tank.
14-15 Charges 75 feet from the after and forward sections, respectively in the starboard side.*
* No examination was made between Shots 14 and 15.
The hand training gear of the starboard sound head became stiff; the repeater was off 2.5 degrees.
The port sound head became noisy when training; the repeater became erratic.
The WDA motor-generator became very noisy, and vibrated excessively.
The RBO speaker in the forward torpedo room was broken.
A voltmeter in the forward battery room was broken.
Several strongbacks in the after battery tank showed signs of springing.
Shot Location of Charge Damage 14-15
Charges 75 feet from the after and forward sections, respectively, on the starboard side. The F.W. tank on the starboard side of the after battery room showed a pronounced displacement, and the hangar bolts were bent.
A hanger holding a resistance box in the forward engine room was broken
An ammeter on No. 1 main engine board in the forward engine room became sticky.
A voltmeter on No. 2 main engine room board in the forward engine room became sticky.
The after torpedo firing mechanism was not working.
18-8. Dragonet represented a submarine on which practically all of the numerous items in the shock protection program had been accomplished. It is therefore significant to note that the shock damage resulting from the 1944 tests on Dragonet was minor, even though the closest shot was fired at a range of 75 feet, whereas the 1941 trials of Tambor, Trout, and Gar, on which only moderate shock protection had been accomplished, resulted in relatively serious shock damage although the closest shot was fired from 100 feet. The Dragonet experiment demonstrates that considerable improvement has been made in the shock resistance of our submarines. However, it si known that the modern submarine can withstand depth charge attacks at ranges somewhat closer than the minimum range employed in the Dragonet tests.
18-9. Several cases are available from war experience which afford a rough means of estimating the shock resistance of our submarines when subjected to near destruction from underwater explosions. The damage narratives of Kingfish, Tunny, Salmon, Scamp, and Halibut, included elsewhere in this report, represent five instances where considerable permanent pressure hull deformation was sustained by submarines on which all or a substantial portion of the shock protection program had been accomplished prior to their damage. A study of that portion of the damage suffered by these vessels which can be attributed to shock alone indicates that while properly shock-mounted vital equipment remained for the most part unaffected, although the submarines in each case were on the verge of being destroyed through failure of their pressure hulls, further improvements in shock protection is necessary particularly
in piping and electrical systems. For example, damage to the control cubicles of Tunny and Scamp very nearly caused the loss of those vessels. It is interesting to note that in none of the cases cited above did sound-isolated equipment mounted on rubber compression type resilient mounts sustain any damage, nor has any report come to the knowledge of this Bureau, so far as can be determined, which indicates that equipment so mounted has ever received disabling shock damage on any U.S. submarine. This is significant since the problem of combining shock protection with sound-isolation is by its very nature a compromise, neither the shock protection nor sound-isolation reaching their optimum possible efficiency when provided concurrently by the same mount. War experience indicated that the degree of both shock protection and sound-isolation provided by properly mounting equipment on compression type rubber mounts is adequate. Various items of directly mounted machinery did sustain shock damage in all of the above actions, e.g., on Halibut the bedplates of the TBL motor generator, the ice machine compressor and the No. 2 air conditioning compressors were cracked. Damage to directly mounted auxiliary machinery has also occurred in cases where no pressure hull damage was sustained. In Tambor's (SS198) action of 3 February 1944, the bedplates of both high pressure air compressors in the pumproom were cracked around their complete periphery and both low pressure blowers were dislodged from their foundations. In Greenling's (SS213) action of 25 February 1945, the bedplate of the No. 2 high pressure air compressor was cracked. In Sargo's (SS188) action of 14 June 1943, the crankcase of the No. 1 low pressure air compressor fractured, disabling the compressor.
18-10. General remarks regarding the shock resistance of various components of our submarins can be drawn from war experience as follows:
(a) Pressure hull structure. (Excellent) Areas of stress concentration under shock loading appear to have been held to a minimum through careful detail design (based on experience gained in controlled explosion tests of full scale models).
(b) Propulsion Diesel engines. (Excellent) With few exceptions, Diesel engines have remained operable even on vessels sustaining severe pressure hull deformation in way of machinery spaces.
(c) Main generators and propulsion motors. (Excellent) No reports have been received where direct shock damage has disabled propulsion motors or main generators.
(d) Propulsion electrical control equipment. (Good) Several cases occurred where propulsion was disrupted due to derangement of control equipment.
(e) Main reduction gears. (Poor) Very susceptible to tooth damage, causing high noise level.
(f) Main battery. (Good) Laminated battery cells have occasionally cracked on vessels sustaining severe shock damage but the number where electrolyte was lost has in all cases been small.
(g) Resiliently mounted sound-isolated auxiliary machinery. (Excellent) No known cases of failure through shock damage.
(h) Directly-mounted auxiliary machinery. (Good) Several instances have occurred where directly mounted machinery has sustained fractured bedplates, misalignment, etc. However, directly mounted auxiliaries have in general been limited to those not essential for submerged operations.
(i) Piping. (Fair) Close attacks almost invariably cause piping leaks, frequently of serious nature.
(j) Auxiliary Power Systems. (Fair) Close attacks have frequently caused damage to or maloperation of relays, switches, controllers, etc.
(k) Torpedo tubes and firing gear. (Fair) Frequent shock damage has occurred to outer doors, gyro setting and depth setting spindles, stop bolts, guide studs, etc.
(l) Instruments, gauges, meters and light bulbs. (Poor) Close attacks generally caused extensive derangements to such equipment.
(m) Periscopes. (Poor) Periscopes were very susceptible to damage of disabling nature. Periscope derangements were reported in over 30 separate actions. In at least three instances, periscope damage forced termination of patrols.
18-11. The Bureau is cognizant of the need for further improvement in the shock protection of our submarines and is currently engaged in a research and development program toward this end. Particular emphasis is being given to correction of weaknesses disclosed by war experience for those components whose performance has been only fair or poor. It is apparent that shock protection is a continuing and expanding problem in view of the present tendency towards submarines of greatly increased hull strength for operation at extreme depths. However, shock protection is not the problem of the design and research agencies alone. It is incumbent upon building and repair activities and ship's force to install and maintain such protection in thorough and complete agreement with the vest and latest data available. Shock protection and sound isolation standards should be as vigorously observed in time of peace as in war.
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1. A detailed study of shock and shock protection methods is contained in NavShips Publication 250-660-26 entitled "Mechanical Shock on Naval Vessels".
2. David Taylor Model Basin conf. Report No. R-283 of April 1945 (Shock Measurements During Depth Charge Tests on USS Dragonet