Sendai as of 1924:
The Sendai class retained basically the same hull and armament of the preceding Nagara Class, the difference was in a rearranged boiler setup, wich led to the four-funneled profile of the ships, a distinctive mark over the Kuma and Nagara classes wich had only three of them.
Eight cruisers where initially planned for this class, Sendai, Jintsu, Naka, Kako, Ayase, Minase, Otonase and an eight unnamed unit, only the first four where actually laid down and three completed because of the Washington Naval Treaty restrictions; Kako was scrapped and reordered as a Furutaka Class heavy cruiser.
Jintsu as of 1925:
identical to Sendai, the only way to spot Jintsu from the lead ship was to look at the RDF equipment fitted between n°3 and n°5 5.5-inch (140mm) gun mount as it was placed differently.
Naka as of 1925:
laid down in june 1922 her hull was destroyed on november 1st 1923 during the Great Kanto Earthquake; Naka was supposed to be launched that very same month, but the damage suffered was irreparable, so the ship was scrapped and laid down again in may 1924; she was eventually launched in april 1925 and commissioned another eight months later.
Naka recieved a clipper bow because of her construction delays after the Kanto Earthquake while both Jintsu and Sendai retained the original stem profile of the previous Kuma and Nagara class light cruisers; the RDF equipment position also differed from that fitted on both Jintsu and Sendai.
Jintsu as of 1932:
During a night training exercise on 24 August 1927, Jintsu rammed and broke in two the destroyer Warabi, sinking her, her bow was also heavily damages and she had to be taken to Maizuru for major repairs; by 1928 when work where done Jintsu had been fitted with a clipper bow like it was done on Naka at the time of her completion, thus leaving Sendai as the only of the three sisters still sporting the old bow.
by 1932 Jintsu also recieved a tpe 2 catapult fitted on the, untill then unused, take-off platform, an Aichi E3A was also embarked as the reconaissance floatplane.
Naka as of 1934:
Naka was the first Sendai class cruiser tu enter the naval yards to be reffited, works lasted from april 16th 1933 to january 31st 1934 and the following modifications where implemented: a lookout platform was added on the minmast, also the fire control platform was enlarged, windows and a fixed roof where given to the bridge and the original 2,5 meter rangefinder fitted above it was replaced with a larger 4meter one; the hangar was converted into office and bridge space, the take-off platform forward of it was retained tough, and a quadruple 13mm machine gun mount was fitted at it's end.
the simgle pole mainmast was replaced by a tripod one, adn the single 90cm ùsearchlights was replaced by two of the same type, a built-up derrick was also added for seaplane handling operations, a kure type 2 catapult was thus fitted between n°6 and n°7 5.5-inch (140mm) gun mounts, the latter of wich was shifted further aft, a Nakajima E4N was embarked as the recon seapane and the upper deck was expanded to starboard between n°6 and n°7 gun mounts.
Sendai as of 1934:
refitted from june 13th 1933 to january 25th 1934 Sendai ended up looking even more different from her sisters, other than still beign the only ship of her class sporting the curved bow she had her foremast reduced in height, two 1,5 meter navigation rangefinder where fitted on both sides of the bridge, two extra lookout posts where also added just below them, the derrick fitted on the mainmast was a pole one instead of a built-up type like those fitted on Naka and Jintsu.
Jintsu as of 1934:
Jintus entered the refit yards in november 1933 and works where completed by july 1934, modifications where much int he lines of Naka, altough some details where different: the fire control platforms where not enlarged as much as it was done on Naka, also the take-off platform in front of the bridge was removed, bridge structure extensions where different; the 3-inch (76mm) anti-aircraft guns where replaced by twin 13mm machine gun mounts, the old aichi E3A was also retaines as the reconaissance seaplane
Naka as of 1935:
less than a year after her main reconstruction Naka underwent a second, minor, one; the flying-off platform was removed and the quadruple 13mm machine gun mount was shifted back in front of the bridge, also the 3-inch (76mm) anti-air guns placed abreast the second funnel where replaced by twin 13mm machine guns
Sendai as of 1936:
refitted once again between 1935 and 1936 to correct topweight issues after the Tomozoru incident Sendai had the fore funnel reduced in height to mach the other three one, giving the ship an even more unique silouetthe, also the former four 90cm searchlights where replaced by three 110cm ones, with the two fitted on the mainmast replaced by a sinlge one fitted on an explanded platform; finally a Kawanishi E7K replaced the Nakajima E4N.
Naka as of 1938:
between february 5th 1937 and january 31st 1938 Naka underwent her second major refit, during the works the bridge structure was further modified ed expanded, foremast platforms where reworked and the mast itself shortened in height, two 1,5 meter rangefinders and two lookout platforms where added on both sides of the bridge, the four 90cm searchlights where replaced by three 110cm ones, the two on the mainmast replaced by a single one, the twin 13mm machine guns fitted abreast the n°2 funnel where replaced by 25mm ones, a 3,5 meter rangefinder tower was added between the third and fourt funnel, a unique feature of the Sendai class, finally an Aichi E11A floatplane replaced the Nakajima E4N.
Jintsu as of 1939:
the second main modernization of Jintsu lasted from december 15th 1938 to november 15th 1939, the refit flowed roughly the same lines of Naka with a few differences around the foremast platforms, the seplane upgrade replaced the early Aichi E3A with a Kawanishi E7K
Sendai as of 1941:
between june 1937 and may 1941 Sendai underwent various smalle r refit instead of a main single one, the first modification was the replacement of the twin 13mm machine guns abreast n°2 funnel with two twin 25mm ones, the second saw bridge and foremast modification with the addition of the 3,5 meter rangefinder tower between the funnels, finally the last one was the fitting of a degaussing calbe and a depth charge projector at the stern; a torpedo tube refit, consisting in the removal of the fore twin launchers and the replacement of the aft ones with quadruple ones was also concieved, but shortage of torpedo tubes prevented it.
Naka as of 1942:
between mach and may 1941 Naka was refitted once again, a depth charge projector was added at the stern, the fore torpedo launchers where removed and the space previously occupied by the mounts themselves and the reloads was enclosed and converted into crew space, the aft twin launchers where then replaced by quadruple ones capable of firing the Long Lance torpedoes, finally a degaussing cable was added.
Jintsu as of 1943:
by the time of her sinking at the Battle of Kolombangara in july 13th 1943 Jintsu had underwent the same modifications done to Naka, then in early 1943 two triple 25mm machine gun mounts where added on deck above the torpedo tubes.
Sendai as of 1943:
refitted at Sasebo from may 5th to june 25th 1943 Sendai recieved a type 21 air search radar on the foremast in place of the type94 director, the mast itself was further modified and some riggings redone, the n°5 5.5-inch (140mm) gun mount was removed and two triple 25mm machine guns added above the aft torpedo tubes, two depth charge rails where also added at the stern; no other modifications where done by the time of her sinking on november 2nd 1943 during the Battle of Empress augusta Bay.
Naka as of 1944:
torpedoed by Uss Seawolf on april 1st 1942, Naka reached Singapore where she was docked for temporary repairs from april 10th to june 3rd 1942, then she steamed to Maizuru when she was drydocked again, repaired and refitted from june 21st to april 5th 1943; her modification where quite similar to those made on Sendai, a type 21 air search radar was added to the foremast, the n°5 5.5-inch (140mm) gun mount was removed but in this case replaced by a twin 5-inch (127mm) one, two triple 25mm machine guns added above the aft torpedo tubes, two depth charge rails where also added at the stern, lower deck portholes where sealed off to improve watertight integrity, finally the aichi E11A was replaced by a Kawanishi E7K.
no other modification where done by february 17th 1944, when Naka was sunk during the US attack on Truck lagoon by planes from the Uss Cowpens and Uss Bunker Hill.