The Sendai-class light cruisers were a development of the preceding Nagara class. Their boilers were better located, and they had four funnels instead of three. Each cruiser class was to be an extension of the previous class with updates and improvements over the preceding class. The Sendai class was to remedy the failings of the single gunned ships preceding them and have an all inline armament. To have been a class of eight ships, the last five were cancelled by the Washington Treaty and the funds repurposed to the Furutaka class.
The old fashioned bridge and foretop mast combination, dated these ships as 'WW1" designs. It was time to leave that past behind and make improvements. On the drawing boards while these ships were under construction was the new Fubuki class destroyers. The Sendai class were to act as destroyer leaders and the armament and control functions were designed with that in mind. The biggest change would be the replacement of the single 5.5" guns with twin and single 5" of the same type and model fitted to the Fubuki and its clone classes. The funnels were trunked together to give twin funnels. The original type A 5" mountings fitted had an elevation of 55 degrees, which gave them a minor AA barrage capability but were not a full dual purpose weapon.
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The renovation of these three ships was compiled over 15 years of service. While in use as destroyer leaders it was found there was not enough room for squadron offices and the solution was to raise the bridge structure by seven feet and break it up as the needed offices. The main rebuild was to enhance the AA capability of the main and tertiary armaments. All six main armament turrets were removed and replaced with one twin dual purpose 5" turret on the bow, while a further four twin open backed mountings replaced the 'B', 'D", 'E', and 'F' turrets. 'C' turret being replaced with a pair of 25mm mountings. This gave the ships an overall armament of 10x5" AA and 17x25mm AA. The 25mm guns kept increasing in numbers till over 40 were fitted. Radar started being fitted from 1942 onwards.
The rebuilding of the three ships virtually had them rejoining the Fleet as it sailed for Hawaii and wars start. They would lead three of the Fleets destroyer squadrons.
