Kawanishi H6K “Mavis”

The H6K was an Imperial Japanese Navy flying boat manufactured by the Kawanishi Aircraft Company in 1936, produced from 1938 to 1941. It was used during World War II for maritime patrol duties with the Allied reporting name “Mavis” and a Navy designation “Type 97 Large Flying Boat”. Its genesis went back to 1934 and it was to be a very long range and versatile model designed for reconnaissance, regular and VIP transport, bombing, naval warfare, with the national airline also using its commercial airliner variant. https://naval-aviation.com/ww2/japan/Kawanishi-H6K-Mavis.php

Design and development

Context

Back to 1933, the IJN questioned the reality of a fight against the USN (as most probable adversary in a war) in the vast expanses of the Pacific. Short of having the required aircraft carriers, another way was to develop a naval air long range bomber force that can operate from isolated islands on which airfield were built. But infrastructures were found underdeveloped, in part due to the interdictions that went with the Washington naval treaty that Japan signed back in 1922 and which stipulates the interdiction of fortifying any island from the signatories, at least to an extent. Plus from 1932, a large part of attention of resources went into the IJA fighting in China, and the Pacific was left to the Navy with a lack of means to maintain laison, reconnaissance and a striking capability across the Pacific.

The next best thing about bombers operating from island airfields was indeed the use of flying boats, that could operate anywhere in the pacific and being served with minimal installations or tenders provided by the Navy.

From specifications to maiden flight

The H6K went back to a Navy requirement of 1934, for a long-range flying boat. It was also the produce of the wealth of information and data collected by the Kawanishi team after visiting the Short Brothers in UK, which at the time was a renowned leading producers of flying boats. Which brought us to Kawanishi kokuki K.K. founded as Kawanishi Engineering Works in 1920, in Hyōgo Prefecture. It branched out from the Kawanishi conglomerate also at the origin of the Nakajima Aircraft Company. Kawanishi created its first model, the K-1 as a Mail-carrying Aircraft in 1921. It also created its own airline called Nippon Koku K.K. (Japan Aviation Co. Ltd) in 1923 and created aircraft for it, until forced shut down by the Japanese government in 1929 and its routes transferred the state airline Nippon Koku Yuso K.K. Thus there was a new split from the Engineering Works to create a purely aircraft design and manufacturing company in 1928 with all the former company’s assets.

The H3K, a copy of the Rangoon gave Kawanishi experience for these large models.

Its most ambitious product became the H6K flying boats but later it was out-done by the H8K and the N1K1 floatplane fighter. It was reborne postwar as Shin Meiwa Industries and went on providing the JSDMF with new flying boats. But back in 1929, and the first successful model of the company was the H3K, a license-built and enlarged version of the Short Rangoon and still large with three prototypes constructed, refined as tests went on. The H3K first flew in October 1930 and was accepted in 1931 as the Navy Type 90-2 Flying Boat and three entered service for long distance patrol and training missions over the Pacific, until retired from 1936. Also in 1930 was introduced the Navy Type 90-3 Reconnaissance Seaplane, 20 built as the Kawanishi E5K1 or Kawanishi Type G, derived from the Yokosuka Navy Type 14-2 Reconnaissance Seaplane.

Another successful model was the E7K one-engine biplane used as reconnaissance model on cruisers in the interwar, already seen there, the E8K reconnaissance floatplane which lost to the E8N, the prototype E10K 1934 night reconnaissance/transport flying boat, the E11K 1937 night reconnaissance flying boat, the E11A and E12K, and E13K 3 seats recced floatplane. All the latter were lost to Aichi. A handful of E15K Shiun were also made, for submerine operations but never really used. By far the most impressive prototype made during the war was the prototype heavy bomber derived from the H8K called the G9K Gunzan (lost to the Nakajima G5N Shinzan). The company also went forward with many naval fighters, the K-11 (1927) J3K (project) J6K Jinpu (project), N1K Kyōfū and famous N1K series. But also trainers such as K6K and K8K in small numbers, and the suicid plane Baika or the gigantic K-200 using turbojets and even larger KX-3.

An interesting portfolio but little production ad many competition lost to others. At least the first true success of the company aside the E7K was the future H6K, improved from in its definitive form of a four-engines monoplane with twin tails on paper as a potential replacement for the H3K and based on the 1934 requirements. It had its hull suspended beneath the parasol wing using V-type struts arrangements and the four radials were clear of water spray. The long fuselage was practical for many reconversions and roles.

After a hard work on the new model, and seeing foreign productions such as the recent Dornier Do-18 (which flew in March 1935) or the older Consolidated P2Y (1929), both showed the parasol was more promising as it reduced drag and offered plenty of lift. The final model was very much unique with its four engines, a first in Japanese aero industry, and it first flew on 14 July 1936. It was seen by representatives of the IJN and accepted as is, originally designated “Navy Type 97 Flying Boat”. Later the denomination was standardized as the H6K with an order for 200 placed in late 1936 without real competition. In all 217 would be built from 1937 to 1941.

Originally, four prototypes were produced leading to the production version. The latter comprised ten preserie H6K2 built, and the bulk of the 124 H6K4 produced with better fuel capacity as bomber/recce. All H6K had the same four radial piston engines, and some were upgraded with the 1000 hp Kinsei 46. Finally the H6K5 adopted the the 1300 hp Kinsei 51/53 engine while 1936 prototype had four 9-cylinder Nakajima Hikari 2 rated for 840 hp. So from these to 1300 hp that erase all early critics of being underpowered, even sluggish.

Design of the H6K

Main:

The H6K was not only the largest flying boat, but also the largest Japanese aircraft at the time, with a wingspan of 40 meters (131 feet 3 inches). It held a record even facing heavy bombers such as the G8N (32.54 m/106 ft 9 in) and even its own successor the H8K which was 38 m (124 ft 8 in) wide. Wing area was a record 170 sq. meters (1,830 sq. feet) for a fuselage 25.63 meters (84 feet 1 inch) long (the H8K was longer at 28m) and a height of 6.27 meters (20 feet 7 inches) on top of the engien cowlings. Empty weight was 12,38 tons (27,295 pounds), with a maximum take-off (MTO) weight of 23 tons (50,705 pounds). It was still way inferior to the H8K and its 32,500 kg (71,650 lb). But the latter had much more powerful engines. The H6K5 was a bit lighter with a MTO of 21,500 kg (47,399 lb).

Top speed at altitude for the 1300 hp Kinsei 51/53 was 385 KPH (239 MPH/208 KT) versus 340 kph on the H6K4 and its Mitsubishi Kinsei 43 engines. Service ceiling was 9,560 meters (31,365 feet) and range 6,775 kilometers (4,210 MI / 3,660 NMI) for the H6K5 but 4,797 km (2,981 mi, 2,590 nmi) for the previous H6K4 and a ferry range, light, of 6,080 km (3,780 mi, 3,280 nmi) which was still enormous compared to any other model in Japanese inventory.

A parasol with four engines, it had symmetrical wings with rounded wingtips and also a rounded tail with two composed rudders compensated and placed at 2/3 end of the outer tail fin. The latter were supported by two struts each, and so were the main wings, comprising a main pair of very long master struts born from the low fuselage hull and going up way clear of the engine under the wings, about 50% distance. They were themselves reinforced by a pair of transverse struts attached mid-lenght and connected to the underwing base between engines and a second immediately short of the attachement points under the outer wings. The two floats were placed just past the underwing attachement of the master struts, supported by a pair of struts and themselves were reinforces by two “V” struts starting from the floats beam.

Despite its great size, the fuselage looked elongated, with an under hull boat shape, two breaks, a forward section pretty large and low, and an upper section going upwards to the tail gunner position. The cockît had two seats, the copilot being handy to handle the heavy controls in case of hard nose escape manoeuvers. The full crew comprised nine men, so pilot and copilot in the cockpit with the radioman and navigator behind, an observer in a small glasshouse/turret, two observers/belly gunners on the waist with blisters reminiscent of the Catalina but smaller, an optional mechanic and a tail gunner.

Armament:

Combat H6K (half indeed were transport ones) were armed with combinations of Type 92 light machine guns, a 20 mm gun for the later verisons and two torpedoes or 1000kg of bombs. Since the models had no bomb bay obviously to avoid water intake, these were installed externally under the wings, to be precise under the master struts, and to be precise at the base of these, close to the fuselage and relaively low mounted, about at the height of the floats. The MGs were mounted in various locations, fixed in the nose (one), another in the spine, one in each waist blister and one in the tail, behind a generous glasshouse. It seems it was replaced n the H6K4/5 by a 20 mm Type 99 model 1.

In terms of torpedoes, only 800 kg (1,764 lb) torpedoes could be carried. According to navweaps, these could only be the standard 1,728 lbs. (784 kg) Type 91 model 1. They were 208 in (5.275 m) long and carried a 331 lbs. (150 kg) Type 97 warhead to 2,200 yards (2,000 m) at 41-43 knots thanks to a 140 HP Kerosene-air wet-heater engine and had a Negative Buoyancy of 227 lbs. (103 kg). The weight discrepancy is perhaps related earlier british airbone torpedoes purchased prior. As for bombs, they were under racks, usually got a total of 1000 kgs the max load was four 250 kgs models. But the H6K was rarely used as bomber or torpedo plane, in the latter case due to its slow speed and large size, making it an easy target.

Transport variants

-16 H6K2-L unarmed transports, 2 H6K4-L VIP transports, 20 H6K4-L unarmed transports with the Kinsei 46 engines, 36 H6K5s with Kinsei 51/53, 16 aircraft were civilian air transports operated by Dai-Nippon Airways (the Imperial Japanese National airline), with on board toilet, galley, and room for 18 passengers. They had civil registration and were used for mail and passenger service to the Pacific.

Transports had a modified fuselage without the blisters, and with a different nose, as well as a serie of seats for 12-16 passengers, in all sixteen H6K2-L unarmed transports, two H6K4-L VIP transports, twenty H6K4-L unarmed transports with the Kinsei 46 engines and thirty-six H6K5s with Kinsei 51/53 engines as well as sixteen used as civilian air transports at the Dai-Nippon Airways or Imperial Japanese National airline.The latter had on board toilet, galley and 18 seats with stewart as flights can go up to eight hours. This made for a grand total of 90, with the remaining 125 being armed combat versions.

Military Variants

H6K1: Evaluation prototypes: 4x Nakajima Hikari 2 engines: 4 built.

H6K1 mod 2: Navy flying boat Type 97 Model 1 prototypes with 746 kW (1,000 hp) Mitsubishi Kinsei 43, 3 converted

H6K2 Model 11: Production model, with 2 H6K2-L officer transport modification: 10 built

H6K4 Model 22: Main production version, some with Kinsei 46 engines, increase fuel capacity: 100 built

H6K5 Model 23: 969 kW (1,300 hp) Mitsubishi Kinsei 51/53, new upper turret: 36 built

More in detail

H6K2 Model 2

The model from April 1940 was called the H6K2 Model 11 as pre-production batch of 1939, ten in all, almost identical to the prototype bu for the composition and placement of the onboard equipment. The 7th and 8th were converted into H6K2-L transports.

H6K2-L

This was the forst dedicated transport variant for IJN personel, based on H6K2. 16 were made from 1940 to 1942, with 10-18 seats depending on the configuration and a crew of 8 or less as there was no armament. There was at least a stewart, the pilot and copilot, navigator, radio and mechanic.

H6KЗ Model 12

Second transport option (two made in 1942) as VIP transport for highest command personnel, with Captains and gradiation of admiral ranks. Yamamoto, Kurita, Nagano among others all flew with these.

H6K4

Also called Model 23 for the engine upgrade. The was the first mass combat variant produced from 1939 to 1942 (127 built) with increased fuel supply from 7,765 to 13,409 liters and better armament. The dorsal turret was replaced by two side blisters with Type 92 MGs, one in the open nose and top turrets, and a 20-mm Type 99 cannon in the tail, replacing the Type 99 Model 1. They also had the Mitsubishi Kinsei 43 engines rated for 1000 hp and from August 1941, the improved Kinsei 46 engines. .

H6K4-L

Transport variant based on H6K4 made in 942-1943, only 20 manufactured differing by small details.

H6K5 Model 23

H6K4 upgraded with the Mitsubishi Kinsei 51 or 53 rated for 1,300 hp and until February 1942, production started at the Naruo plant with 36 built. Instead of an open bow firing post they had an enclosed turret with a light MG.

Detailed specs

H6K4 m22
Crew:
Fuselage Lenght	25.625 m
Wingspan	40 m
Wing area	170 m2
Height	6.27 m
Empty weight:	11,707 kg (25,810 lb)
Gross weight:	17,000 kg (37,479 lb)
Max takeoff weight:	21,500 kg (47,399 lb)
Propellers:	3-bladed metal, fixed pitch.
Engine:	4× Mitsubishi Kinsei 43 14-cyl. ACR engines 750 kW (1,000 hp) TO
Top speed:	340 km/h (210 mph, 180 kn) at 4,000 m (13,000 ft)
Cruise speed:	222 km/h (138 mph, 120 kn) at 4,000 m (13,000 ft)
Range:	4,797 km (2,981 mi, 2,590 nmi)
Ferry Range:	6,080 km (3,780 mi, 3,280 nmi)
Wing Loading:	100 kg/m2 (20 lb/sq ft)
Power/mass:	0.1731 kW/kg (0.1053 hp/lb)
Guns/Machine guns	4× 7.7 mm Type 92 nose, spine, waist blisters, 20 mm Type 99 tail turret
Bombs/Torpedoes	2× 800 kg (1,764 lb) torpedoes/1,000 kg (2,205 lb) of bombs

The H6K in action

H6Ks arrived in operations from 1938 onwards, and saw action first in the Sino-Japanese War, patrolling for the IJN along the coast. Its long range and safety (it could land anywehere at se in case of engine failure) had it adopted for ad-hoc transport before tailored verisons appeared. It was ideal and widespread to patrol over the Pacific after Pearl Harbor also, with four Kōkūtai operating a total of 66 H6K4s. Sixteen already were used by the Imperial Japanese airline for passenger service, also to the Pacific, used by Politicians, but also army and navy VIPs.

The Battle of Wake on 12 December 1941 saw a lone H6K bombing the island, until shot down by a resident scrambled F4F Wildcat. On 15 February 1942, one was intercepted by a US P-40 Warhawk 190 km west of Darwin after attacked Allied shipping and the resilience of the H6K was shown as it managed to shoot down its assaliant, the P-40.
The H6K became also popular over southeast Asia and southwest Pacific notably in the maze of islands in the Carolinas and Philippines, Marianas. They showed an excellent endurance up to 24-hour straight patrols, and long-range reconnaissance and bombing missions far from their bases or operating mid-way thanks to the presence of tenders. They operated for example from the Dutch East Indies, and still could reach Australia; bombing any point there or over the Indian Ocean.

However, the H6K was not fast enough to escape better armed, faster and longer range allied fighters arriving in 1943. In front-line service it had been already replaced by the Kawanishi H8K with the production of the transports ending in 1943. It was maintained in service in quieter areas.

At first, their effectiveness was very high, but with the airbone density put up by TF 38, “Mavis” became a sitting duck between a poor protection of fuel tanks, slow speed, reduced agility, in short an easy prey. By late 1942 attack missions with bombs and torpedoes were abandoned for long-range reconnaissance in relatively quiet sectors, and ASW patrol, still repalcing bombs by depht charges, where they found a new utility, notably thanks to their very long time in the air.

From the second half of 1942, they started to be replaced by the H8K and were all transferred to escort and ASW patrol. Three H6K in 1942-1943 were equipped with search radars, first used in the Imperial Japanese Air Force for night patrols as well. “Mavis” stand for a Songbird.

Gallery

H6K-5, 8th Kokutai 1941

H6K-5 24 Flotilla Marshalls 1942

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H6K Type 96 transport variant H6K8

H6K-3 in flight

H5K in Surabaya, which participated in the Indonesian independence war

Sources

Doubilet, David. “The Flying Boat”. Sport Diver Magazine. Volume 15, Number 8, September 2007.
Francillon, René J. (1979). Japanese aircraft of the Pacific War. London: Putnam.
Green, William (1962). Warplanes of the Second World War: Volume Five, Flying Boats. Macdonald.
Lawrence, Joseph (1945). The Observer’s Book Of Airplanes. London and New York: Frederick Warne & Co.
“Kawanishi’s Parasol Patroller”. Air International. Vol. 29, no. 6. December 1985.
“Pentagon Over The Islands…The Thirty Year History of Indonesian Military Aviation”. Air Enthusiast Quarterly. No. 2
Richards, M.C. “Kawanishi 4-Motor Flying-Boats (H6K ‘Mavis’ and H8K ‘Emily’)”. Volume 11, Profile Publications
Van der Klaauw, Bart. Water- en Transportvliegtuigen Wereldoorlog II (in Dutch). Uitgeverij de Alk.
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