Japan’s Floatplane Cruisers
By Mike Bennighof, Ph.D.
April 2022

For a generation, Japanese naval planners intended to fight the U.S. Navy in a climactic final battle in the Western Pacific Ocean. They did manage to do so, though not with the intended results, and not in the form they had imagined.

To choose the proper time for a clash with the Americans, the enemy fleet would have to be sighted long before it reached its objectives. While the value of aircraft for scouting purposes had been recognized from the very earliest flights, Japanese planners did not want to dedicate carrier planes to the reconnaissance role. Every spot on the flight deck would be dedicated to strike aircraft and fighters. Instead, float planes carried by cruisers would seek out the enemy.

Tone during Indian Ocean operations, 1942.

In 1934, the Imperial Navy laid down a pair of cruisers with a design seemingly influenced by then-current notions of hybrid cruiser-carriers. But Tone and Chikuma were not true hybrids. Originally designed as light cruisers with 12 6.1-inch guns in four triple turrets grouped forward, during construction they were secretly altered to carry eight 8-inch guns in double turrets. Secondary weapons numbered eight 5-inch dual-purpose guns, plus a dozen 25mm anti-aircraft guns. They also had a heavy torpedo armament, a dozen 24-inch tubes mounted near the stern to protect the bridge in case of a torpedo explosion.

Designed as modified Mogami-class light cruisers, the planned 8,500-ton diplacement rose to 11,215 on completion. Like other Japanese cruisers, they had very light armor protection but did feature many of the creature comforts of the Mogami class: steel-tube bunks instead of swinging hammocks, cold-water drinking fountains, air conditioning, and of course several special compartments to store pickles.

While the odd gunnery arrangement kept the entire after deck clear for aircraft handling, this was only a side effect and not the designers’ primary intent. Japanese gunnery officers had been unhappy with the performance of their Type A (heavy) cruisers for some time. “Treaty cruisers” of all nations had been shaped by the 1922 Washington agreement’s limits on tonnage and armament, mixed with desires for high speed. The results were lightly-armored ships, with great length to generate high speed without excessively heavy machinery.

Lightly-built, very long ships like the Japanese heavy cruisers (note their non-existent armor protection in Second World War at Sea series games) twisted and warped after pounding in heavy seas. Sometimes this became excessive enough to damage their structural integrity — Mogami for example had to be completely rebuilt after her maiden voyage.

But even when the damage did not affect the ship’s handling, it still could degrade her long-range gunnery. A warp of just a few inches in the gun mounts’ positions would throw off ranging, given the great distance between the forward and aft turrets on the 631-foot Nachi or Atago (almost as long as a battleship). So for Tone and Chikuma, the design placed all of the main turrets forward in a close group to enhance accuracy.

Chikuma prepares to fire a broadside, 1941.
The aircraft arrangements on the after deck could handle eight seaplanes, but did not include a hangar. Japanese designers considered the speed gained in launching aircraft to be worth the weather damage inevitably incurred by the seaplanes. With the big guns placed on the other end of the ship, the aircraft would not be subject to blast damage and aircraft could be launched during a gunnery action, something not possible on a conventionally-designed cruiser. The two cruisers rarely operated their full complement, at times carrying a few as one aircraft.

Tone completed in November 1938 and Chikuma in May 1939. They operated off southern China in their early years, and then with the First Air Fleet in the early stages of the Great Pacific War. A seaplane from Chikuma scouted Pearl Harbor before the Japanese attack, and both cruisers deployed their seaplanes to scout for the carriers during the operations in the East Indies and Indian Ocean, at Coral Sea, and at Midway.

Neither cruiser suffered any damage until the fighting for Guadalcanal. Again they accompanied the carriers, and American planes damaged Chikuma at the Battle of Santa Cruz in October 1942. But they remained with the carriers and were not committed to the savage surface actions off Guadalcanal, and then spent most of 1943 in the central Pacific. Both went for refits during the year, receving an additional 57 25mm anti-aircraft guns, most of them mounted on the broad aircraft deck. That greatly reduced their seaplane-handling capacity, but by this point, Japan lacked planes and pilots to give them full complements anyway.

Chikuma under air attack at Rabaul, November, 1943.
In March 1944 the two cruisers along with Aoba entered the Indian Ocean to attack Allied merchant shipping, one of the very few uses of Japanese surface ships in this role. On 9 March Tone sank the 7,840-ton British freighter Behar, crewed mostly by Indian seamen. Before abandoning ship, radio officer Henry Gordon Cumming sent off a distress signal.

Tone took 114 men aboard, and ten days later, Capt. Haruo Mayuzumi ordered them arrayed on the aircraft deck. Apparently randomly, Mayuzumi selected either 65 or 69 of them (witnesses could not agree after the war as to how many died during the Tone’s shelling of their ship), who were shot on the fantail when Mayuzumi refused them the honor of death by sword and pitched into the Indian Ocean. These included 10 of her 14 officers, and most of her Royal Navy gunnery and sonar crews.

Mayuzumi did not survive the war, but his superior, Vice Admiral Naomasa Sakonju, took responsibility in his 1947 war crimes affadavit. “In view of the fact that the Allies are lately killing Japanese prisoners of war at Guadalcanal by running tanks over them and are often bombing and torpedoing Japanese hospital ships, causing many casualties,” the admiral wrote, “the H.Q. came to a conclusion that the Allies are aiming at the reduction of Japan's manpower, and H.Q. decided to retaliate.”

Both cruisers accompanied the carriers at the Battle of the Philippine Sea, but at Leyte Gulf were part of the surface force that fought the action off Samar. Chikuma was caught by American planes the next day and destroyed; Tone survived the battle but was sunk at her moorings in Kure by American carrier planes in August 1945.

The two cruisers appear in several Second World War at Sea series games, and presented a difficult design decision: Should they receive their full potential scouting capability, or the same as other Japanese heavy cruisers? Since the number of aircraft carried varied from operation to operation (and sometimes within a single operation) we gave them an extra scout plane, with special rules sometimes reducing that.

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Mike Bennighof is president of Avalanche Press and holds a doctorate in history from Emory University. A Fulbright Scholar and NASA Journalist in Space finalist, he has published a great many books, games and articles on historical subjects; people are saying that some of them are actually good. He lives in Birmingham, Alabama with his wife, three children, and his Iron Dog, Leopold.

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