Half-way between armoured cruisers and battlecruisers

Tsukuba and Ikoma, massive ships armed with 12 in (305 mm) pieces were the result of a request from the navy after the observation of the Russians long-range gunnery at the Battle of the Yellow Sea, but also replacement of the Hatsuse and Yashima, blown up by mines in Port Arthur. After completion they were briefly the world's most powerful cruisers in service (before the British Invincible class) and crucially the first capital ships entirely built in Japan on a Japanese design.



These ships seemed more akin armoured cruisers than battlecruisers but were not exempt of defects:On completion, after a very fast construction (too much, given what was noted later), they were too slow and above all too weakly armed to be compared to their European equivalents, but were nevertheless classified as such by the General Staff.

Design

Prior to that class, the Japanese acquired perhaps their most heavily armoured cruisers to date, based on the popular Giuseppe Garibaldi-class design (a private venture by Ansaldo Nyd): The Kasuga class of 1904. They won recoignition and international attention in the Russo-Japanese War of 1905 and their actions drive towards the concept of battlecruiser. Engineers and designers under the naval staff supervision had this in mind when drawing the first sketches of the new ship. They were financed by the Japanese Diet temporary special budget of ¥48,465,631 affected to the 1904 War Naval Supplementary Program. They were aimed at taking part in the line of battle and overwhelming enemy cruisers, a notable change in IJN's doctrine.

The Tsukuba class can be described summarily as bulkier, stubbier British Cressy-class armored cruisers. They diverged in having a clipper-style bow and modern Miyabara water-tube boilers, reduced to 20 (vs.30) to free some length for the new turrets and massive ammunition without much power loss. Armor thickness was preserved, but with an improved layout. The waterline armor belt (Krupp cemented armor) was 7 inches (178 mm) between turrets and only 4 inches (102 mm) on both ends. The upper strake of 5-inch (127 mm) armor extended between the barbettes and over the 6-inch casemates.

1-inch (25 mm) transverse bulkheads were later see as serious weaknesses in the ships' protection. Main gun turrets front was 9.6 inch (244 mm) thick, sides 9-inch (229 mm) and 1.5-inch (38 mm) roof. Main barbettes were 7 in thicks and decks ranging from 1.5 in to 2 in (51 mm) on the deck slopes. The forward conning tower was 8 ins (203 mm), with a 3-inch roof. The major construction problem observed was their large metacentric height (4 feet 5 inches or 1.34 m). As a consequence they made bad gun platforms because of their very quick roll.

Powerplant

Both ships had two 4-cylinder VTE (vertical triple-expansion) steam engines, connected to single propeller shaft each. They were fed by 20 Miyabara boilers as stated above, claiming a working pressure of 16.8 kg/cm2 (1,648 kPa; 239 psi). These innovated by having a fuel oil spraying system onto the coal for extra power. In total these VTE developed 20,500 indicated horsepower (15,300 kW). Design speed was 20.5 knots (38.0 km/h; 23.6 mph) but during trials they reached 20.4 to 21.6 knots (37.8 to 40.0 km/h; 23.5 to 24.9 mph) with forced heating at 22,670–23,260 ihp (16,910–17,340 kW). Up to 1,911 long tons (1,942 t) coal was carried, and 160 long tons (160 t) of oil.


Tsukuba design - Brassey's 1915.

Armament

Centerstage were the four 45-caliber 12-inch (41st Year Type) guns (305 mm). These battleship-standard guns were mounted in pairs with an hydraulically powered centerline turrets fore and aft. The Elevation/Depression was −3° to +23°. They could load ammo at an angle of +5°, up to +13° in exceptional conditions. They offered a 2,800 ft/s (850 m/s) muzzle velocity with AP shells, with a maximal range of 24,000 yd (22,000 m). Secondary armament comprised a set of 12 Elswick "Pattern GG" 45-caliber 6-inch (152 mm) guns. They were all mounted in armored casemates, the lower ones in the middle deck being condemned in case of bad weather.

They fired 100-pound (45.4 kg) AP shells at 2,706 feet per second (825 m/s) muzzle velocity. For close-quarter combat they were given 12 quick-firing (QF), 4.7-inch guns mounted in bow and stern casemates to deal with incoming TBs, the remainder being posted on decks, shielded, firing 12.5-pound (5.7 kg) projectiles at 2,359 feet per second (719 m/s). In addition they had four 40-caliber QF 12-pounder 12-cwt guns, mostly used for saluting purposes and four QF 3-pounder Hotchkiss guns. However the ships differred by having three three submerged torpedo tubes of different calibers, 533 mm for the broadside ones on the Ikoma.

Tsukuba in 1907

Career

Both ships were to be built at Yokosuka Naval Arsenal, named after famous Japanese mountains. But after fearing the nearby Russian 2nd and 3rd Pacific squadron coming from the Baltic Sea to raid the harbour, they were eventually ordered at the less-exposed Kure Naval Arsenal, which previously had little experience in this area. They needed brand new slipways, which were built. This gave considerable responsibility and experience to the yard, which became evtnually the main IJN yard and naval base until WW2. Tsukuba was completed in a record two years, whereas Ikoma took more time, the end of the war signalling a lower priority.

Lacking of modern cranes and heavy lifting equipments, the yard also had grave shortages of steel plates and rivets, which were imported from the US. Tsukuba suffered most from these problems. After commission in 1907, she left for the the US, articipating in the Jamestown Exposition Naval Review in May. Then she sailed for Europe, touring many ports and stopping in UK to be fitted with the lattest Vickers fire-control system, that calculated the firing data and communicated to the firing contols and turrets in real time. Crucially, both ships were reclassified as battlecruisers in 1912. Just before the war they had an armament overhaul, with ten 6-inch and eight 4.7-inch guns being installed in total.

Tsukuba served in wartime with the 1st South Seas Squadron. This unit was in charge of chasing the German East Asia Squadron in the Central Pacific. 7 October, she landed a party which occupied Ponape in the Caroline Islands. Ikoma joined the squadron in November and sailed for Fiji. She became a gunnery training ship in 1916. Both ships joined in 1917 the 2nd Division. On January 14, 1917, the Tsukuba suffered a violent accidental explosion of one of its ammunition bunkers that tore its flanks, killing 305. She sank on the shoals of Yokosuka Bay and was later salvaged and broken up.

In 1919, Ikoma as a training ship for gunners, was rearmed with 10 x 152mm, 8 x 120 mm, 8 cm/40 3rd Year Type anti-aircraft (AA) guns, and 6 x 45 mm. She was re-rated as first class cruiser in 1921, disarmed in 1922 following the Washington treaty limitations, and stricken from the lists in 1924, after only 16 years of active life. Both ships would remain strange hybrids, not powerful and fast enough to be considered as true battlecruisers, not armoured enough to be battleships, but certainly much too powerful to be considered as standard armored cruisers. They innovated in some area, but their rushed construction had some consequences.

Features (Kurama)

Displacement: 15 400t Fully Loaded

Dimensions: 137.2 x 23 x 8 m

Propulsion: 2 shafts VTE, 20 Miyabara boilers, 20,500 hp, 20.5 knots

Armour: Belt 203 mm, see notes

Crew: 844

Armament: 4 x 305, 12 x 152, 12 x 120, 4 x 45 mm, 2 x 40 mm AA, 3 TT 457mm sub.

Sources/read more

navypedia.org/ships/japan/jap_cr_ikoma.htm
naval-encyclopedia.com/ww1/pages/japan/marine_jap1914c.htm
naval-encyclopedia.com/ww1/pages/japan/tsukuba.html
https://en.wikipedia.org/wiki/Tsukuba-class_cruiser
Naval Warfare, 1815-1914
Cruisers and Battle Cruisers: An Illustrated History of Their Impact By Eric W. Osborne

Illustration of the Tsukuba, by the author