The last IJN cruiser

She was one of the last cruiser of the Empire, the very last being the fourth Agano class. Initially designed as a hybrid scouting cruiser on the same principle as IJN Tone, yet lighter, cheaper and faster to build, she was tailored to carry and operate specifically six Kawanishi E15K long-range seaplanes, fitted with a 45-meter (147 feet) long aft catapult, to screen forward of oceanic submarine fleets.

Her armament comprised two triple 6-in guns (155 mm) at first, same as the Mogami-class, but as the war progressed, she was rearmed with two surplus twin 8-in turrets, so just four in all. At her completion in February 1943, planned long range operations, notably against the Panama Canal by large Japanese carrier submersible were very compromised. Also, her own planned seaplanes were still not operational. The admiralty decided to fit her instead a more standard catapult and standard model floatplanes. The large hangar was no longer used, and she operated with just two aircraft.

Due to her reduced military value, IJN Oyodo saw little fighting. Twelve ships of the same class were initially planned for the 1939 and 1942 naval plans, none being built, or even laid down. IJN Oyodo received a better AA as the war progressed: In in 1945 this went up to fifty-two 25 mm AA in triple, twin or single mounts. This did not prevented her eventual fate: In 1945 she was anchored in kure, short of fuel, when USN aviation straddled then sank her in the harbor, on July 28, 1945. As the the last survivors of the Imperial Navy's cruiser, she was broken up after the war, but the modern JMSDF took inpiration of her for large ASW helicopter desoroyers of the 1970-90s.

Development

Design

Hull construction & general characteristics

Propulsion

Armor Protection

The armor protection of Oyodo was based on the need to withstand direct hits from 6-in semi-armor-piercing (SAP) shells, and 250-kg bombs from 3000 m. The CNC steel main armor belt was 60 mm thick, and covered compartments over the machinery spaces, the rear electric generators and ammunition stores located between frames 92 and 155. It was 2.35 m wide above the waterline. The ship having a design draft of 5.95 m, it was supposed to rise by 1.56 m above the waterline.

An armored middle deck also made of CNC plates was 30 mm thick for the amidship section, 28 mm (1.1 in) on both edges, and 2 meters in width. Forward the armored deck started where the citadel ended, enclosed with a 35 mm transverse bulkhead connected from below to the lower deck. The aft 35 mm bulkhead (1.4 in) descended to the hold deck level. The 5-meter section between the lower and hold deck was given an increased thickness, up to 50 mm, as it was the aft wall next to the ammunition store.

The citadel ran from frames 55 to 92, divided into two unequal parts. The forwards section was between frames 55 to 83 and covered the ammunition storage, so quite serious protection including an internal armor belt which was 2.6 m wide (10.2 feet), assembled from CNC wedge-shaped plates. It was 75 mm thick (3 inches) at the top, 40 mm (1.57 in) at the bottom. From above, it was combined with a 50 mm (2 in) armored lower deck also in CNC, connected from below and resting on the double hull bottom.

The aft citadel section, frames 83 to 92, protected the 6 in and secondary guns along with thr second radio room. It was joint to a 28 mm (1.1 in) armored lower deck. The forward part of the citadel was also bonded to the main armored deck by a 60 mm, tapered down to 25 mm bulkhead, the forward and aft section separated by a 10 to 16 mm (0.6 in) bulkhead, made of ordinary D steel plating.

The 6-in turret barbettes had walls of CNC steel 20 mm thick (0.8 in), with 25 mm (1 in) support rings. Below the middle deck, the barbettes were downgraded to a 35 mm thick ring, which was conical with 120° angled plates. Beyond the armored lower deck thickness was reduced from to 25 mm, but continued by a tube with 25 mm walls, inclined to 60°. Outside the barbette, elevators for the 25 mm and 100 mm ammunition went in tubes respectively in front and behind the barbette of B turret. About one meter above the lower deck, they were covered by CNC plates 55 mm thick (2.16 in) 35 mm front and rear plates.

The bridge was protected by 40mm CNC steel plates forward, 20mm D steel plates on its sides and rear. It was enclosed at the top by 20mm CNC plating. Between the bridge and control posts, under the armored deck they were communication tibes with 8 mm D steel plates.

The funnel bases were protected by D steel metal sheet, 10 mm thick (0.4 in) front and rear. The sides received 16 mm plating, but also 30 cm below (11 in) and 70 cm (27 in) above the middle deck. Ventilation ducts of the engine rooms had the same protection all along their truncation. The steering control room was protected on all sides by CNC plates 40 mm thick (1.57 in) down to 20 mm at the front and 25 mm at the rear. The middle deck above them used a sandwich protection, with two layers, one upper 20 mm CNC plate and a lower 16 mm D steel plate.

The shell elevator was protected up to the middle deck level with 35 mm CNC plating (1.37 in) on all sides. The communication shafts between the engine rooms and steering gear were covered by 10-16 mm D steel plating. The aviation fuel tanks had 16 mm D steel plating as well.

IJN oyodo's underwater protection was poor however. First off due to the reduced beam of just 16.6 m, there was of course no armored anti-torpedo bulkheads. Designers instead relied on high subdivision of the hull into many watertight compartments, an old and proven solution, many of these side compartments filled with oil or seawater. The double hull bottom had 94 compartments and space between this double hull and main hold deck comprised 159 subdivisions, while the rest of the hull above the hold deck had 28 watertight compartments that could be filled artificially by automated valves, used for counter-flooding. A total of up to 613.3 tons of water was supported to correct a listing port or starboard.

The machinery compartments were separated by a longitudinal bulkhead making it possible to continually operate avec after a single torpedo hit. However, with her tight waist, IJN Oyodo had little margin for stability. Calculations showed it could withstand the flooding of only one engine room and one boiler, listing to 15°. This is what happened on July 28, 1945, but the crew at the time was unable to counterbalance the multiple torpedo hits in time by proper counter-flooding, so the cruiser capsized as a result.

Armament

Author's profile illustration Oyodo 1944

Specifications

Displacement 8,164 t. standard -10 252 t. Full Load
Dimensions 192.10 m long, 16.60 m wide, 5.9 m draft
Machines 4 propellers, 4 turbines, 6 boilers, 110,000 hp.
Top speed 35 knots
Protection 50 to 25 mm
Armament 6 x 155 mm (2 × 3) guns, 8 x 100 mm (4 × 2) guns, 12 x 25 mm AA guns, 2 planes
Crew 600

IJN Oyodo in combat

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Naval History