Japanese Battleship Mutsu - Investigations Into The Loss

Investigations Into The Loss

An official accident commission led by Admiral Shiozawa Koichi was convened three days after the sinking to investigate the loss, with its preliminary conclusions issued on 25 June 1943. The commission considered a number of possible causes:

• Sabotage by enemy secret agents. Given the heavy security at the anchorage and lack of claims of responsibility by the Allies, this could be discounted.

• Sabotage by a disgruntled crewman. While no individual was named in the commission’s final report, its conclusion was that the cause of the explosion was most likely a suicidal crewman in No. 3 turret who had recently been accused of theft.

• A midget or fleet submarine attack. Extensive searches immediately following the sinking had failed to detect any enemy submarine and the Allies had made no attempt at claiming the enormous propaganda value of sinking a capital ship in her home anchorage; consequently, this possibility was quickly discounted. Eyewitnesses also spoke of a reddish-brown fireball which indicated a magazine explosion and was confirmed during exploration of the wreck by divers.

• Accidental explosion within a magazine. While the Mutsu carried a large number of projectiles, immediate suspicion focused on the Type 3 anti-aircraft shell as it had been suspected of causing a fire before the war at the Sagami arsenal. Known as a "sanshiki-dan" or “sankaidan” these were fired by the main armament and contained 900 to 1,200 25 mm diameter steel tubes (depending upon sources), each containing an incendiary charge. Tests were conducted at Kamegakubi Naval Proving Ground on several shells salvaged from No. 3 turret and on shells from the previous and succeeding manufacturing batches. Using a specially built model of the Mutsu’s No. 3 turret, the experiments were unable to induce the shells to explode under any perceived service or artificial service conditions.

• Fire. Compared with other nation’s warships in wartime service, Japanese battleships contained a large amount of peacetime flammable materials including untreated bedding, wooden decking, wooden furniture and wooden sheathing (for insulation). When she had been modernised in the 1930s, some of the Mutsu’s electrical wiring may have dated from her original commissioning. While fire in the secure magazines was a very remote possibly, a fire in an area adjacent to the Number 3 magazine could have raised the temperature to a sufficient enough level to ignite the highly sensitive black-powder primer stored in the magazine and thus instigate the explosion.