IJN Sōryū (1935)
Japan, 1937-42. Fleet Aircraft Carrier
The first purpose-built IJN fleet carrier
Another important landmark in the development of IJN air power, was the construction of the IJN Sōryū. Until then, following a parralel with the USN, IJN Hosho was the earlier equivalent to the USS langley, Converted IJN Akagi and Kaga, the answer to USS Lexington and Saratoga, and the IJN Ryūjō, the equivalent of the USS Ranger, a light carrier playing with the Washington's treaty inconsistencies. She was the result of the London treaty, opening a new alley for Japanese medium carriers.
The IJN Sōryū was often paired with IJN Hiryū and indeed, they proceeded from the same program, as half sister-ships. But they diverged considerably, and the former paved the way to the excellent Shōkaku-class aircraft carrier completed in 1941, and in fact for a whole wartime generation of Japanese fleet aicraft carriers. IJN Sōryū's career started immediately with the Sino-Japanese war and her actions with the Kidō Butai, followed its fate at Midway in June 1942.
The London Naval Conference of 1930 at last allowed the Japanese to strengthen their carrier fleet as the parity with the USA and Great Britain and Japan shifted from 5:5:3 at Washington to 10:10:7 which was more favorable and opened an option for two more, larger aircraft carriers. Both ships were indeed built under 1st Supplementary Program of 1931.
IJN Sōryū was one of two new large carriers approved for construction, under the 1931–32 Supplementary Program with IJN Hiryū. This was a first as she was to be designed from the keel up as a medium fleet aircraft carrier. The main advantage of the time she was started, was to incorporate lessons learned from the light carrier Ryūjō
. The latter as a reminder was built to exploit a loophole of the Washington treaty defining aircraft carriers by their tonnage; Below 10,000 tons.
The idea was to built swarms of these light carriers, outside the cap fixed in that category. Of course the treaty's revision led to the closure of this in the 1930 London treaty, just as the first ship was built. So she remained alone, but experience with her led to an interesting starting point to develop a larger fleet carrier, while still relatively light, but maximizing aircraft capacity while using available treaty tonnage.
During the Shanghai incident of 1932, 1st Koku Sentai, IJN KAGA and HOSHO were deployed to the Chinese coast, first opportunity for Japanese carriers to be tested in battle conditions, gaining invaluable operational experience. Around this time the impractical triple-deck arrangement on AKAGI and KAGA was abandoned and the two carriers were indeed scheduled to be rebuilt by 1935 and 1938. But the fleet still needed its first truly successful medium sized Japanese carrier. IJN SORYU and HIRYU were authorized under the Second Replenishment Program of FY 1934 which accentuated the expansion of naval aviation. In addition to two purpose-built carriers it included two fleet oilers TSURUGIZAKI and TAKASAKI, designed for rapid conversion to carriers (later SHOHO and ZUIHO). The general aim was to achieve parity with the United States naval air arm.
Of course, all aforementioned carriers were conceived in accordance with the tonnage restrictions of the Washington Treaty. It was hoped that by the time of HIRYU's completion in 1937 the restrictions would be lifted, but in January 1936 the Japanese delegation withdrew from the new London Treaty talks. It was decided not to comply with the limitations of both Washington and London Naval Treaties any longer. After 1 January 1937, warship construction could proceed without further constraints.
1933 - start of the design process
First off, it was obvious in 1930 that the design of Ryūjō was top-heavy, especially after the admiralty decision to double the number of aircraft. At that stage, in 1933, it was clear on her first trials that she had serious stability issues and was downright dangerous. She was rebuilt in 1935. Still in 1933, the engineers at least started with a better basis, freed from the below 10,000 tonnes cap. The ship could have a larger, beamier hull, a slightly taller forward freeboard to cope with heavy weather and better hangar-prow combination, while accommodating a more reasonably shaped hangar, and not inheriting a tall tandem hangar.
The "Blue Dragon", translation of her name, was still on specifications tailored for speed, and an emphasis on aircraft capacity over self defence. Targeting 16,000 tonnes, the admiralty still hoped to have three instead of two over the fleet's carrier authorized tonnage at the time. In 1935, the Japanese indeed ripped their treaties altogether. The reasoning was the same as on the American USS Ranger, designed earlier (In 1930).
The beam was augmented by 50 cm (from 20.8 to 21.3 m or 69 feets 11 inches), and the overall length jumped to 227 m (746 feets 5 in) instead of 179 m, engineers preferring to stretch the design to add more capacity, which also favored a better length-width ratio and thus, again, overall speed. On Soryu, this ratio was of 10,65 versus 9 on Ryujo. For speed and agility it became evident that four shafts produced some advantages. Therefore, a four turbines arrangement was preferred, with eight boilers instead of six, resulting on a brute output of 152,000 shp versus 65,000 shp on Ryujo, almost double.
The end result, combined to the better ratio was to provide these ships a top speed well in excess of 32 knots, whereas Ryujo was limited to 29 knots, just sufficient to act with battleships. For Soryu, it was expected to use these carriers in night operations with the latest IJN heavy cruisers, and therefore a top speed of 34-35 knots was targeted. The new design was also required to carry more planes, in this case 63 plus nine in reserve so an overall 72, versus 48 on Ryujo (and that was with spares and in 1933, the size of aircraft downgraded this massively until 1942).
1934 - The design is refined
With all this in mind, the engineers responsible for the design submitte their final proposal in early 1934, wich was approved and blueprints prepared by the chosen naval yard of Kure Naval Arsenal. Construction at last started on 20 November 1934 when her keel was laid down. This was more than one year and a half after USS Ranger had been launched already. In Comparison, the latter was slightly shorter, but beamier, slower, but carried more planes, a grand total of 86 as built, which was remarkable for such light tonnage. In between, design proceeded on the next generation of aircraft carriers in the USN, the Yorktown class
, and both navies were capped by the same global 135,000-ton Treaty limit.
The new design, worked out from 1933, targeted 90 planes, but on a better balanced 19,000 tonnes basis. Meanwhile, the Japanese proceeded with IJN Hiryu ("Flying dragon"), a virtual copy of the first, with two years of gap, but with enough difference not to be considered as a true sister-ship. No naval historian today attached them in the same class anyway.
With their post-1935 "treaty out" approach, Japanese engineers were free at last to design in 1936 the "ideal fleet carrier", the Shokaku class, dubbed by many historians as "arguably the best aircraft carriers in the world" when built. They started with almost double the tonnage (32,000 tonnes), nine meters more in beam but still a slightly inferior aviation complement to the taller and roomier Yorktown, with "only" 72 operational planes and 12 spares so a total on paper of 84. The output was superior, so that despite their larger tonnage they still managed to reah more than 35 knots on trials, and the best defensive armament so far.
It is important to note that the Japanese outright lied in official publications on their CVs, officially for 10,050 tons standard. Their true tonnage was not revealed until the end of WWII. Nevertheless, if the "Soryu class" was overly successful, at least from the Japanese perspective, they were lacking good aircraft facilities, compared to contemporary HMS ARK ROYAL and USS YORKTOWN. Their aircraft-handling facilities were not modern, but improved on the next Shokaku class.
In fact it was hoped that by the time of HIRYU's completion in 1937 the restrictions would be lifted, but in January 1936 the Japanese delegation withdrew from the new London Treaty talks. It was decided not to comply with the limitations of both Washington and London Naval Treaties any longer. After 1 January 1937, warship construction could proceed without further constraints.
Detailed Design of IJN Sōryū
Deck and hull plan in 1941
The final design of IJN Sōryū called for a long hull as precised earlier, with a remarkable set of differences compared to Ryujo. Her prow was taller from the start (three stages high) but the height of her main two hangars was quite low, allowing the ship to avoid topweight issues. The internal distribution was in reality more complex with a separate hangar and workshop containing the spare planes and a repair space, plus ammo parks (gasoline tanks, aerial torpedoes, bombs, machine-gun cartridge bands, etc.), which were lower, so as to keep a balanced metacentric height. This was still not however a massive departure from Ryujo as rebuilt.
Hull and protection
If Ryujo was an unsatisfactory compromise, in short, a failed aircraft carrier, Soryu was the prototype for all following IJN aicraft carrier designs: Fast, lightly built, designed to deliver blows but not taking them. The hull measured 227.5 m (746 ft 5 in) overall but 210 (688 ft 9 in) at between parallels, 222 m (728 feets) at the waterline, for a beam of 21.3 m (69 ft 11 in), also at the largest hull section, not on the flight deck, and 7.62 m (24 ft 11 in) of normal draught.
Armour protection was not the overriding priority there. It was to be adjusted where and when the most needed and not more. It never passed 2.2 inches in thickness, and this was the deck over the ammunition magazines and aviation gasoline storage tanks (55.88 mm) with Ducol steel, down to 1 inch (25,4 mm) over the machinery, whereas the belt was only 1.6 to 1.8 inches thick (41-45,72 mm) with an internal anti-splinter bulkhead.
Soryu after launch, fitting out at the Kure Naval Arsenal in 1936
ASW compartimentation was of course pushed just enough to include a full separation of the machinery, between the four turbines, outer and inner shafts, and the boilers surrounded by many compartiments designed to absorb any torpedo blast, and at the bottom, a double hull running for about 70% of her lenght. But she had no bulges.
This ASW protection was more optimized against lighter aerial torpedoes than regular marine torpedoes, and it was believed their high top speed was a good argument against torpedo hits. Protection against aerial bombs however was limited to the "sandwich effect" of the two decks below the main and secondary hangar, plus the waterline deck. It was not sufficient at Midway, but because of the way ammunitions and fuel was stored and used inside the first hangar itself, where the damage was done. At that stage, no thoughts were given to armored decks. This was a close prewar British idea, and a very good one as shown by events in the Mediterranean.
Fantail view, IJN Soryu on trials at 35 knots
The machinery was without contest the main selling point of the design. It was enormous for an aircraft carrier at the time for that tonnage, beating all standards. IJN Sōryū was propelled by four shafts, two outer and two inner, drove by four geared steam turbine sets, with a total of 152,000 shaft horsepower (113,000 kW). Steam came from eight Kampon water-tube boilers. These turbines and boilers were just borrwoed from the contemporary Mogami-class cruisers.
This sort of raw power combined with her slim, cruiser-type hull gave her a speed of 34.5 knots (63.9 km/h; 39.7 mph): This made her in 1937 the fastest aircraft carrier in the world at the time, when commissioned. She carried 3,710 metric tons (3,650 long tons) of fuel oil for an overall range of 7,750 nautical miles (14,350 km; 8,920 mi) at the cruis speed of 18 knots (33 km/h; 21 mph). The boiler uptakes were trunked together on her starboard side amidships, still below flight deck level, so to exhaust just below that level with two funnels curved downwards to avoid releasing smoke. The whole package was virtually repeated on IJN Hiryu.
IJN Sōryū was to be lightly armed, focusing on anti-aircraft armament as it was believed in 1934 onboard fighters had the primary duty to defend their carrier. Hwever ro procure a long range AA defence against bombers, and capabilities in a still possible naval fight, Soryu was fitted with twelve 127 mm/50 (5 inches) in three twin-gun mounts on either side. Two were closer together aft port and two forward on starboard. The Type 89 dual-purpose guns rested on projecting sponsons. They had a max range of about 14,700 meters (16,100 yd), but a practical ceiling of 9,440 meters (30,970 ft) when elevated at 90°. Maximum rate of fire was 14 rpm, 8 rpm sustained. They were assisted and directed by two Type 94 fire-control directors also mounted on sponsons, one of each side, but the starboard director mounted on the island was able to cover all sides and therefore control all this artillery if needed.
The second layer of active AA protection consisted of fourteen twin-gun mounts (28) license-built Hotchkiss 25 mm (1 inch) Type 96 AA guns. Three were located on a platform below the forward end of the flight deck, the rest were located in sponsons along the flight deck, three on the opposite side of the island, followed by three others mid-ship, then five heavenly spaced in between the aft 127 mm mount on the other side. However as in the previous designs, all three main sponsons were mounted below the flight deck level and could only cover half of the ship.
The ubiquitous 25 mm was a known standard issue light AA weapon suffering from severe design shortcomings, like being unable to be trained or elevated fast enough, with mediocre sights for high-speed targets, excessive vibration, muzzle blast and noise. Effective range was 1,500–3,000 meters (1,600–3,300 yd), with 5,500 meters (18,000 ft) of ceiling at +85° and 110-120 rpm with 15-round magazines. To serve these, five Type 95 directors were installed on each side of the flight deck and one in the bow. In 1938, gun shields were added to protect both the 127 and 25 mm mounts.
Section of the island cutaway drawing, Extract from Kagero's book on IJN carriers.
Sōryū's flight deck was as customary shorter than her hull, at 216.9 m (711 ft 7 in) overall, but for 26 m (85 ft 4 in) in width. There was the customary overhang at both ends, and it supported by pairs of pillars. fore and aft. Her island was built on starboard on a extension protruding beyond hull, not encroaching on the flight deck. It was a small and narrow model reminiscent of what was done on Akagi, with a main enclose bridge and open brige above, and two small utility rooms below. On top of it was located a serie of binoculars for the watch and aft the large main telemeter for the 5-in guns.
The flight deck was criss-crossed by nine transverse arrestor wires starting at the level of the exhausts, and going back about 60m short of the end of the flight deck. These could could stop a 6,000 kg (13,000 lb) aircraft. In addition, there were two folded safety nets in case of an overshoot, one located abaft the forward exhaust and another forward.
The flight deck due to the particular way the ship was tretched out, was low over the water, only 12.8 meters (42 ft), the lowest since Hosho. The designers reduced indeed the height of the hangar, with a main upper hangar 171.3 long (562 feets) and 18.3 metres wide (60 ft) but just 4.6 meters (15 ft) in height, while the lower hangar was shorter at 142.3 m (467 feets) by 18.3 m (60 ft) and just 4.3 m hight (14 ft 1 in).Total area for parking was 5,736 Sq. metres (61,742 sq ft). These hangars were so low in fact, contrary to US paractice allowing spare planes to be just suspended under the ceiling, that the tails and folded wings sometimes reached the ceiling. The Nakajima B5N "Kate" torpedo bomber when introduced was a particular difficult fit inside, condemned to the 1st hangar, or the flight deck as it could neither have its wings spread or folded inside.
There was also a recovery crane starboard aft, close to the aft offset lift, to recover water crashed planes and floatplanes. It was carried folded into a recess of the flight deck. The latter was colored, with a central section, starting roughly 10 meters forward of the island, and ending after the rear lift, in clear pine wood, and metal deck forward and aft. There was a white bearing mark forward and central white line running all the way to the aft section, and a white/red painted aft section for landing, with wings extensions for aiming.
The 60 strong air group was distributed from the two hangars to the flight deck via three elevators: The forward one was the largest at 16 by 11.5 meters (52 ft 6 in × 37 ft 9 in), rectangular in shape to fit in a "Kate" wings unfolded. It was located abreast the island, centerline but the two others were squarish and offset to starboard, a better fit for fighters. The first was 11.5 by 12 meters (37 ft 9 in × 39 ft 4 in), the rear one, smallest, was 11.8 by 10 meters (38 ft 9 in × 32 ft 10 in). All three had a lifting capacity uo to 5,000 kilograms (11,000 lb). To serve this air group, IJN Soryu's total gasoline capacity was 570,000 liters (150,000 U.S. gal).
Her air group was initially planned to include eighteen Mitsubishi A5M, twenty-seven Aichi D1A2 dive bombers, and twelve Yokosuka B4Y Type 96 torpedo bombers but the Nakajima A4N1
was issued instead.
(Dedicated Illustrations in preparation)
Completed in 1937, IJN Soryu was given only biplanes, transitioning to monoplanes from 1938 and until 1941. She operated in short succession the A2N, A4N, and finally A5M, A6M fighters in WW2, but also the D1A, D3A diving bombers and the B4Y and B5N torpedo bombers. At Pearl Harbor, she carried 21 Mitsubishi A6M Zero, 18 Aichi D3A and 18 Nakajima B5N for a total of 57, plus 11 in ready reserve to replace losses, and 9 more to be assembled to replace the latter (or be cannibalized). This basically was the same air group at Midway, barely six months later.
-The Nakajima A2N (1929)
was already an old biplane introduced in 1936. Pilots trained with it, the very first squadrons tested comprised some Navy Type 90-III (A2N3) Carrier-based fighter, which moslty flew from the Hōshō, Kaga and Ryūjō. They were quickly replaced by the A4N or some possibly flew for early tests in 1937, according to navypedia.org.
-The Nakajima A4N1 (1934)
was the replacement for the A2N, introduced in 1936 and quickly became the main fighter of Soryu from 1937 to 1938, used extensively over China. It was not given any codename by ONI, as considered obsolete in 1941. It was a stopgap before she received her planned A5M, in short supply.
-The Mitsubishi A5M "Claude" (1935)
first IJN monoplane, but with open cockpit and fixed undercarriage was introduced in the IJN in 1936, so as IJN Soryu was still fitting out. It however replaced the A4N in 1938 when available. By that stage, it was probably the A5M3, replaced by the A5M4 variant in 1939. It was still used in 1941.
-The Mitsubishi A6M "Zeke" (1940)
arrived just in time, to be precise, the A6M2, for Pearl Harbor. They became the mainstay of the carrier until her loss in June 1942, as there is no clue the A6M3 replaced it. The latter was mostly available from November-December 1942.
-The Aichi D1A "Susie" (1934)
was her first dive bomber, a biplane, but brand new when she was completed. It was retired in 1942, but on second-hand units, whereas on Soryu it was retired well before the attack on Pearl Harbor, meaning Soryu lacked for a short time a dedicated dive bomber, before the "Val" arrived, with pilots fresh from training.
-The Aichi D3A1 "Val" (1938)
was introduced from 1940 and replaced the D1A on all IJN carriers gradually in 1941. Given the schedule for the December attack, pilots trained intensively all the winter, on land bases and on IJN Soryu. This was still the frontline dive bomber on board at Midway.
-The Yokosuka B4Y "Jean" (1935)
was the only torpedo biplane onboard IJN Soryu, introduced when she was fitted out. It equipped the carrier when commissioned and soldiered on for most of the Sino-Japanese war until declared obsolete.
-The Nakajima B5N "Kate" (1937)
was the arguably most modern torpedo plane in the world when introduced in 1938. It had better caracteristics than the Devastator. It was probably available in sufficient numbers to replace the B4Y in 1940 already, leaving plenty of time for Soryu's pilots to train for Pearl Harbor.
Nakajima A4N1 fighter, which only served two years onboard Soryu.
Mitsubishi A5M4 "claude", Soryu 1939. In 1941, she was replaced by the A6M.
Comparison with IJN Hiryu
The most striking aspect with IJN Hiryu was her taller bow, four stage high (four rows of portholes) compared to just two on the initial Ryujo design, and three on Soryu. No risk ever she "plowed" in heavy weather of her deck being washed up to the hangar by tall waves as it happened previusly. Interestingly also, the ratio between the hangar's height coppared to Ryujo's was reverse. It was 50% on Soryu. The hangar was much lower, although seating higher up over the main deck.
For armor, IHN Hiryu was much better protected, with 90 millimeters (3.5 in) over the machinery spaces and aviation gasoline storage tanks and even 150 millimeters (5.9 in) over the magazines, quite a stretch.
Author's illustration of IJN Sōryū in May 1942
Sources/ Read more
⚙ Specs 1937
|Dimensions||227,5 m (746 ft) x Beam 21.3 m (69 ft) x Draught 7.6 m (24 ft)|
|Displacement||16,200 long tons standard, 17,100 long tons FL|
|Propulsion||4 shaft Kampon geared steam turbines, 8 Kampon boilers 152,000 shp|
|Speed||34 knots (63 km/h; 39 mph)|
|Range||7,750 nmi (14,350 km; 8,920 mi) at 18 knots (33 kph, 21 mph)|
|Armor||Belt 41 mm (1.6 in), see notes|
|Armament||6×2 12.7 cm (5 in) DP, 14×2 25 mm (1 in) AA, 70 planes|
Conway's all the worlds fighting ships 1922-1947
The model's corner
Model kit 1/700, Tamiya's boxart
Tamiya 1/700 waterline color set boxart