A severe earthquake during cooling of several cast 20" naval artillery barrels hoplessly deformed them, destroyed the masters,and nearly destroyed the foundry, too. Only ten barrels survived intact, enough for five twin turrets of the type proposed for the two Satsuma-class large battleships that needed three turrets apiece. When the Yokozuna project was contemplated, there still weren't enough turrets to supply the needs of the ship and she was forced to use triple 18" instead. One of the two remaining 20" turrets was deemed neccessary as a spare for Satsuma, but the other was to go to sea on a specially-built ship.

Officers were impressed by the British Terror when it patrolled off China before the war, and they often wondered what kind of ship would be needed to combat it. With its single 15" turret, Terror could stand off and shell incoming cruisers at long range, so the Japanese thought that in the absence of aircraft, even the slow-moving and slow-firing heavy guns of such a ship would warrant a disproportional commitment of forces to oppose it. The Aki was intended to be such a ship; able to bombard cities or installations far inland along coastal Asia, or escort a convoy and take on anything short of a battleship that tries to interfere. The huge guns on such a small ship made a good impression when seen from dockside, too, so Aki was used to recruit interest in the Navy during ports-of-call. The ship had heavy ballast along the keel and a tremendous, bulging beam for such a short ship to keep it upright when the heavy guns recoiled. Aki pushed aside so much water so fast that sailors swore there was a "dry spot" directly astern of the ship as the parted water tried to close back in after her passing.

Aki patrolled around the Home Islands and escorted convoys to and from Indonesia. Both a surfaced American submarine preying on a convoy Aki was accompanying and Aki herself got a rude shock when the 20" guns fired. Aki's deck crewmen had their shirts ripped off by the blast and suffered flash burns, concussions, and ruptured eardrums. The sub was hit by a "short" and blew up in a fountain of spray. Six months later, swarms of new Douglas Skyraider dive bombers from the USS Coral Sea found Aki hugging the Honshu coast and thoroughly pummeled the small craft until it went under.