Yahagi, the second Japanese warship of that name, was the third of the four Agano-class vessels (the other three being Agano, Noshiro and Sakawa).
Construction of the Agano-class cruisers was approved by the Japanese parliament (Diet) in March 1939 under the Fourth Naval Armaments Enhancement Program (Dai-Yon-Ji Kaigun Gunbi Jūjitsu Keikaku), also known “Four-in-Circle Program (Maru Yon Keikaku), or simply Maru 4. Under the terms of the program, the four light cruisers (kei jun’yōkan),also referred to as type B cruisers (otsu-gata jun’yōkan, or simply otsu jun) and officially classed as second-class cruisers (ni-tō jun’yōkan), were to fulfill the role of destroyer squadron flagships. At that time destroyer squadrons (DesRon), called literarily torpedo squadrons (suirai sentai), consisted of four four-ship destroyer divisions (DesDiv, or kuchiku-tai).
The design plans of the new ships were prepared at Navy Technical Department (Kansei Honbu, or Kanpon) by a team led by Shōshō (Rear Admiral) Keiji Fukuda. The final version of the plans was approved in October 1939. The cruisers featured graceful, slender deck lines, a single funnel and a compact superstructure. Continuous main deck ran along the entire length of the hull, rising towards the bow and dropping slightly in the stern section. The ships were powered by six Kanpon Ro-Gō oil-fired boilers feeding steam to four turbine sets, which drove four propellers. Propulsion system generated a total of 100,000 shp, which translated into a top speed of 35 knots. With the bunkerage of 1,406 tons, the ships had a range of 6,000 NM at a cruising speed of 18 knots.
Relatively light armor weighing 656 tons was designed to protect the key areas of the vessel (mainly machinery and magazine spaces) against 127 and 152 mm shells. Propulsion system compartments (boiler rooms, engine rooms and electric generator rooms) were protected by a 60 mm armor belt, a 20 mm middle deck and by two transverse 20 mm watertight bulkheads. The main artillery ammunition magazines, adjacent to the bulkheads, featured 55 mm armor plating on the sides, 20 mm armor on the roofs, 20 and 25 mm plating in the front section (forward magazines) and 20 mm in the back (aft magazines). The wheelhouse armor protection consisted of 40 mm plating in the front, 20 mm on the sides and floor, 30 mm on the roof and 16 mm in the back. Steering gear and rudder rooms in the aft section of the ship featured 20 and 30 mm armor on the sides, 20 mm on the roof and 16 mm in the front and back. Other vital structural elements and pieces of equipment were also protected by armored plates. However, there was no anti-torpedo protection due to the small beam of the cruisers’ hulls. The only anti-flooding measure in the event of hull breach were watertight transverse and longitudinal bulkheads.
The main battery consisted of six 152.4 mm 50-cal Type 41 15-cm guns (50-kōkei 41-shiki 15-senchimētoru hō) mounted in three triple turrets – two superimposed turrets forward of the superstructure and a single aft turret, behind mainmast. A twin turret mounting heavy 76.2 mm 60-cal Type 98 8-cm high-angle guns (60-kōkei 98-shiki 8-senchimētoru kōkaku-hō) was mounted on each side of the superstructure. Agano-class light AA armament fit differed from ship to ship and underwent changes during the vessels’ service lives as a result of refits and modernizations (see below). Two Type 92 Model 4 quadruple torpedo launchers (61-senchimētoru 92-shiki 4-gata 4-rensō gyorai hassha-kan) were installed on the ship’s centerline underneath the aircraft platform and catapult. The ships carried 16 610 mm Type 93 Model 1 Modification 1/2 torpedoes (93-shiki 1-gata kai 1/2 gyorai), eight in the tubes and eight at the ready for a quick re-load. In addition, the cruisers were equipped with two rails for Type 95 Modification 1/2 depth charges (95-shiki kai 1/2 bakurai) with a total supply of 18 charges. Later on newer Type 2 Mod. 1/2 DCs (2-shiki kai 1/2 bakurai) depth charges were used. The ASW suite consisted of Type 93 Model 2 hydrophone (93-shiki 2-gata suichū chōonki) and Type 93 Model 3 sonar (93-shiki 3-gata sagushingi).
The cruisers carried two three-seat Type 0 reconnaissance seaplanes built by Aichi (Rei-shiki sanza suijō teisatsuki, E13A1, or “Jake” in the Allied code). The machines were stowed on the aircraft platform located amidships and were launched using a 19-meter Kure-Type Mark 2 Model 5 catapult (Kure-shiki 2-gō shashutsu-ki 5-gata). Upon the return from a mission the aircraft would land alongside the ship before being hoisted onto the deck by a 15-meter crane attached to the mainmast.