Submarine aircraft carriers
by Fred Lane
There has been a recent upsurge in interest, mainly on the internet and in journal articles, about submarine aircraft carriers. Most of these involve the giant Japanese I-400 class but they tend to neglect earlier attempts, going back to 1915, when a German submarine became the first to launch a seaplane on a bombing mission.
The Japanese Sen-Toku I 400 class submarines were the most capable submarine aircraft carriers ever built. They displaced 5223/6560 tons surfaced/submerged on a 122 x 12 x 7 metres (400 x 39 x 23 feet) hull. Propulsion was four 3000 hp diesels, giving 18.75 knots on the surface, while two 1200 hp electric motors could drive the boat at 6.5 knots submerged. They could carry three Aichi M6A1 Seiran seaplanes and were armed with eight 533 mm (for Type 95 modified Long Lance) torpedo tubes forward, one 140 mm (6.5 inch) gun aft and three triple-barrel 25 mm machine cannon plus one single .25 mm cannon. The aircraft loaded directly from the hangar onto a 37 metres (120 feet) long catapult. Complement varied between 144 and 220.
First wartime use: 1915
The German U-12 sailed from Zeebrugge, Belgium, on 15 January 1915 with a Friedrichshafen FF-29 seaplane armed with small bombs lashed down on its foredeck. They intended to close the English coast before launching the aircraft but the swell was too steep to ensure safe passage of the delicate aircraft. U-12 submerged shortly after leaving harbour, allowing the aircraft to float off and fly away. The FF-29 reached the English coast and returned safely to Zeebrugge without suffering or inflicting notable damage.
Other experiments were conducted by both sides. For example, on 24 April 1916, in an attempt to intercept raiding Zeppelins, the British submarine E-22 carried a couple of small Sopwith Schneider Scout seaplanes out to sea on the after-casing, then submerged to allow them to fly off. The seaplanes returned to Felixstowe after their launch but the E-22 was torpedoed the next day by a German submarine and the experiment was never replicated.
In 1916 the German U-12 carries a fragile Friedrichshafen FF-29 seaplane on its forward casing.
Limited all-weather application
This method of lashing an unprotected seaplane onto a submarine’s casing continued until at least the1930s, when the Dutch submarine K-15; performed the trick of flooding the forward ballast tanks until the deck was awash, then loading and getting under way with a Fokker C-VII-W seaplane tied down on her foredeck. Of course, dead calm seas are required for such an evolution.
The Dutch replicated the German 1916 experiment in the 1930s with this Fokker C-VII-W seaplane.
All these exercises aimed to help increase an aircraft’s range or extend the submarine’s scouting horizon, but they compromised one of the submarine’s greatest assets: the ability to submerge in an emergency. With an aircraft tied to its casing, this was grossly impaired. Logically, a number of attempts followed, chiefly by mounting a large submersible hangar on a submarine, while parallel development continued to construct a fold-up seaplane with good range, endurance and payload. Many navies experimented with different submarine and aircraft capabilities in the 1920s and 1930s. The USN, for instance, built a watertight aircraft hangar abaft the conning tower of their submarine S-1. This housed a tiny Martin MS-1, which was derived from a German WW I-era design.
One (then) big British submarine, the M-2 (90.1 metres long, 1620 tons surfaced) was launched in 1917 with an enormous 12-inch (305 mm) gun but this submarine gun size was subsequently outlawed by the 1922 Washington Treaty. Around 1927, an aircraft hangar, a hydraulic catapult and a small Parnall Peto two-place reconnaissance seaplane re[laced the gun. The concept was proven, but it was a cumbersome arrangement. The delicate aircraft was easily damaged, and so was the hangar door seal. Additionally, the aircraft launch and recovery process was slow, requiring the submarine to remain on the surface for extended periods.
The big British M-2 submarine launches a Parnell Peto seaplane from its catapult.
M-2 foundered with all hands in 1932 and divers subsequently found its hangar door open with the aircraft inside. It was assumed that either the aft dive planes malfunctioned or the hangar was swamped, perhaps by a rogue wave, as an overeager crew opened the hangar door.
The Americans also experimented with the aircraft-carrying submarine concept.
USS S-1 carried a tiny Martin MS-1 in 1923.
The French Surcouf commissioned in 1934 as the biggest submarine in the world. Measuring 110 metres (361 feet) long, Surcouf displaced 3304 tons (surfaced) and 4218 tons (submerged). The submarine combined both British M class initiatives by mounting two large eight-inch (203 mm) guns forward of the conning tower and incorporating a water-tight hangar aft that initially housed a two-seat Besson MB411 spotter/reconnaissance sea-plane. The spotter was desirable because the guns had a 24-mile range while the submarine’s rangefinder was only good for 6.8 miles. Surcouf sank with all hands in the Caribbean after a probable collision with a merchant ship in 1942.
The French Surcouf carrried twin eight-inch guns and a Besan411B spotter-reconnaissance seaplane.
The Italians also dabbled in the submarine aircraft carrier field in the 1920s, constructing a hangar in the submarine Ettore Fieramosca. A number of aircraft were constructed to fit this hangar, but they employed none operationally.
The Japanese, meanwhile, persevered with productive research into the submarine aircraft carrier concept. Experiments began as early as 1923 with a small Heinkel seaplane. By 1935 the locally-built Watanabe E9W1 “Slim” aircraft had joined the submarine force. From 1941 onwards, a much improved Yokosuka E14Y1 “Glen” seaplane was frequently carried by the 44 submarines built for this purpose. It was a Glen from the Type B1 submarine I-25 that scouted Sydney, Melbourne and Hobart in February and March, 1942.
The aircraft type most carried by submarines in WW II was the Japanese Yokosuka E14Y1 “Glen” seaplane. These aircraft performed valuable reconnaissance duties.
Japanese wartime research culminated in the deployment of three big Sen-Toku I-400 class submarines, the type that arouses most interest in recent submarine aircraft carrier discussion. It was planned to build 18 of these monsters in 1942 but by 1943 this number was reduced to five and only three were completed. They were the largest of all submarines until nuclear-powered craft appeared in the 1960s. Importantly, two of the I-400s could carry three Aichi M6A1 Seiran seaplanes each, plus spares, aircraft ordnance and aircraft stores.
These Seirans were not the simple reconnaissance or spotter types carried in earlier submarines, but high performance aircraft designed to penetrate a defended war zone to deliver a useful load of torpedoes or bombs. Intending to attack the Panama Canal’s Gatun Locks from the east, the Seirans practised with targets constructed ashore in Toyama Bay. For the submarines, the Gatun Locks raid involved a non-stop transit from Japan, around the Cape of Good Hope and across the Atlantic. The I-400s had an amazing maximum range of 37,500 miles at 14 knots.
The Aichi M6A1 Seiran was unknown to Allied intelligence until after WW II. It had a crew of two and an empty weight of 3301 kg (7277 pounds). It could carry one torpedo or 1800 pounds of bombs over 642 miles. Its inverted V 12-cylinder engine developed 1400 hp. Seirans could fly at a handy 256 knots and they had a service ceiling of 32,000 feet. A total of 28 were constructed, but only one survives, housed in the Udvar-Hazy Centre, near Dulles airport, VA.
They also had a 98-foot (30 metres) catapult that could launch a heavily-laden Seiran. In the hangar, with the aircraft’s wings folded back, the floats detached and the elevators, fin and rudder folded down, the whole aircraft maintained a cross-section no greater than the aircraft propeller. The seaplane’s floats could carry extra fuel and be jettisoned, if necessary. It was claimed that a worked up crew of four could range, rig, fuel, arm and launch the first aircraft within seven minutes, and all three Seirans in about 45 minutes after surfacing. In a calm sea, the aircraft might be recovered and stowed from alongside using the ship’s folding hydraulic crane in about the same time, but not all Seirans were expected to return from high value sorties, such as the Gatun Locks raid. Instead, after using the fuel stowed in them, the floats could be jettisoned before the aircraft reached a highly defended zone.
These big submarines had a number of unusual construction features, apart from their huge size. The hull was essentially a pair of cylinders lain side by side. The pressure hull’s midships cross section was a figure eight but this tapered to a single cylinder aft and a vertical figure eight forward to accommodate the craft’s eight torpedo tubes in two compartments, one above the other. There was also a separate aircraft engine overhaul and test bed under the hangar.
One report said that underwater steering was difficult at slow speeds. This seems logical, given that the conning tower was offset two metres to port and the aircraft hangar, with its large frontal area, was offset to starboard. As may be expected from such a large submarine, it also tended to take more time than more nimble vessels to submerge.
The four diesel engines produced a total 7,700 hp but their arrangement was unique in the Japanese navy. Two diesels were coupled in pairs to each propeller shaft, which could also be powered by a 1200 hp electric motor when submerged. A rudimentary snorkel was added during construction, permitting limited underwater cruising on the diesel engines.
Starting with the Friedrichshafen FF-29 in 1915, the submarine aircraft carrier development concept executed a full cycle, from bombers through fighters and scouts and back to bombers. Nowadays, the I-400’s role has been usurped by missile-firing submarines.