THE RECONSTRUCTION OF THE BATTLE FLEET OF THE IMPERIAL JAPANESE NAVY BETWEEN THE WARS

The interwar period was shaped by the two Naval Arms Limitation Treaties, which prohibited the building of new capital ships and placed strict limitations on the reconstruction of existing units. These constraints flew in the face of established practice, which dictated that major warships be constantly updated to take into account progress in technology, particularly with regard to weaponry and propulsion, in order to adapt their offensive and defensive capabilities to the most recent advances and thereby prolong active service life.

The Washington Treaty determined capital ship tonnage allocations for the five major naval powers according to the following ratio: Britain and the USA 5; Japan 3; France and Italy 1.75. Japan was permitted to retain ten capital ships (four battlecruisers of the Kongo class and the six battleships of the Fuso, Ise and Nagato classes), one of which (Mutsu) had been rushed to completion, for a total of 315,000 tons. Fourteen incomplete or projected capital ships of the Eight-Eight Fleet had to be abandoned. The older ‘semi-’ and ‘pre-dreadnought’ ships completed before the Kongo class were all to be scrapped or relegated to training duties. This gave the IJN ten capital ships versus eighteen for the US Navy – a tonnage ratio of approximately 60 per cent – and compelled Japan to subject its strategy for a possible war against the USA to a fundamental review.

Under Washington Treaty rules, the service life of a capital ship was to be 20 years, and a replacement ship could be completed only after this period had passed. In addition, a so-called ‘naval holiday’ was agreed, meaning that no new capital ship could be commissioned before 1932. This was a lengthy period that would preclude the incorporation of new technology into the battle fleets of the major powers, leaving only obsolescent vessels in service by the end of the decade.

Many of the capital ships to be retained had been designed around 1910. Ten years later, not only had naval technology advanced, but the advent of aircraft and the submarine had radically changed the conditions under which future naval warfare would be conducted. The capital ships designed before the First World War had little protection against the new threats from the air and from below the surface. Moreover, the lessons drawn from the only major engagement between the British and German battle fleets, the Battle of Jutland, had effectively drawn a line between ships designed prior to 1916 and those designed in its aftermath. However, under the terms of the treaty, only Britain was permitted the construction of new capital ships (Nelson and Rodney), while the US Navy was allowed to complete two battleships armed with 16in guns in exchange for the completion of the Japanese Mutsu, which was similarly armed. The current construction programmes of France and Italy were effectively terminated, and their surviving capital ships, all of which had been completed before Jutland, would become quickly outdated compared to their more modern British and American counterparts.

Despite the constraints on new construction, both Britain and the USA were anxious to protect their older ships against the new dangers posed by submarines and aircraft, so the treaty included provision for a limited reconstruction within a limited increase of displacement. In Part 3 Section I (‘Rules for Replacement’) the contracting powers agreed that no retained capital ship should be reconstructed except for the purpose of providing means of defence against air and submarine attacks, and that for this purpose existing tonnage could be equipped with bulge or blister or anti-air deck protection, provided the increase in displacement did not exceed 3,000 long tons (3,048 metric tons) for each ship. No alteration in side armour, in calibre, number of and general type of mounting of main armament was permitted. However, there were some exceptions for France and Italy and one for Britain – a major reconstruction of the battlecruiser Renown was underway which involved fitting both vertical and horizontal protection.

The IJN began the modernisation of its capital ships with the oldest units, the battlecruisers of the Kongo class. Work on Haruna began in 1924, followed by her three sisters – reconstruction of the fourth ship, Hiei, had to be paused then modified following the London Conference of 1930. These four ships were successively modernised in accordance with the terms of the treaty: the horizontal and underwater protection was reinforced, but the propulsion system had to be left as it was. The speed of the ships declined accordingly, while protection was strengthened almost to battleship standard. Following reconstruction the three ships of Kongo class were reclassified as ‘battleships’ on 1 June 1931, and from this time use of the ‘battlecruiser’ classification was discontinued.

The London Treaty of April 1930 extended the ‘battleship holiday’ until 1936, thereby precluding new construction. It also stipulated that the number of capital ships remaining in service be reduced to the level envisaged by the Washington Treaty of 1922; excess vessels would either be scrapped or used for other permitted purposes. For Japan this meant that the former ten ships had to become nine, and Hiei, which had only recently begun her reconstruction, was to be redesignated a training ship; this would involve stripping one part of her armour and armament and reducing her service speed. Both arms limitation treaties were due to expire at the end of 1936 and there was no prospect for their renewal.

It was informally agreed at London that modernisation of the older battleships remaining in service could be more extensive than originally envisaged at Washington, and the IJN promptly embarked on radical reconstructions of her older battleships, beginning with Fuso and Yamashiro. The reconstruction, which was executed in two phases – for Yamashiro these were contiguous – lasted from 1930 to 1935. The four ships of the Ise and Nagato classes were then taken in hand, and the three Kongos that remained in active service underwent a second reconstruction.

This second series of modernisations involved marked structural changes that extended to the general arrangement and the distribution of equipment and fittings. The appearance of all ten ships was radically altered. The IJN term used for this work was kaizo (‘modernisation conversion’) in order to distinguish it from the customary kaiso (‘refit’). The extent to which the reconstruction increased the defensive capabilities of the older ships is open to question; the dated design of these units placed serious constraints on the effectiveness of ‘add-ons’ such as extra armour plating and bulges. However, the IJN invested massive resources, both financial and human, in the reconstructions, and the naval architects deployed considerable ingenuity in devising the required improvements, which were intended to bring Japan’s battle fleet up to a uniform standard and enable it to defeat the US Navy in the envisaged ‘decisive battle’.