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ARTICLE BY TIM THORTON
© 1987 WARSHIP MAGAZINE
© 2001 SPACECRUISERYAMATO.COM
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Just before dawn on Christmas day 1943 the Gate class submarine USS Skate was patrolling about 180 nautical miles northwest of Truk. The main operational base of the Japanese Combined Fleet, when her surface search radar picked up three contacts at 23,0(X) yards steaming southeast at 19 knots. As the contacts closed, they showed themselves as one large vessel and two smaller ones, the latter apparently being escorting destroyers. In the growing light of the tropical dawn, Skates Captain, McKinney, dived his boat although he lay to the west in the darkest part of the horizon and fired a salvo of four torpedoes at the largest target. With his submerged speed limited to 10 knots he had no real alternative Course of action and although he was unable to identify it he was rewarded by the gratifying sound of one detonation. However, the ship did not stop and Skate continued her patrol unmolested and unaware that she was the first American vessel privileged to catch sight of the giant battleship Yamato, the pride of the Imperial Japanese Navy.
Such was the parlous state of the Japanese merchant marine, even at this stage of the war, that Yamato had been dispatched as a distinctly unusual fast transport from Truk to Japan on 12 December 1943. She had arrived at Yokosuka with out incident five days later and for the next three days was loaded with supplies and soldiers. With two escorting destroyers she then sailed for Truk and her meeting with Skate. Morison, in his official work on US naval operations in World War II, implied that she was on passage from Truk to Kavieng. In New Ireland when the attack was made but this is incorrect in both the light of Japanese records and the calculations of the distance and time involved.
After the attack, Yamato's speed was unimpaired and she continued on to Truk anchoring later the same day. Eventually, after a vacillating delay of five days, her cargo of troops and stores was off loaded. Any extension of her mission was cancelled. This may well have been a prudent decision because had she completed her sortie she would probably have been attacked by aircraft from two US carriers which were deployed to intercept just such traffic. As her subsequent experiences during the Battle of Leyte Gulf and in the South China Sea demonstrated, she had immense resilience under attack, but such an encounter in a less critical operation would have been, to say the least, ill-judged.
Fortunately, nobody was killed in the attack and as her cargo was being off loaded divers were sent down to inspect the damage and make temporary repairs. Truk had always been more of an anchorage than a fleet base and had few repair facilities, despite allied assumptions to the contrary, and so a return to Japan was inevitable after the torpedo attack. With one escorting destroyer, Fujinami, she sailed from Truk for the last time on 10 January 1944 and docked at Kure six days later. It was to be four months before the damage was repaired and a minor refit completed, though, happily for Japan, she missed no significant action.
Such a catalogue of events is not remarkable. Any ship at sea in wartime is likely to he open to submarine attack and the hit on Yamato was simply
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one more example to add to many others. What was far more significant for the leaders of the Imperial Japanese Navy was the extent of the damage. The explosion had occurred on the starboard side beneath the after l8in main triple gun turret. Quoting from her Captains report to the Navy Ministry the damage was as follows:
"A hole about l6ft (5m) deep extending downwards from the
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MIDSHIP SECTION OF YAMATO CLASS BATTLESHIP
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top of the bulge connection and S2ft (25m) in length between frames 151 and 173. Water flooded into No 3 turret upper magazine from a small hole in the longitudinal bulkhead caused by caving in of the waterline armor"
Put simply, her underwater defenses had been breached by a single torpedo and she had shipped over 3000 tons of water, something which her designers had worked assiduously to avoid. The resultant concern deepened when it was learnt that the torpedo had been running shallow and had struck only four
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SECOND HOLD DECK PLAN
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feet below the surface, where the explosive effect, which increases with the depth of water, had not been particularly great. What is interesting is how and why the design failed on a ship which the Japanese had always intended to he the pinnacle of battleship excellence and one certainly capable of stopping a single torpedo. From the time the, Japanese Naval General Staff ordered the Bureau of Naval Construction to study
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such a proposal in the autumn of 1934 it was clear the vessel would he enormous since the only sure way of building in superiority in the three key elements of speed, firepower and protection, was to increase the size of the vessel. The first calculations of her principal designers, Yuzura Hiraga and Keiji Fukuda, proved too ambitious even for the Admirals of the Naval General Staff It was speed that was sacrificed since the length and depth of the hull proposed, which were vital prerequisites for high speed, would simply not fit Japanese parts without unacceptable additional expense. On the other hand, a shorter and beamier hull which was still thought likely to confer a speed comparable with future US ships was not without compensations which they did their best to exploit.
Despite this she still had a draft of l0.8m when fully loaded and some dredging was required at the approaches to naval bases and dry docks. Nonetheless, the final trial displacement of 69,100 tons was still close to twice the size of any operational battleship at the time, though the battleships of other navies were naturally following the same inexorable trend, This final design remained, despite the compromise, well balanced. Just over 58 percent of displacement was consumed by the three key considerations:
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33.2 percent or 22,895 tons being allocated to armor, 16.9 percent or 11,661 tons to weaponry and 7.7 percent or 5300 tons to machinery. The only slight deviation from what might be thought the norm was that the figure for machinery was a little low, the accompanying reduction in speed required for the weight saved to he used for protection. It is the underwater element of this defense which must now be considered.The ideal of all-round protection had long since been abandoned as shell
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JOINT BETWEEN MAIN BELT AND INTERNAL
ARMORED BULKHEAD OF THE YAMATO CLASS
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and torpedo attack had proved too destructive. Along with other navies the Japanese adopted the 'all or nothing' principle; protection was limited to those areas vital for survival and for fighting; in short, the main machinery and gun turrets. The result was an armored central raft which left the bow and stern sections virtually unprotected. The smaller an area this raft represented the stronger could he the armor, and this was not of inconsiderable importance given thatasinglel2in cube of steel plate weighed a quarter of a ton. In the case of Yamato her great beam, which at the maximum was 127.7ft (38.9m), proved a great boon because her four main turbines and their associated boiler rooms could all be placed side by side across her hull. As a consequence, the area to be shielded shrank to a surprisingly short section of the hull, amounting to just 53.3 percent of the waterline length of 839ft (256m). This was a great achievement and her broad hull also conferred sufficient buoyancy for her to float even if all the unprotected spaces were left open to the sea after enemy action.
Damage from the initial kinetic energy of incoming shellfire could best be minimized by thick armor plate with a hardened exterior, hut such a system could never hope to defeat a torpedo's explosive charge of several hundred pounds detonating in direct contact with it and amplified by the surrounding
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DAMAGE TO YAMATO TORPEDO DEFENSES 12.25.43
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water. Matters could actually he made worse since the heavy armor tended to fracture and the broken shards could rip deep into the hull. Volume was the best protection against torpedoes since it allowed for the expansion of the explosive gases while the remaining force rapidly dissipated with distance. There was never enough internal volume in a hull to provide much space for this and designers generally had to be satisfied with providing the minimum. In the case of Yamato the constraints were severe despite her great beam because of her chosen
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machinery layout. This was exacerbated by the choice of a reliable but bulky set of boilers, which ran at relatively low pressure. They were used because replacement beneath the 200mm armored deck would have proved extremely difficult hut the corollary was a narrower torpedo defense.
The width of this around her machinery spaces was on average 5.lm, and was narrower than that of almost all her contemporaries in other navies despite her displacing considerably more. Two examples will suffice to illustrate this. The American North Carolina, on a displacement calculated in a comparable manner at 45,298 tons, had a system 5.6m deep, while the German Scharnhorst on only 35,398 tons still managed a depth of 5.4m. For Yamato, it was therefore essential that within the comparatively narrow space remaining the best possible arrangement was used.
In order to counter a torpedo explosion, a space outside the true hull was required which would be strong enough to detonate the weapon well away from a stronger yet flexible main bulkhead beneath. The Japanese developed empirical formulae to determine the thickness of protective bulkheads and bulges based on tests with models and full scale systems. Once established they were then used with much confidence and for Yamato the main bulkhead was to be 75mm ducol steel. When a full scale plate of this was duly tested in 1939 against a blast of 400kg of TNT, the results were encouraging since it did not split open although its watertight integrity was lost.
Unfortunately, the designers also had to counter what was considered to be the great danger of long range plunging shell fire which might dive under the main armor belt and into the ship's vitals. The physical requirements for resisting the kinetic energy of shellfire and the explosive force of torpedoes could not be easily reconciled in a single system, but in Yamato there was not the room to separate one from the other. The enormous 410mm main belt was inclined on average at a 20 degree angle which increased the thickness of armor which any steeply falling shell would have to penetrate, and this angle conveniently provided space outboard for the anti-torpedo bulge without altering the form of the hull. The belt would have run into the 75mm anti-torpedo bulkhead below, but such was the fear of shell fire following tests on the underwater trajectories of projectiles that this was radically increased in thickness till, over the main machinery, it tapered from no less than 200mm down to the original 75mm at the ship's bottom.
Given a larger bulge outboard this main defense would have been far more formidable but by linking it to the main belt the bulge was only 3m wide at most at mid-draft. By comparison with foreign practice this placed it uncomfortably far forward thus failing to take full advantage of the limited depth available and compounding the weakness by reducing its ability to deform under pressure, just when such a feature was more essential. In the USN, the South Dakotas had a similar arrangement but they were designed within stringent Treaty constraints, a worry Yamato's designers did not have. In the US Montana class, a planned vessel of similar dimension to Yamato, the holding bulkhead was placed much deeper and not tied to the belt at all.
This late adoption of a thicker, lower bulkhead caused a new problem: how to join it to the main belt above without jeopardizing the great inherent strength of either half. The solution, as can be seen, was far too flimsy and relied for its transverse strength, which would be tested most in a torpedo strike, on the shearing resistance of tap and three-ply rivets. This was to prove the Achilles heel of the system.
The percentage of explosive force which would break on this flawed main defense did not rely only on the volume of the intervening outboard space. The composition of this space could be significant, and in all Yamato's foreign contemporaries part or all of the outboard void was tilled with liquid, generally fuel oil. This was not a fire risk and since it was incompressible it could spread and reduce the shock of any explosion, and in addition diminish the danger from splinters. The Imperial Navy were well aware of this system and thought liquid layers next to the main bulkhead were the best type, but like the Royal Navy, their primary teachers, they also experimented with using closed steel tubes which were packed into the outboard spaces to fulfill a similar function.
In practice they rusted and proved inefficient energy absorbers although in a Japanese report of 1936 their use was calculated to reduce 'the thickness of the protective plate to 70 percent generally'. For Yamato, even this expedient was dispensed with partly because of the chronic steel shortage exacerbated in large measure by her and her sister's construction, Although fuel oil was stored in the double bottom this was the only use of liquid and its defensive properties were not taken into any calculation. Yamato's outboard explosion chambers were left watertight and empty of anything more tangible than air.
Since the advantages of liquid loading were understood, this result is difficult to comprehend, despite assertions at the time that the heavy armor would be sufficient by itself. Pumping arrangements for such spaces could increase flooding since the valves between tanks were liable to fracture after an explosion. but since Yamato ~ range of 7200 nautical miles at 16 knots was low, taking into account the vast size of the Pacific. the extra storage would have proved beneficial in more senses than one. It has also been suggested that they needed to he left empty for possible use in counter flooding, and they were certainly equipped for this, and yet if they were partially filled, flooding with seawater on one side would have had less effect. It is possible that the need to keep her draft shallow influenced the decision. This is supported by the fact that a proposal to reduce the individual volume of compartments outside the citadel was rejected because the extra weight would have had an adverse effect on her draft.
Two longitudinal bulkheads were included between the main armor and the outboard main machinery spaces in order to contain any flooding and, in addition, the anti-torpedo bulkhead was thickened, The two bulkheads were designed to be capable of deforming without rupturing and, to add elasticity, the flooring was offset on opposite sides of the bulkhead, hut it was still too stiff and it could suffer little deflection without rupture, at least of butts and floor connections'. There was also a fear that because the fire rooms were closed, the air pressure inside the boiler rooms would he too great, and so the two inner defenses were braced still more by heavy beams placed transversely at the upper operating level. Any movement of the bulkheads, however, would simply cause them to be punctured by the beams permitting water to enter the fire rooms. Opposite the magazines forward and aft of the machinery such expedients were not required but there was only one holding bulkhead behind the main armor, reflecting the narrowing of the hull which confined internal volume still more. If this last over-rigid barrier was breached, internal flooding was inevitable.
This, in essence, is what happened when Skate's torpedo struck. Running shallow it hit the bulge where it was less than 2m wide and the main belt took most of the blast.
This did not fracture, hut the weak joint below did shear, indenting both sides into the ship by about I m. This in turn led to the last defense being holed as mentioned in her Captain's report. Had the torpedo been running deeper and hit closer to the suspect joint, damage would have been far greater. The resultant flooding caused a list of two to three degrees hut since the outboard voids were fitted with sea cocks of 10in diameter, which could be operated remotely, counter flooding on the port side quickly put her hack on an even keel.
When these weaknesses were realized by the technical teams investigating the damage, they suggested that a new plate be installed across the lower corner of the upper void between the two inboard bulkheads and inclined at 40 degrees. This was proposed for the full length of the machinery spaces but it was hopelessly inadequate and in the event was only fitted in the region that had been damaged.
Of greater interest are the factors surrounding the decision taken in 1939 to increase the armor thickness of the side defenses, and accept what was always suspected to be a weak joint between it and the main belt. The importance of Yamato in prewar Japanese naval plans cannot he overestimated. The state economy could not hope to compete with that (if the USA in quantity, so quality and superior technology were, not for the first or last time, seen as the solution, and Yamato was the embodiment of this ambition. The navy had already constructed an elaborate and detailed plan to defeat the US Pacific Fleet, and it was intended that the big guns of the Combined Fleet would deliver the loop de grace. This planned scenario does much to explain why Yamato's designers changed emphasis towards favoring an anti-shell protective defense.
The unwillingness to wait for a suitable joint to he developed can also be understood in relation to the Imperial Navy's overall plans for the future. To establish her necessary technological lead, secrecy was vital to forestall any American response, but despite inordinate Japanese zeal some rumor of what was happening inevitably crossed the Pacific'. The US had, therefore, recommenced naval building and any delay in the construction of Yamato would have sacrificed the Imperial Navy's slender lead.
Rivalry between the army and navy also played its part, not only over funding such items as expensive battleships, hut also in determining national policy. In such a climate delays in construction could have given the army the upper hand and so had to he avoided.
Two major ironies overhang this discussion. The obvious one is that all this time, effort and money spent on battleship construction was to prove virtually irrelevant. The future of naval warfare lay with carrier air power not the big gun. Both Yamato and her sister Musashi were to succumb to aerial attack, their main l8in guns virtually unused in anger. The second is that the torpedo fired by Skate was armed with a warhead of 635lb of a new explosive, 'Torpex'. This had twice the explosive power of TNT and with its introduction in 1943 it upset, at a stroke. all the careful calculations on which Yamato's designers had labored for so long. It will be recalled that constraints were such that defenses could only ever he just sufficient: With the introduction of Torpex the most robust anti-torpedo system was likely to be breached. It was a pity that Yamato's design sacrificed underwater defense in depth when the threat in this sphere had increased so markedly. Nonetheless, even with the flaws discussed here her defenses were still powerful and her resilience under assault later amazed her American foes. However, her torpedo protection was without doubt the weakest element in an impressive design and it is not surprising that torpedoes were eventually to dispatch both vessels.
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