16-1. A thorough search of available records indicates that throughout the entire war there were but fifteen reported cases excluding losses, in which permanent deformation of the pressure hull was sustained by U.S. submarines as the result of non-contact underwater explosions of bombs and/or depth charges. In none of these fifteen cases was the pressure hull ruptured or torn nor is there any instance on record where a U.S. submarine has survived pressure hull rupture which occurred through war damage while the vessel was submerged.1 This is to be expected since rupture of the pressure hull of a submerged submarine should normally be lethal inasmuch as the flooding of one or more main compartments will render submerged control impossible, and the boat must then either surface, where it probably would be destroyed by enemy action,2 or must proceed to the bottom. Even assuming the bottom to be at a depth less than the collapse depth of the pressure hull and compartment bulkheads, experience shows that, under war conditions, self-salvage of the boat or ultimate escape and survival of personnel is quite unlikely.3
Hull Damage and Strength
16-2. In three of the fifteen cases where pressure hull deformation is known to have been sustained, the boats were damaged so extensively that they were subsequently scuttled by their own crews to prevent enemy capture.4 In the other twelve cases,5 the boats successfully evaded the enemy after receiving damage and returned to base. Ten of these submarines were repaired and returned to service while two6 were scrapped.
16-3. Since the strength of submarine hulls is naturally a subject of great interest to both operating and repair personnel, a brief discussion of the elementary design criteria of such hulls, and their probable mode of failure, has been included below. The information has been obtained from the considerable investigations conducted by the David Taylor Model Basin and this Bureau.
16-4. Weight considerations are a determining factor in submarine design since the sum of the weights of the various components of the ship, i.e., hull, propulsion machinery, armament, control gear, etc., must equal the weight of the water displaced by the intact volume of the ship when submerged. It is apparent that weight saved by the use if an efficient hull structure can be utilized for the benefit of the other components or for further increasing the strength of the pressure hull to permit a greater operating depth. Submarine pressure hulls are therefore usually made circular for as great a proportion of the length as possible. For the sake of simplicity, in the preliminary design study of a projected submarine the diameter of the pressure hull can be considered as the principal variable determining the amount of weight which will be available for the various ship components. The length of the hull is usually fixed by the arrangement study, i.e., the space requirements of armament, stores, crew, propulsion machinery sufficient to achieve the desired surface and underwater speeds, etc. The diameter and length determine the submerged buoyancy which, after subtracting the weights of the other components, determines the amount of weight that can be utilized for the hull structure and therefore the collapse depth of the vessel. In actual practice, the preliminary design phase may consist of a series of studies to determine the effect of varying length, diameter, hull form, types of propulsion plants, arrangements, etc., on such operating characteristics as speed, range, and submergence depth, in order to achieve the best compromise possible. Once the diameter of a projected submarine and its operating depth (including a reasonable factor of safety) have been decided upon, the detailed structural design of the pressure hull can proceed.
16-5. Under external pressure, the collapse strength of a transversely framed cylindrical vessel of circular cross-section is determined by the physical properties of the structural material used for the shell, the shell thickness/diameter ratio (t/D), and the unsupported length between frames diameter ratio (L/D). By varying these factors, failure of the shell plating can be made to occur, under sufficient external pressure, in one of the following three manners; (a) by stresses in the shell reaching the yield strength of the material; (b) by instability or buckling of the shell at stresses below the yield strength of the material; and (c), by what can be considered as a combination of failure by both instability and yield. Assuming constant diameter (D) of the transverse section and adequate but not excessively heavy framing, the relation between the thickness of the shell (t) and the unsupported length between frames (L) will then determine which of the above modes of failure will govern. If the shell is relatively thick and the unsupported length between frames relatively small, the shell will not fail until the material
is stressed to its yield strength by the combined effect of tangential and longitudinal stresses. The tangential stress resulting from radial compression (ie., the compressive "hoop" stress) is the determining influence in this mode of failure. The longitudinal stresses which result from end compression of the cylinder, from flexure of the entire hull as a beam, and from inward bending of the plating between frames under radial hydrostatic pressure, will have only minor influence in bringing about collapse in this case. This type of failure is analogous to the compression failure of a short, thick column. The first manifestation of failure will be the characteristic formation of a series of in and out bulges in the circumferential belt of the shell between frames (See Photo 16-1). However, if the shell is relatively thin and the unsupported length between frames is relatively large, buckling of the shell between frames similar to that just described will occur before the shell has been stressed to the yield strength of the material. In this latter case, the buckling action is called failure by "elastic instability" and collapse will occur at a much lower hydrostatic pressure than in the first case. The action is analogous to the failure by elastic instability of a long slender column. It is of interest to note that in the first type of failure the buckles form after the shell commences to yield plastically, whereas in the second case buckling starts while stresses are still in the elastic range.
16-6. Assuming adequate rigidity, the frames have considerable influence on the strength of the vessel by maintaining the original circular hull form. For a constant shell thickness and diameter, and assuming frame spacing initially of such distance that failure of the hull will occur by elastic instability, then as the frame spacing is decreased (i.e., as the L/D ratio becomes smaller), the collapsing pressure of the vessel will increase, since, although failure will still occur by elastic instability, the "hoop" stresses in the shell when failure occurs will become progressively larger and will approach the yield strength of the material. The collapsing pressure will continue to increase as the frame spacing decreases until the L/D ratio is reduced to that value at which the frames carry a sufficient portion of the load to prevent the shell from buckling until the combination of the "hoop" and longitudinal stresses in the shell material reach the yield strength. The collapsing pressure for the shell then becomes practically independent of further decrease in frame spacing. Once their spacing has been determined, the frames are designed so that they will collapse at a pressure slightly above the pressure that will cause buckling of the shell between the frames.
16-7. A sturdy column is one which will not collapse until stressed to the compressive yield point. The sturdiness of a column depends upon its slenderness ratio (L/r), where (L) is the length of the column and (r) is the least radius of gyration. Generally speaking, the sturdiness increases as the slenderness ratio decreases. Likewise, the sturdiness of a pressure vessel, assuming rigid framing, depends upon
an analogous slenderness ratio which can be expressed in a manner corresponding to the column theory as below:
Slenderness ratio of pressure vessel =
When this ratio is modified to include the physical properties of the shell material (S = yield strength, E = modulus of elasticity), it is referred to as the "thinness factor" and is expressed as:
Thinness factor =
The sturdiness of a pressure vessel increases as the thinness factor decreases. It can be seen that increasing the yield strength of the material, while holding the other factors constant, increases the thinness factor and therefore the prospect that the vessel will fail by elastic instability before the "hoop" stress in the shell due to hydrostatic loading approaches the yield point. If a light and efficient construction is to be attained, the "hoop" stress in shell material should approach the yield stress before buckling occurs and this requires that a proper balance be achieved between t, L, and D for the yield strength of the steel selected. High strength steels, for example, require relatively close frame spacing if the full yield strength of the material is to be realized.
16-8. Circular submarine hull designs currently in use by the U.S. Navy are considered as thin shell vessels stiffened by rather closely spaced frames that are sufficiently rigid to maintain the circular form until the shell between frames starts to fail. It is intended that the various strength factors be so balanced that when the hydrostatic collapse depth is passed, the hull will commence to fail by buckling of the shell plating between frames at a "hoop" stress close to the yield strength of the steel. This buckling will first occur as a series of in and out bulges or "lobes" distributed around the circumferential belt of the shell between frames, the bulges in adjacent frame spaces being at an angle of approximately 45 degrees to each other.7 Complete failure will follow by rupture of the shell plating at some point of high stress concentration, induced by the plating tearing away from a frame and merging two or more bulges into one large bulge, bending of the frames out of their normal plane or circular shape, or by a tendency of the hull to twist and close up the frame spacing in accordion-like folds due to end pressure. High stress concentrations at hatches, or any type of stress raiser, may very possibly cause a rupture even before the bulge pattern between frames becomes evident.
16-9. In the hope of attaining added safety during attack, and also to better avoid detection, the pressure hull of the SS285 Class was designed to permit a much greater depth of submergence than that of any previous submarines in the U.S. Navy. The increased strength was attained partly by increasing the thickness of the pressure hull plating and partly by use of steel of greater strength, i.e., high tensile steel rather than medium steel. In addition, H-frames were used instead of the I-frames and T-frames employed in previous designs, since H-frames, due to their wider flanges, possess greater torsional and transverse rigidity for a given sectional area and are therefore less susceptible to buckling or upsetting (Photo 16-1>).8 It is interesting to note that only two submarines of this Class reported sustaining permanent pressure hull deformation as the result of underwater explosive attack, although many were subjected to close detonations of bombs and depth charges.9 No SS285 Class submarines are known to have sustained any indications of failure due to overdepth.
16-10. Near the forward and after ends of U.S. submarines, the inner circular pressure hull terminates and the outer hull becomes the pressure hull. These single hull ends conform to the lines of the submarine, gradually fairing into a shape which approaches that of an ellipse in cross section. Frames of the single hull portion are internal while those of the cylindrical double hull portion are external to the inner hull. The internal frame, together with a strip of shall acting as a flange, is designed to take the full load, and the shell, except that portion considered as the frame flange, although very heavy and capable of withstanding large loads, is considered primarily as an envelope to keep out the water. The internal frames are designed so that their neutral axis will, as nearly as possible, approximate a circle. The single ends are made slightly stronger than the circular hull to provide some protection against collapse of end structure by reason of trim angle. Past experience and tests have been the guide in progressive development of non-circular hulls rather than theoretical formulae. Several cases have occurred where the single hull ends of U.S. submarines have sustained extensive plating and moderate frame deformation
under severe depth charge or bomb attack.10 No serious weaknesses were disclosed in these single hull ends, although areas of plating proved susceptible to local failure. On both Tunny (SS282) and Halibut (SS232), the section of the top pressure hull plating between the forward escape trunk and the forward torpedo loading hatch was markedly depressed, whereas adjacent plating was not affected;11 on Kingfish, local depressions occurred in way of the shell plating for engine lubricating oil stowage tank No. 5.12
16-11. Flats bounding tanks exposed to full sea pressure, such as those of the forward and after ballast tanks on SS285 Class and previous submarines where the torpedo room decks are the tank tops, must be capable of withstanding, without excessive deformation, the maximum pressure corresponding to the collapse depth of the other portions of the vessel. The stiffening for such flats is usually provided by an intersecting system of floors and longitudinals. The plating is divided by these floors and longitudinals into approximately square panels and the welding connecting the flat to the stiffening system makes thee members integral components. The design of flats, as in the case of non-circular hulls, is based to a large extent on empirical data derived from tests and service experience. There are three cases known to the Bureau where forward or after ballast tank flats were deformed inward as the result of close bow or stern depth charge or bomb detonations (Photo 16-2).13 In none of these instances was the distortion such that any portion of the plating was torn loose from the stiffening members. In each case, the adjacent single hull plating was also deformed, and to a greater extent than the flats in two of the three cases. It is interesting to note that although Tunny (SS282) twice sustained extensive shell deformation in her single hull ends, in way of the forward torpedo room in her action of 26 August 1943 (Section VI), and in way of the after torpedo room in her action of 1 September 1944 (Appendix I), in neither case was damage to the torpedo room flats reported.
16-12. The primary function of the main divisional bulkheads on U.S. submarines is to preserve the watertight integrity of the several compartments of the vessel in the event of damage and subsequent flooding from sea. Unlike bulkheads of surface ships, they are neither intended to serve as racking members nor are they primarily designed to support loads on decks above. These bulkheads make no contribution to the collapse strength of the hull and are not intended to take any load
except in the case of flooding or where portions of the bulkheads form the boundaries of pressure tanks. Escapes were enabled from both Squalus (SS192) and Tang (SS306) when watertight boundaries which prevented further flooding were established at bulkheads by quick action on the part of the crews. Although in both of these instances the holding bulkheads were not subjected to pressures near the maximum for which they were designed, it is interesting to note that the doors and fittings remained substantially intact under deflection. The bulkheads on Squalus showed no signs of permanent set on subsequent examination although it withstood a head of about 230 feet of water for several weeks. One interesting case of bulkhead damage was that which occurred on Halibut (SS232) in her action of 14 November 1944, during which the forward portion of her pressure hull received severe damage as the result of close depth charge attacks. The after bulkhead of the forward torpedo room was deflected forward, both port and starboard, but only in that portion at the outer periphery which formed the junction between the inner and outer hulls (Photo 16-3).
16-13. While the factors controlling the strength of submarine hulls to withstand hydrostatic pressure are now well understood, no accepted theory exists as to the strength of a submarine pressure hull under the condition of combined hydrostatic loading and dynamic loading that occurs when a submarine is undergoing attack by explosives at a considerable depth. Experimental data now available is not sufficient to serve as a guide in the formulation of such a theory. However, new and significant information was obtained from the series of deep sea explosive tests conducted on small scale submarine models by the Underwater Explosive Research Laboratory of the Bureau of Ordnance (paragraphs 15-9 and 15-14).
16-14. The modes of failure of submarine structures under combined explosive and hydrostatic loading can be divided into two general classes as follows: (1) immediate splitting or tearing of the pressure hull due to direct action of contact or close detonations; (2) deformation of the pressure hull due to explosive loading, but without splitting or tearing occurring, followed by collapse or tearing due to hydrostatic loading on the weakened structure (if the depth is great enough). The combined effect of hydrostatic loading due to depth, and explosive loading from bomb or depth charge detonations, is greater than either alone, and the lethal distance of a given weight of explosive increases rapidly as th depth of a submarine is increased.14 Tang (SS306), Tullibee (SS284), Robalo (SS273), and Flier (SS250) are known to have been lost through destruction of a portion of their pressure hulls from close or contact detonations. As stated previously, no U.S. submarines are known to have survived war damage which breached their pressure hulls while submerged. It should be noted that the range at which a given weight of explosive will cause permanent deformation only of a submarine pressure hull is but slightly greater than the range at which the
same weight of explosive will cause rupture or complete failure of the hull to occur. The distance increment between the range at which permanent deformation only will occur, and that which will cause complete destruction, considerably decreases as the submergence depth of a submarine increases.
16-15. It is considered that the most serious structural hazard to a submarine from underwater detonation which are located at distances slightly greater than the limiting distance which would in any event cause splitting of the pressure hull, is the possibility of rupture or tearing occurring at a "hard spot" or discontinuity of structure which produces local areas of high stress concentration. There is hardly any portion of a submarine hull structure which is entirely free form these stress raisers, and as a consequence the hulls are vulnerable to failure at such points. Photos 16-4, 16-5 and 16-6 show typical pressure hull failures in areas of stress concentration as obtained during tests conducted by the Bureau on full scale submarine sections at the Navy Yard, Portsmouth, in 1942. The detail specifications for all recent submarines outline methods of attachment of the ends of stiffeners to the strength hulls to insure a measure of protection against dynamic loading from underwater explosions. Liners and pads are provided to protect the hulls against tearing when dented or deflected in the way of the ends of stiffener brackets. Welding is so designed that the stiffeners will pull away from the pads before the pads tear the hull plating. The design details are based on information obtained from many explosion tests, and war experience had shown that the methods are generally satisfactory. An elastic connection is less liable to sustain damage than one that is rigid and forms a hard spot. Hard spots, abrupt changes in area of members, nicks, and unsupported edges of holes are potential starting points of failure and should be avoided. Efforts are made to reduce the locked-in welding stresses both in the design and building of submarines.
16-16. The damage which occurred to Scamp (SS277) in her action of 7 April 1944 illustrates features of both correct and incorrect detail design. The pressure hull of Scamp broke free from several frames in way of the heaviest damage (Photo 9-8). This was obviously far less serious than the same damage accompanied by a tear in the hull plating would have been. The frames on Scamp were I-sections. It is perhaps possible that had H-section frames been installed, as on the SS285 Class, the frames might not have broken free from the hull since inward bending of the wide faying flanges of an H-beam will absorb considerably more energy and deformation than the relatively narrow faying flanges of I-section frames. With either type of frame, however, it is preferable to employ welding of such strength that the frames will break free without tearing the pressure hull. This is insured by the welding design and is facilitated by the fact that the frame welding can only be as heavy as the flange thickness will permit, and, with present frame sections, the maximum strength of such welding is less than that of the shell plating. Undesirable designs for conditions
of high explosive loading were illustrated by two failures which occurred on Scamp: (1) tearing of the outer hull tank top plating at frame 79, as detailed in paragraph 9-24 and Photo 9-7; and (2), tearing of the main engine air induction line at its welded bracket supports, as detailed in paragraph 9-22 and Photo 9-5. Both of these failures were caused by high stress concentrations. Scamp's narrative demonstrates the serious effects such apparently minor design defects may have on the survival of a submarine when attacked.
16-17. The collapse strength of a submarine circular hull is reduced markedly when there is any appreciable departure from a truly circular section. Hydrostatic tests of submarine models demonstrated that those models having even a small degree of out-of-roundness failed at pressures below that required to cause failure of more perfect models. Where the out-of-roundness was limited to a lobe length (see paragraph 16-8) between two adjacent frames, the first bulge to appear as pressure approached the failure value was almost always at this initial buckle or dent, and the bulge patterns spread out from this point. Deep sea explosion tests on SS285 Class pressure hull models conducted by the Underwater Explosives Research laboratory demonstrated that out-of-roundness was even more detrimental to submarines under combined explosive and hydrostatic loading than under hydrostatic loading alone.15 Current specifications require that at no place shall the maximum radius of the circular hull exceed the minimum radius by more than one-half the shell plating thickness. Tests show that if this tolerance is not exceeded, no appreciable reduction in strength will result. The importance of maintaining the circularity of the pressure hull to within the above tolerance can not be overemphasized. Checks of hull circularity should be made in all cases where submarines have sustained pressure hull deformation.
16-18. Dishing of pressure hull plating between frames due to overdepth operations has occasionally occurred (Salmon Photo 7-5). While it is true that such dishing cannot be considered to have weakened the hull from the standpoint of hydrostatic loading alone, and that the submarine should be able to again safely descend to the depth at which the dishing originally occurred, the resistance of the dished hull to bending stresses and explosive loading is definitely decreased. Once a dish has formed, the plate in the vicinity has absorbed energy which it cannot again absorb and is therefore more susceptible to failure under explosive attack. In all cases where marked pressure hull dishing has occurred, whether due to hydrostatic pressure or explosive attack, the original geometry of the structure should be restored and new plating is recommended.
16-19. Expedients should not be adopted which will in any way weaken submarine hulls or introduce areas of doubtful structural strength. The welding of pressure hull structure back into place falls in the category of restrained welding in which there are always locked-in stresses.
These stresses may result in the immediate formation of cracks (which for grooved "tee" joints may not be discernible by magnafluxing or other methods of inspection), or in the failure of the structure under loads considerably below the full strength of the material. Certain alternations and war damage repairs occasionally leave no other course than to cut the hull plating and structure and to install replacement material. Patches, except when worked flush with plating in a cut-out section, are stress raisers, and because of the hard spots introduced are potential danger points even though perfectly welded. The original geometry of the hull should be restored in all cases. Any attachments such as padeyes, cleats, etc. are stress raisers and should be avoided. Even slight nicks, such as those caused by striking a welding arc, will tend to produce notch effects. Experience during the war has demonstrated that the utmost care must be taken on submarine pressure hull repairs and that X-ray or radiographic examinations must be conducted if defects are to be avoided. It is also extremely important to check the physical and chemical properties of the replacement material when any doubt exists as to its meeting the required specifications for the hull area concerned.
16-20. The Bureau has issued rigid instructions on the methods to be employed in cutting and welding of submarine pressure hull structures and structural fittings subject to deep submergence pressure. The reader is invited to refer to the following letters for further information: BuShips conf. ltr. SS/S11-1(515-692) of 8 May 1944; BuShips conf. ltr. SS/S29-6(515) of 15 May 1944; and BuShips conf. ltr. SS/S11-1(515) of 2 April 1945.
16-21. The marked improvement of welded construction over riveted construction in resistance to war damage has been repeatedly demonstrated in both surface vessels and submarines. There is no reported instance where welds in U.S. submarines pressure hulls have failed as the result of war damage. This fact of course does not preclude the possibility of such failures having occurred, since the results would very likely have proven fatal to the submarine concerned. That welding has proved so satisfactory on our submarines is attributable to the constant emphasis placed by the U.S. Navy on improving welding methods and materials, adequate training of welders, qualified supervision, frequent inspection, and the correct design of ship welding details. Experience has demonstrated the importance of proper welding sequence in the control of shrinkage, distortion, locked-in stresses and cracking during construction. Poor workmanship engenders fracture since failure may originate at a small notch such as is occasioned by peened-over cracks, undercut welds, porosity and inclusions in the weld, improper joint preparation resulting in incomplete backside root chipping on grooved joints, etc. The importance of maintaining the welding standards set forth by the Bureau for production and repair welds for submarine pressure hulls cannot overemphasized.
16-22. It is interesting to note that there are several submarines which have been quite heavily depth charged and on which laminated pressure hull plates were subsequently found. In theory, laminated plates involve a loss in strength. However, this assumes the lamination to be complete; that is, that there is a free plane of cleavage which would permit the separated plates to move freely relative to each other. In practice, the laminations are not free planes but are irregular and partially fused surfaces. It is believed that such surfaces will act in shear substantially as if joined and that no Important loss in strength will result. When encountered, they should not be regarded as cause for alarm. Nevertheless, laminated plates should never be worked into submarine structure, and when found during construction or repair should be removed if possible.16
16-23. War experience indicates that riveted and bolted access plates as fitted in submarine pressure hulls have been generally satisfactory in strength compared with other portions of the pressure hull, but that when extreme shell deformation occurs, the joints of such plates tend to open and may develop serious leakage. For example, on Grenadier (SS210)17 during the action leading to her loss on 22 April 19843, the riveted plate over the maneuvering room tore partially free from adjacent plating when the pressure hull in that area was damaged. This allowed a stream of sea water to cascade upon the main control cubicle beneath and was one of the factors which caused loss of propulsion. Riveted plates, however, appear to withstand moderate deformation without tearing or developing excessive leakage. On both Salmon (SS182) and Kingfish (SS234), the riveted plates in way of the areas of pressure hull damage were themselves considerably deformed but remained tight or nearly so.18 However, on Kingfish, several riveted keystone frame joints started to separate (Photo 5-2).
16-24. Riveted and bolted plates, when properly designed and fitted, are not structurally weak from the standpoint of normal hydrostatic loading. However, double butt-straps act to create hard spots and thus may weaken adjoining structure rather than in themselves constituting a weakness, and riveted construction is not fundamentally desirable from the standpoint of watertightness when subjected to large deformations. In addition, the reinstallation of riveted plates, once removed, usually involves considerable time and expense. New butt-straps and plates are frequently required; oversize holes must be welded
up and redrilled; caulking edges may have to be built up by welding; faying surfaces on adjacent pressure hull structure occasionally develop ripples; obtaining metal to metal contact along the longitudinal seats is often a long and arduous process; and reriveting may be substandard. It now appears possible that the use of riveted portable plates may be discontinued in favor of welded plates on both new construction and submarines in service. However, it is intended that bolted access plates will be retained in those locations where repeated removal may be necessary, such as for battery and parts replacement.
16-25. The rupture of the forward torpedo room 37.5 pound HTS pressure hull plating on Dragonet, as the result of striking a rocky promontory while submerged, is an excellent example of the type of failure to be expected when heavy concentrated loading is applied to restrained plating panels. In Dragonet's case, it is quite likely that the force of grounding was sufficient to cause the shell to fracture in any event. This is indicated by the fact that one of the frames supporting the plating also broke. However, the plating was greatly restrained in the areas of rupture by extremely strong and rigid internal frames, spaced on 24 inch centers, which acted to prevent vertical movement and tended to increase the likelihood of early fracture. The capacity of restrained material to absorb energy is reduced markedly since the ability of the material to flow plastically is less than that of unrestrained plating. The fact that Dragonet was operating in water at a temperature about 29°F.19 may have been a contributing factor in the failure of the pressure hull plating. It has been definitely established that low temperature greatly reduces the ability of both MS and HTS to absorb energy. The temperature below which the capacity of steel to absorb energy decreases rapidly is generally referred to as the transition temperature. Below the transition temperature, fractures show a brittle, crystalline surface in contrast to the fibrous type associated with the same steel at higher temperatures. The few test results available seem to show only a negligible change in yield strength as the temperature is lowered. Therefore, under static or slowly applied loads, it is not considered that appreciable change in strength is to be expected. Temperature effects are reflected in impact and notch tests rather than in static tests. Since the transition temperature has been found by actual tests to vary from -20°F. to well over 100°F. with different samples of MS and HTS, although all of the samples satisfied the specifications, it is impossible without actually testing the material in the region of the failure on Dragonet to determine its transition temperature. The appearance of the fractures on Dragonet suggests that low temperature may have been a factor in the plating failure, although as previously stated, the force of grounding was no doubt sufficient to have caused the failure. It should be noted that the average transition temperature of high tensile steels, as in the pressure hulls of SS285 Class and subsequent submarines, (less SS361-364), is considerably lower than the average for the
medium steels in previous submarine classes, and therefore the use of HTS material can be considered as a definite improvement over MS in this respect.
16-26. Ballistic protection on U.S. submarines has been limited to the installation of 20 and 30 pound STS for the protection of bridge personnel and 40 pound STS for ready service ammunition. In addition on SS285 Class submarines the entire conning tower, including the ellipsoidal end bulkheads, were fabricated from 40 pound STS for this purpose. Ballistic protection was authorized in April 1943 for all fleet type submarines subsequent to SS197 when the need for shielding the vulnerable bridge, conning tower and ready service ammunition was demonstrated by several aircraft strafing and small caliber surface engagements, such as Growler's action of 7 February 1943.20 Generally speaking, and disregarding a possible high velocity of approach of the firing weapon, 20 pound ST will provide protection against normal impact of .50 caliber projectiles fired from a range of 1000 yards and 40 pound STS will provide similar protection 100 yards. In at least two instances the STS plating on our submarines was penetrated.21 With the exception of the conning tower hatch on Growler, there were no instances where either conning towers or pressure hulls were penetrated by small caliber enemy projectiles. This is attributable to the low incidence of such attacks rather than to the meager protection afforded by the small amount of STS plating provided for our submarines. The design problem in ballistic protection of submarines is one of calculated risk in any event since weight and stability considerations will not permit extensive use of STS in thicknesses sufficient to insure defeat of small caliber projectiles fired at close range.
16-27. The structural design of modern submarines is the result of evolution over a period of many years. Research, particularly with full scale caissons, initiated by the Bureau and accomplished mainly at Portsmouth, N.H., and model tests and theoretical analyses by the David Taylor Model Basin have contributed greatly to present knowledge and design practices. While marked advances have been achieved in the past several years, further improvements appear definitely possible. The elimination, where possible, of hard spots and potential points of stress concentration in pressure hulls is a matter which should be borne in mind by the operating personnel in establishing requirements and features which require pressure hull penetration. Hull openings should be held to an absolute minimum. Theoretical investigations and further controlled testing of models and full scale vessels are required to widen the field of knowledge on the behavior of submarines under explosive attack. Steels are currently under development which it is hoped will combine the desirable qualities of high yield strength; high ductility; notch toughness; increased energy absorption characteristics at the high strain rate conditions which result from explosive loading; weldability; and low transition temperatures. Submarine hull designs for operation at depths of as much as 1000 feet may then become feasible within the percentage of submarine total weight which can be devoted to hull structure.
Photo 16-1: View of Model No. 107 after hydrostatic test at U.S. Navy Experimental Model Basin. Note typical example of bulge formation in shell and failure of frame having insufficient rigidity. The need for wider frame flanges is clearly indicated, such as provided in the H-frame sections used on SS285 Class submarines.
Photo 16-2: View showing deformation which occurred to MBT No. 1 tank top flat in forward torpedo room of Halibut (SS232) as a result of her action of 140 November 1944. Washboard effect on port side is shown.
Photo 16-3: View showing buckled area of after bulkhead of forward torpedo room on Halibut (SS232) as a result of her action of 14 Nocember 1944. Bulkhead was deflected forward in peripheral strip between the junction of the inner and outer hulls from about 40° below top center to about 90° from top center, both port and starboard.
Photo 16-4: Typical example of tearing of inner shell at stress concentration formed by incorrect junction of bulkhead stiffener as obtained by explosive tests on a full scale submarine partial length section at the Navy Yard, Portsmouth in 1942.
Photos 16-5 and 16-6. Typical failures of inner hull as obtained by explosive tests on a full scale submarine partial length section at the Navy Yard, Portsmouth in 1942. Above: tear in pressure hull plating at tank top margin plate. Note that I-frames are torn from pressure hull. Below: tear in pressure hull at junciton of I-framem with tank top margin plate.
Table of Contents
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1. (a) Dragonet's (SS293) forward torpedo room was holed as the result of striking a reef while submerged, but she was not under attack at the time. See Section XIV for Dragonet narrative. (b) Bergall's (SS320) pressure hull was torn open above the waterline by an 8-inch shell hit (dud). The vessel was on the surface when struck and returned to base running surfaced for the entire distance. See brief of damage in Appendix I (13 December 1944).
2. One such possible case is the loss of Argonaut (APS01) on 10 January 1943. See brief in Appendix II.
3. The loss of Tang (SS306) on 24 October 1944 is the only known instance where personnel have made underwater escapes from a U.S. submarine sunk through war action.
4. Perch (SS176), Grenadier (SS210), and Sculpin (SS191). See Sections IV and VIII for narratives on loss of Perch and Grenadier.
5. Gudgeon (SS211), 26 January 1943; Kingfish (SS234), 23 March 1943; Gato (SS212), 1 April 1943; Tunny (SS282), 26 August 1943; Gato (SS212), 20 December 1943; Scamp (SS277), 7 April 1944; Crevalle (SS291), 14 May 1944; Tunny (SS282), 1 September 1944; Salmon (SS182), 30 October 1944; Halibut (SS232), 14 November 1944; Redfish (SS395), 19 December 1944; Greeling (SS213), 25 January 1945. See narratives in Sections I, VI, VII, IX, and briefs in Appendix I.
6. Salmon (SS182) and Halibut (SS232).
7. A series of 14 distinct and separate inward bulges (1/2 lobe) should theoretically occur around the complete circumferential belt between each frame space on SS285 Class submarines.
8. David Taylor Model Basin Conf. Report No. 515 of May 1943 (Hydrostatic Test of a 1/12-Scale Model Pressure-Hull Section of the SS285 Class of Submarines).
9. Redfish (SS395), depth charged on 19 December 1944 during her second patrol and Crevalle (SS291), depth charged on 14 May 1944 during her third patrol. See briefs of damage in Appendix I.
10. Tunny (SS282), 26 August 1943, Section VI, and Halibut (SS232), 14 November 1944, Appendix I, are excellent examples of damage to single hull ends.
11. See paragraph 6-15.
12. See paragraph 5-12.
13. Kingfish (SS234), 23 March 1943, Section V; Halibut (SS232), 14 November 1944, Appendix I; and Redfish (SS395), 19 December 1944 Appendix I.
14. See paragraph 15-14.
15. OSRD conf. report No. 5417.
16. BuShips ltr. SS283/S11(515) of 4 November 1944.
17. See Grenadier narrative, Section VIII.
18. See Kingfish narrative, Section V, and Salmon narrative, Section VII.
19. Temperature measured at engine injection (salt water).
20. See Growler narrative, Section XIII.
21. See briefs of damage in Appendix I for Skate (SS305), 6 October 1943; and Pogy (SS266), 19 April 1945.