15-1. The principal Japanese anti-submarine weapons were the depth charge and bomb, both designed to achieve their destructive effect by non-contact underwater explosion in close proximity to the target. An explanation of the behavior of such non-contact underwater explosions has therefore been included in this report.
Behavior of Underwater Non-Contact Explosions
15-2. The action of an explosive is the result of a purely internal chemical conversion which liberates a large quantity of gas and heat within a very shorty period of time. The total quantity of energy produced by the detonation of a conventional explosive is much less than that given off by combustion of equal weight of any of the standard fuels. However, the rate at which the energy is released, due to the velocity with which the reaction travels through the explosive, is very much greater. Detonation is an action characteristic of high explosives. Substantially complete and virtually instantaneous transformation of a high explosive charge into energy and decomposition products is called a "high order" detonation. All underwater explosive weapons are intended to detonate high order.
15-13. The sequence of events which occurs in the water as the result of an underwater explosion is of considerable interest. When a high explosive material, such as used in a depth charge or depth bomb, is detonated underwater, the initial action that occurs is the creation of a detonation wave which travels through the explosive material at a very high velocity, approximately 20,000 feet per second, and coverts the solid charge to a dense gas at a high temperature and pressure without appreciable change in volume. It has been estimated that the initial pressure of the gas bubble resulting from underwater detonation of explosives having properties similar to TNT is of the order of one to two million pounds per square inch. Under this tremendous initial pressure, and considering the detonation to have occurred at a sufficient depth so that surface effects do not appreciably alter the subsequent phenomena, the gaseous product of the detonation then suddenly starts to expand, suddenly compresses the water around the charge, and thereby sets up in the water a shock wave. The shock wave consists of the shock front, which is a discontinuity of pressure typified by an almost instantaneous pressure rise, followed by an approximately exponential decay of pressure with time and by, theoretically at least, a long tail of pressure too low to be of practical interest (Figure 15-1). This shock wave travels faster than the subsequent expansion of the gas bubble, leaves the vicinity of the explosion and is propagated radially outward in all directions (Photo 15-1). Except for the regions of very high pressure in the immediate vicinity of the explosion, the shock wave obeys the laws of spherical propagation of sound reasonably well, i.e., as the wave advances it peak pressure falls off a little faster than the inverse of the distance from the center of the explosion, the positive time duration of the pressure pulse increases slightly with increasing distance, and the wave travels only a little faster than the speed of sound in water (about 4900 feet per second). The water, after the wave has passed, remains substantially undisturbed. This initial shock wave or pressure pulse, the positive pressure portion of which lasts for only a few milli-seconds in passing any particular point as it travels radially outward, is believed to account for the major portion of the destructive effect of a
close underwater detonation and all of the remote action or destructive effect of a distant underwater detonation. When received from a considerable distance, the shock wave is discernible as a "metallic ping" on a submarine pressure hull. In a general way, the cause of damage to a ship from underwater explosions may be thought of as the sudden delivery at the ship of energy at high potential, i.e., the energy delivered by the shock wave. The energy contained in the primary shock wave is approximately one-quarter of the total energy released by the detonation of the explosive charge.
15-4. Simultaneously with the dispatch of the shock wave, the enormous initial pressure created by the transformation of the explosive charge into gas imparts a high velocity in a radial direction to the surrounding water and the gas bubble starts to expand outward with an initial velocity greater than that of an ordinary gun projectile. The kinetic energy thus imparted in mass flow to the water may be a significant damaging factor in very close explosions but is otherwise of only academic interest since the water velocity falls off rapidly with distance from the center of the explosion. Because of the high momentum imparted to the water, the bubble expansion continues beyond the point at which the gas pressure within equals the absolute hydrostatic head of the surrounding water and atmosphere. Eventually, however, the opposing external hydrostatic pressure brings the expansion to a halt, the pressure within the bubble at this point being of the order of only a few pounds per square inch, and reversed flow of water commences inward toward the center of the gas bubble. The gas pressure then increases rapidly as the size of the bubble diminishes, but the motion again overshoots the point of hydrostatic equilibrium due to the inward momentum of the water. At the extreme compression on this first contraction the gas pressure within the bubble reaches a peak estimated to be about 10,000 pounds per square inch. This high pressure initiates a second pressure wave or pulse in the surrounding water, having maximum intensity at the time of minimum bubble diameter. While of considerably less intensity than the initial shock wave, this second pressure wave is of many times greater duration and consequently may be a significant damaging factor to some types of targets.
15-5. The cycle of expansion and contraction may then repeat several more times in the manner described above until all of the energy has been dissipated or the bubble has either vented itself by breaking the surface of the water or has broken up into numerous small bubbles of no consequence. Due to energy losses, the bubble oscillations rapidly dampen out, successive maximum bubble sizes becoming smaller and successive minimum bubble sizes becoming larger. With each successive contraction, pressure waves or pulses of progressively decreasing intensity will be emitted. Photo 15-2 is an excellent high-speed camera study of gas bubble oscillations produced by the underwater detonation of a No. 8 blasting cap, roughly equivalent to 1/2 gram of tetryl. Figure 15-1 below depicts the phase relationship between the gas bubble oscillations and the pressure waves.
Figure 15-1. Phase relationship between gas bubble oscillations and the resultant pressure pulses transmitted through the surrounding water. The dotted lines indicate a condensation of the time scale. Time scale used for initial pressure pulse is expanded compared to that used for second and third pulses.
15-6. A secondary effect associated with an underwater explosion, which may in certain circumstances by of importance, is that of pressure wave reflection from the surface or from the bottom. If an explosive charge is detonated at a location such that either the sea surface or the bottom is at a distance comparable to or less than the distance to the target, then reflected waves from either the surface or the bottom may greatly intensify or diminish the direct pressure wave at the target by interference. The pressure wave is reflected from the sea surface as a tension wave, subject to the restriction that water can support only a limited negative pressure prior to the occurrence of cavitation. If the detonation is shallow, surface reflections can therefore very materially diminish the main pressure pulse by interference .However, if the detonation occurs close over a hard ocean floor, i.e., rock, sand, etc., the pressure pulse upward and to the sides will be considerably increased over that for a similar detonation in free water where the ocean floor is relatively distant, the amplification being caused by reflection of energy from the hard bottom. The pressure pulse will theoretically be doubled for charges which are detonated on a perfectly rigid surface. This factor would appear to operate in favor of anti-submarine forces attacking a target running close to hard ocean floors.
15-7. The surface phenomena over an underwater explosion is of interest. Three intrinsic effects can be noted under certain conditions.1
(a) Almost simultaneously with the explosion, the surface of the water is agitated by the pressure wave and a light spray may be thrown up. This effect is not noticeable if the explosion is very deep.
(b) During the first second or two after a shallow explosion, the water rises in a flattish "dome" which is often white in color and may attain a height of 50 feet or more. As the depth of the explosion is increased, the maximum height of the dome diminishes, and finally, at relatively great depths, no dome is formed (e.g., there is none from 300 pounds of TNT 150 feet deep).
(c) Plumes of spray may be thrown up by the gaseous products of the explosion venting through the surface. If the charge is only a few feet below the surface, the plumes break through the dome while the latter is still rising, and may attain a height of many hundreds of feet. As the depth of the explosion is increased, the plumes become less marked and also appear later; they may break through the dome at the instant the latter has attained its greatest height, or when it is sinking again, or the plumes may not appear until after the dome has disappeared. Finally, at great depths, no plumes are formed, but a minute or so after the explosion a mass of creamy water pours up to the surface.
15-8. The phenomenon of repeated impact blows resulting from single underwater explosions, due to the intense pressure waves created by succeeding bubble contractions, was frequently noted by submarine personnel during the war.2 The outflow of water ahead of the expanding gas bubbles has also been commented on in numerous war patrol reports, being typically described as a "rush" of water about the ship or through
Photo 15-1: German photograph showing gas bubble and shock front 0.000014 second after detonation of 30 milligrams of explosive similar to TNT. This photograph is the result of a double exposure, the first being taken before detonation to show the exact position and shape of the explosive charge.
Photo 15-2: High speed photographic history of gas bubble oscillations resulting from detonation of a No. 8 blasting cap underwater. This series of photographs is from David Taylor Model Basin conf. Report No. 512 of May 1943.
the superstructure which could be distinctly heard inside the pressure hull after depth charge attacks. There are many cases on record where submerged submarines have been forced up or down due to these water movements, occasionally by as much as fifty feet. For example, during her fourth war patrol, Balao (SS285) was forced down from 390 foot depth to 590 foot depth by a series of close depth charge detonations, each detonation causing the boat to increase depth by about 20 or 30 feet. Hawkbill (SS366), in her action of 18 July 1945, was forced up from periscope depth by depth charges detonating under the hull and broached with a 20 degree up angle, exposing 60 feet of the bow. Various submarines have been heeled over by underwater explosions. Grenadier (SS210), for example, reported a 10 to 15 degree heel as a result of a close bomb detonation (paragraph 8-5). The following description of depth charge detonations have been excerpted from various U.S. submarine war patrol reports and are representative of how such explosions sound to submarine personnel. Bluegill (SS242), while being depth charged at 340 foot depth on her third patrol, reported "The characteristics noted on far depth charges . . . were first a detonator click . . . and then bang!" Bluegill also described the more distant explosions as "a deep rumble increasing to the final boom." Cabrilla (SS288), while being depth charged at about 350 foot depth on her fourth patrol, reported "On almost all depth charges the familiar small detonation was followed by a large explosion and then swishing and falling waters were very evident". Salmon (SS182), while being depth charged at 150 feet depth during her first war patrol, reported for relatively distant depth charge that "As each charge went off there would be two pings on the hull which sounded as though two small nuts had been dropped, then after a period of one to two seconds the depth charge would be heard". Flying Fish (SS229), depth charged at 250 foot depth during her second patrol, compared a depth charge detonation to a "thud similar to that expected when a 100-pound weight is dropped on the hull" followed shortly by the sound of the explosion and then "water would be heard rushing through the superstructure". Pargo (SS264), depth charged at about 350 foot depth during her fourth patrol, reported depth charge detonations which "rocked the boat in three separate and distinct pressure waves" and with a sound effect of "whoom -- whoom -- BO-om". Others described the sound of depth charges exploding in the distance as like a child playing with a toy hammer, very mild and weak, and the sound of fairly close charges as a "hard metallic click-bang". The metallic "click", "thud", or "ping" may be interpreted as the mechanical response of the hull structure to the arrival of the pressure pulse from an explosion and should have been distinguishable from the subsequent audible frequencies generated by the explosion, i.e., the "bang" or "boom", only for an explosion relatively distant from the hull. The increment of time between the arrival of the pressure wave and the audible frequencies will not be perceptible at very close ranges, the two arriving practically simultaneously. That this is so is evidenced by the fact that submarine personnel considered themselves safe if the "click" preceding the "boom" was distinguishable, since they found by experience that the depth charge would then detonate at a range which would not cause much, if any, damage. Contrary to the general impression, the pistols of Japanese depth charges contained no mechanism which could cause a "detonator click". The
sound of "falling waters" referred to by Cabrilla was no doubt caused by the surface effects of the depth charge detonations. The "two pings" referred to by Salmon were perhaps due to a bottom reflection of the pressure pulse striking the hull shortly after the arrival of the direct pressure pulse.
15-9. The mechanics of damage by underwater explosions are not sufficiently well understood at this writing to permit precise evaluation of the relative effectiveness of the initial shock wave and each of the succeeding pressure waves in producing damage. It is known that for most practical situations, the initial shock wave of a non-contact underwater detonation can be considered the principal agent of damage. However, it has been demonstrated that under certain conditions it is possible for the additional pressure waves from succeeding compressions of the gas bubble to contribute further damage. For example, deep sea explosion tests by the Underwater Explosives Research Laboratory of the Bureau of Ordnance on small-scale (1/23) partial-length models, representing simplified SS285 Class submarine circular hull sections, showed that under combined hydrostatic loading (due to depth of the water) and explosive loading a well defined critical charge distance existed, such that a smaller distances general collapse of the test cylinder occurred whereas at greater distances only small local damage was observed.3 With the explosive charge just within the critical distance, it was found that general collapse did not occur until the arrival of the pressure pulse from the first contraction of the gas bubble. With the charge at approximately two-thirds this critical distance, however, the initial shock wave alone was of sufficient intensity and duration to cause collapse. It should be noted that due to the scale effect in interpretation of small model tests, the secondary pressure pulses from detonations of conventional depth charges and bombs in attacks on full size submarines would be somewhat less effective than indicated by the above tests.
15-10. For an explosion set off at given distance from a circular submarine structure, secondary pressure pulses will have the greatest damaging effect when the charge is detonated below the hull.4 This is attributable to the tendency of the gas bubble to rise vertically under the influence of its buoyancy, especially in the contraction phase where relatively high velocities of ascent may be reached. It is therefore possible that the pressure pulse generated at the end of the first cycle of the bubble oscillation may be initiated close enough to the submarine to produce new or additional damage although the detonation itself was more distantly removed. Gas bubble oscillations resulting from detonations located near the side or above a submarine can generally be expected to rise and therefore increase their distance from the hull.
For a charge which detonates relatively close to the side of a hull, however, vertical rise may be offset by the tendency of the gas bubble to migrate to the nearby rigid hull. Even for charges which detonate close above a submarine, the gas bubble may oscillate several times in very close proximity to the hull instead of rising.
15-11. Although considerable research and experimental work has been carried on by many nations, including the United States, in an effort to determine the damaging ranges of various anti-submarine non-contact explosive weapons, present experimental and theoretical knowledge is still not sufficient to compute accurate damage versus distance relationships for such weapons. The figures given by different investigators among various countries are considerably at variance. Unfortunately, the great amount of submarine war damage experience available to this country furnishes no precise quantitative information regarding the behavior of submarines when attacked with underwater non-contact explosives since the charge sizes, distances and relative points of detonation are not known in any of the cases. The damaging range for a given weight of specified explosive will of course vary widely among the different types of submarines, depending upon the energy absorption characteristics of the hull structure, the design and mounting of vital systems and machinery, and the circumstances of the attack, i.e., the orientation and distance of the charge with respect to the hull and the depth of submergence of the submarine.
15-12. A study of available data indicates that the detonation of 300 pounds of TNT will almost certainly cause pressure hull rupture and therefore presumed lethal damage when the center of detonation is within 25 feet of the pressure hull of a U.S. light-hulled submarine (SS284 and previous). Between 25 and 50 feet, rupture or marked permanent deformation of the pressure hull and/or serious damage to vital machinery and systems may generally be expected, depending upon the position of the charge relative to the hull and the submergence depth of the submarine. Moderate to minor damage to equipment and fittings can be expected from 50 to 100 feet and beyond. It appears that 40 feet would be a fair range to delineate the boundary between the zones of serious and moderate damage of 300-pound TNT charges used against U.S. light-hulled submarines at relatively shallow depths. The British estimated from their war experience that pressure hull rupture would be certain to occur on submarines of contemporary construction with the following combinations of explosive weights and ranges: 150 pounds of TNT at 14 feet; 300 pounds of TNT at 20 feet; 1200 pounds of TNT at 40 feet. German tests on U-446 in June and December of 1944 indicated that the detonation of 275 pound of S1 explosive located 33 feet below the surface would cause marked permanent pressure hull deformation when within a distance of about 60 feet from the surfaced submarine.5 French calculations based upon research tests conducted in 1934 indicated that a double-hulled Paskal type submarine submerged to a keel depth of 50
feet would sustain permanent pressure hull deformation from the detonation of a 440-pound charge (type of explosive not stated) when placed at approximately the same depth and closer than 50 feet to the pressure hull.
15-13. Figure 15-2 below contains a plot of the damaging-range curve developed by the Bureau in 1943 for light-hulled SS212 Class submarines (11/16-inch medium steel pressure hull) when subjected to non-contact underwater detonations of varying weights of explosives comparable in power to TNT. The data upon which this curve is based are sparse and incomplete and the accuracy of the curve should be regarded accordingly. Furthermore, it does no take into account the orientation of the charge with respect to the hull or the depth of submergence of the submarine, both of which have a marked effect on the range within which damage will occur. The region below the curve represents charge weight and distance relationships at which marked permanent deformation of the pressure hull and serious damage to vital machinery and systems may generally be expected. The curve should properly be thought of as a "band" rather than an exact line of demarkation. Included in the same figure is a second curve giving distances at which hull rupture can be expected to occur for target submarines having an assumed 7/8-inch high tensile steel pressure hull, equivalent to that of the SS285 Class. This latter curve was developed by the Operations Evaluation Group from meager experimental evidence plus theoretical considerations and differs appreciably from the Bureau of Ships curve since it is based on positive rupture of a 7/8-inch thick HTS pressure hull rather than on permanent deformation only of an 11/16-inch thick MS pressure hull as in the case of the Bureau of Ships curve.6 The Operations Evaluation Group curve compares closely with the lethal ranges arrived at by the Underwater Explosive Research Laboratory after expanding the results of their small scale SS285 model tests (paragraph 15-9) to apply to a full size SS285 Class submarine (submerged to a depth of 50 feet).
Figure 15-2. Damaging Range Curves for U.S. Submarines
(For Shallow Submergence Condition Only)
15-14. The effect of depth of submergence on the strength of submarines under dynamic loading (also termed pulse, shock or explosive loading) from non-contact underwater detonations is naturally a matter of much interest to both design and operating personnel. In the case of the SS285 Class small model tests cited in paragraph 15-9 above, the lethal distance of a given weight of explosive was observed to increase rapidly with depth. For example, the maximum charge distance at which cylinder collapse resulted was two and one-half times greater at 673 feet than at 200 feet.7 Qualitatively such an effect is to be expected, since the stress due to explosive loading is superimposed on that already present in the structure due to hydrostatic pressure and therefore the lethal distance should increase rapidly as the absolute hydrostatic collapsing depth of the hull is approached. In addition, marked permanent deformation of a pressure hull might be sustained from explosive loading at 300 foot depth, for example, without further collapse of the hull occurring due to hydrostatic loading, whereas at a greater depth the same degree of deformation could result in complete hull failure due to hydrostatic loading following the explosive attack. There is considerable question however, as to the accuracy with which such small scale models represent full scale submarines in their resistance to underwater explosive attack.8
15-15. It is apparent that there is need for further research and experimental studies in order to determine the exact behavior of non-contact underwater explosions and their effect on present and projected submarines under every conceivable condition. It is considered that such studies would be principally of value for the optimum development and employment of underwater anti-submarine explosive weapons. In addition, however, further study of hull deformation short of rupture, and shock effects, should make possible continued improvement of the strength of submarine pressure hulls under dynamic loading, increased shock resistance of machinery and vital systems, and perhaps improved basic characteristics of submarines and their operating doctrine.
15-16. The question naturally presents itself as to what hazard, if any, exists in the possibility of close underwater explosions causing detonation of torpedo warheads when loaded in torpedo tubes having their outer doors open, or of the .50 caliber, 20mm and 40mm and 3, 4 or 5-inch fixed ammunition carried topside on submarines in pressure-proof ready service cases. It is known that projectiles and torpedo warheads are generally subject to detonation under the following conditions: (a) when subjected to the roasting effect of high temperatures applied for protracted periods of time (generally 15 to 30 minutes for TNT-loaded torpedo warheads); (b) when struck by high velocity fragments; sensitivity to detonation in this latter case being dependent upon the velocity of the fragment, the temperature of the fragment,
the wall thickness of the warhead or projectile concerned and the sensitivity characteristics of the explosive itself; and (c) sudden and violent crushing or pinching of the container walls of dent sensitive explosives. It is believed that condition (a) can be ruled out since even should the gas bubble from a nearby detonation envelop an exposed projectile or torpedo warhead while in a late stage of its expansion, the temperature of the gases would then be relatively moderate and the heat would not be applied for a sufficient length of time to cause donation. Should the gas bubble make contact during its early and extremely high temperature phase, the point of detonation would then be so close to the hull that destruction of the ship would doubtless in any event occur.
15-17. With regard to condition (b) above, carefully conducted tests and the combined war experience of both submarines and surface ships indicate that for all practical considerations little to no hazard exists in this respect. It has been ascertained that high velocity fragment attack upon an exposed torpedo warhead or projectile will generally not cause detonation when the fragments pass through a few feet of water (4 ft. or more) This is due to the fact that fragment velocities are decreased so rapidly upon passing through the water as to become less than the critical velocity necessary for detonation of even a relatively sensitive explosive such as Torpex. For example, it has been demonstrated in a series of tests, that the detonation of a Mk. 13 mine (675 pounds TPX) would not cause the detonation of an AM-M64 GP bomb (262 pounds TNT) even though both were suspended only four feet apart in free water.10 In this case the bomb was completely enveloped by the gas bubble from the adjacent mine detonation and was also subjected to intense fragment attack. As another example, the British submarine HMS Triumph, while proceeding on the surface on 26 December 1939, struck a floating contact mine almost directly under her bow.10 The resulting detonation demolished the forward 18 feet of the hull back to the muzzle doors of the lower torpedo tubes, and caused immediate flooding of the forward torpedo room plus other damage farther aft. Although the forward torpedo room were loaded with torpedoes, no warhead detonations occurred. No information is available as to whether or not torpedoes were loaded in the two bow eternal torpedo tubes, which on the Triumph Class are located in the superstructure about ten feet forward of and four feet above the lower nest of four tubes, and therefore presumably would have been more heavily damaged. TNT-filled torpedo warheads are difficult to detonate by fragment attack since the required critical fragment velocity is very high. Numerous cases are on record where TNT-loaded warheads on destroyers have been punctured by strafing or fragment attack from close bomb hits without themselves detonating. Submarine torpedo warheads are currently being loaded with HBX-1, an explosive having a fragment sensitivity comparable to that of TNT.
15-18. Condition (c), detonation by sudden and violent crushing or pinching of the container walls of dent sensitive explosives, appears
to offer definite hazard to exposed torpedo warheads when loaded with a relatively sensitive explosive such as TPX. This is due to the fact that the strength of the warhead container walls may not be sufficient to prevent crushing action under close underwater explosive attack. In Gato's action of 1 April 1943, for example, the warhead of a torpedo stowed in one of the after tubes was severely crushed by a close depth charge attack aft (Photo 15-3). The tube was flooded at the time, with the outer door closed, but apparently the pressure waves from the depth charge explosion forced the outer door open slightly and thereby admitted full pressure to the warhead. It is not known whether the explosive charge in this warhead was TNT or TPX, both types being still in use by our submarines at this period of the war. If the filler had been TPX, there is appreciable likelihood that the warhead would have detonated and caused the loss of the ship. It appears that torpedo warheads containing dent sensitive explosives should be made of somewhat greater strength than the pressure hull so that destruction of the hull can be expected before appreciable deformation of the warhead container will occur. On Gato the pressure hull adjacent to the torpedo tubes was depressed by as much as 2 inches, indicating that the hull and warhead container were of roughly equivalent strength. Since HBX-1 is understood to have a much lower dent sensitivity than TPX, the hazard with HBX-1 torpedo warheads would be appreciably less. It should be noted that, under explosive attack, substantially the full effect of the attack may reach torpedo warheads even though the outer doors are closed, due to the possibility of gasket damage or fluttering of the door under successive pressure waves.
15-19. Several instances have been reported by U.S. submarines which would seem to indicate that luminous gases, visible shock waves or smoke from close underwater detonations may have entered the interiors of the ships concerned, either through packing glands or hatches which opened momentarily. Icefish (SS367) reported "Got to 420 feet just as a barrage of 4 depth charges of blockbuster intensity went off on top of us. The cordite fumes from these filled the conning tower and both periscope packing glands leaked so heavily the bilges could not accommodate the water. . . ."11 On Kingfish (SS234), an EM 2/c stationed in the motor room during the depth charge attack of 23 March 1943, reported: "I was looking at the stern tube as one of the depth charge bombs burst and a ball of fire came through the tube. It came through the stern tube in the shape of a blunt streak two feet wide and extended into the boat about 3 feet. It was an instantaneous streak, flash and it was gone12 This phenomenon was also observed by one other witness. Also on Kingfish, it was reported that two observers saw flame shoot out of one of the after torpedo tubes coincident with a close detonation in that vicinity.13 At the same time, the air pressure
in the tube was reported to have increased to 130 pounds pr square inch and smoke and burned explosive fumes were detected in the after torpedo room. Several after torpedo tube inner door gaskets were slightly damaged. Bream (SS243) reported ". . . men in forward torpedo room saw blue flame spurt into the room" during depth charging.14 Bluegill (SS242) reported "Forward torpedo room saw flame off its seat during many explosions."15 Guitarro (SS363) reported "Blue flame showed around the periscopes and the after engine room hatch" following mass detonation of an ammunition ship 1900 yards distant.16
15-20. The descriptions of the phenomena noted above are remarkably consistent with each other, although occurring under different conditions. The odor of smoke and explosion fumes could presumably be explained only by the gas bubble having been in contact with the vessel at some time during its history. The reports connect these odors with the first events of the explosion. The connection between the odors and the gas bubble becomes confusing when it is considered that the lethal distance of the average depth charge (300 to 600 pounds of TNT or equivalent is greater than the maximum radius of the bubble (about 20 to 25 feet for the charges cited above at periscope depth, and about 12 to 15 feet at 300 foot depth), and particularly so when it is noted that the vessels sustained no damage of great consequence. It is possible that the bubble in a later phase may actually be involved in the reports of smelling fumes, since the period of oscillation of gas bubbles from depth charge detonations is about one half of a second under typical conditions, and such an interval of time may not have been distinguished in making the report.
15-21. The observations of flame are even more confusing, since they seemingly point to contact of the hull with a gas bubble while unexploded products are still in combustion, or at least while temperatures are still high. This would almost certainly be limited to the gas bubble in its first cycle. Guitarro's experience suggests that the gas bubble may not be involved since in her case it was impossible for the bubble of the ammunition ship explosion to have come in contact with the submarine. Shock wave phenomena emanating from explosions in air can sometimes be visually detected by observers in the same media. It is perhaps possible that some similar visual effect may be transmitted from an underwater shock wave to the atmosphere within a submarine through slack packing glands or fluttering hatches and torpedo tube doors. For example, rapid expansion of saturated air upon the passage of a shock wave will produce
momentary precipitation of water vapor and this would appear as a instantaneous light haze. Air within a submarine is generally at or near saturation when running silent with the air conditionining system secured. This explanation seems to be a relatively unlikely one but is felt to be the most reasonable hypothesis which the facts reported, assuming them to be true, will support. It is not improbable that the reports of flame may have been due to vagaries of vision. It is known, for example, that slight changes in the pressure, volume or velocity of blood in the retina will cause some people to see momentary grayish or purplish luminous clouds that sweep over the field of vision. Others may see rivers of light or a succession of scintillations that resemble fireworks.
Photo 15-3: View showing damage sustained by torpedo warhead on Gato (SS212) as the result of a close depth charge explosion aft in her action of 1 April 1943. Torpedo was stowed in an after tube, with the tube flooded and outer door closed.
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1. David Taylor Model Basin conf. Report No. 480 of October 1941.
2. Also observed as early as 1861 by Lt. Col. H.L. Abbot, USA, while investigating submarine mines for the Corps of Engineers, U.S. Army. Observed during Trout (SS202) depth charge tests off Portsmouth, N.H in 1941.
3. Office of Scientific Research and Development conf. Report No. 5417.
4. Both the Germans and French considered that for detonations occurring under a submarine, the limiting range within which serious damage could be expected would be double that for similar detonations located horizontally out from the side of the submarine -- NavTechMisEu conf. Technical Report No. 227-45 of August 1945.
5. NavTechMisEu conf. Technical Report No. 227-45 of August 1945. The German S1 explosive is similar in power to TPX.
6. Operations Evaluation Group conf. Research Report No. 40 of 28 April 1944.
7. Static collapsing depth of these SS285 scale models was about 870 feet.
8. David Taylor Model Basin conf. Report C-WW/S1-8; C-S81-3 of March 1947.
9. Office of Scientific Research and Development conf. Report No. 4810.
10. ONI Attaché Report No. 992 of 4 September 1940.
11. C.O. Icefish conf. ltr. SS367/A16-3/(011) of 13 November 1944 (Report of War Patrol No. One).
12. Comdt. Navy Yard, Mare Island, Supplementary Report on Kingfish War Damage, Appendix I (Statement of Witness to Flash through Starboard Stern Tube).
13. BuOrd (Re6) memo dated 4 May 1943, (Report on Inspection of Kingfish Damage and Interview of Ship's Officers by Dr. A.B. Focke).
14. C.O. Bream conf. ltr. SS243/A16-4, Serial (03-45) of 22 March 1945 (Report of War Patrol No. Five).
15. C.O. Bluegill conf. ltr. SS242/A16-3, Serial 039 of 25 November 1944 (Report of War Patrol No. Three).
16. C.O. Guitarro conf. ltr. SS363/A16-3, Serial 044 of 16 November 1944 (Report of War Patrol No. Three).