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{ { Infobox ship characteristics
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|Ship type=Protected cruiser
|Ship displacement=11,000 tons<ref name=Con>Conway's, 1860–1905</ref>
|Ship length=*
( o/a) |Ship beam= |Ship height= |Ship draught=*
|Ship draft=
|Ship power=
|Ship propulsion=*2 shaft triple expansion engines:
(Ariadne, Spartiate, Amphitrite and Argonaut: ) Belleville boilers |Ship speed=* (Ariadne, Spartiate, Amphitrite and Argonaut: ) |Ship range=* at (bunker capacity 1900 tons)Brassey's (p.220-233) quotes a coal capacity of 1,000 tons. |Ship endurance= |Ship complement=677 |Ship sensors= |Ship EW= |Ship armament= 16 × single QF gunsJanes Fighting Ships 1900 claimed on page 108 that the Diadems had 6-in 45 calibre guns. The 6-in BL Mk VII was 45 calibres. This was later corrected in subsequent editions to the 40 calibre QF guns 14 × single 12-pounder (76 mm) QF guns 3 × single QF 3-pounder (47 mm) guns 2 × 18-inch (450 mm) torpedo tubes (underwater) 8 Maxim machine guns |Ship armour=*Casemates and gun shields Hoists Deck Conning tower fore tube to fore conning tower aft conning tower Armour was Harvey Nickel steel, except for armoured deck. |Ship armor= |Ship aircraft= |Ship aircraft facilities= |Ship notes= }} |}
The 'Diadem-class cruiser was a class of "first class" protected cruiser built for the Royal Navy during the 1890s that served in the First World War. The class consisted of eight ships, built at a cost of around £600,000 each.
They were considered "good sea boats" but criticised for their lack of a heavy calibre main armament, bulky unprotected sides, average speed and lack of manoeuvrability despite being considered large ships when completed. They were the last British first class protected cruisers; the armoured cruiser taking the large cruiser role in future."
Design
The Diadems were designed by Sir William White, and are typical of British design practice of the period, with high freeboard (by the standards of the time) and the use of 'double-decker' casemates, with the hull forward and aft cut away to allow fore and aft fire from the casemate guns. Casemates provided protection for the guns and allowed a large number of guns to bear on the broadside; however, the lower (main deck) guns were almost unusable in heavy seas.
According to a contemporary (1900) edition of Jane's Fighting Ships the hulls were wood sheathed and coppered. and "There are coal bunkers on the lower deck, and above and below the armour deck. The pair of 6 in guns forward and aft are served by one hoist to the pair, each casemate has its own hoist: those to the double casemates being double. The machinery weighs 1630 tons: the boilers occupy a space of about 132 feet (40 m). Working pressure is [300 to 250 psi (2.1 to 1.7 MPa)]. "The ships are almost absolutely identical in appearance. The Diadem and Andromeda have no steam pipes abaft the first two funnels, while all the others have steam pipes before and abaft each funnel."
History
The Diadem class was intended, like the preceding , for trade protection, and were intended to be "capable of dealing with any cruiser existing or building". The Powerfuls had proved costly and demanding in manpower, and the new class had to represent a more economical solution, that could be built and operated in greater numbers. To achieve this, the specification was reduced relative to the Powerfuls: power and speed were reduced from () to (about ), the thickness of the protective deck reduced from to , and the armament simplified by replacing the single guns fore and aft with pairs of guns, mounted side by side. These changes saved about £100,000 per ship (approximately 15%) relative to the Powerful design.
With eight ships, each of 11,000 tons and costing £600,000, the Diadems were an important class. However, they proved controversial. The naval journalist Fred Jane characterised them as "much-discussed" and complained that "all the weak points of the Powerfuls were exaggerated in them". In particular, contemporary critics raised objections to their indifferent speed, poor manoeuvrability, and lack of heavy guns, and the fact that, while most of the armament was well protected in casemates, the fore and aft pairs of guns, which commanded the greatest fields of fire, were protected only by gunshields.