The battleship that broke the 15-inch naval gun barrier

Countries & Cities · Published 12 October 2026 Swipe more like this →

In short: In the early 20th century, naval supremacy was measured in inches. The Nagato-class battleships were the first dreadnoughts launched with guns larger than 15 inches, making them the most powerful afloat at the time. Their evolution from coal-fired giants to modern warships mirrors the rapid, violent pace of naval innovation.

Constructed for the Imperial Japanese Navy near the conclusion of World War I, Mutsu and her sister Nagato represented Japan's final pre-Treaty capital vessels. Representing a major innovation, these ships were the initial class equipped with 41 cm (16.1 in) weaponry. To achieve this power, the ships utilized an American "all or nothing" armor scheme, which prioritized protecting the ship's core with maximum thickness while leaving non-essential areas unarmored. They also featured a unique "heptapodal" seven-legged mast designed to maintain rigidity for range-finding even under heavy shellfire.

The ships underwent significant transformations between 1919 and 1945. Originally designed with 21 boilers, they were modernized in the mid-1930s to use 10 oil-fired Kampon boilers and more modern turbines. This reconstruction included lengthening their sterns and rebuilding their superstructures into a distinctive "pagoda mast" style. While the US Navy initially believed the ships could only reach 23 knots, sea trials revealed they were capable of much higher speeds, with Nagato reaching 26.7 knots during her initial trials.

The class's history is marked by both technological triumph and tragedy. Mutsu was destroyed in June 1942 when an aft magazine detonated, killing 1,121 crew and visitors; the IJN later attempted to conceal the cause to maintain morale. Nagato served as the flagship for Admiral Isoroku Yamamoto during the attack on Pearl Harbor in 1941. After being converted into a floating anti-aircraft platform in 1944, Nagato was eventually sunk by a second U.S. nuclear weapon test during Operation Crossroads in 1946.

Technologically, the ships were pioneers in underwater protection. A substantial fuel oil reservoir sat behind a heavy torpedo bulkhead, which supported a deep water-tight space next to the hull. This system was proven in full-scale trials to successfully resist 200-kilogram (440 lb) torpedo warheads. By the mid-1930s, their main guns were further upgraded to increase their maximum range from 30,200 to 37,900 meters.

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