Contemporary deck plan and port elevation of Diadem - class cruiser. In the deck plan, the "6 inch" annotations refer to the calibre of the guns.
|Operators:|| Royal Navy
Royal Canadian Navy
|Preceded by:||Powerful class protected cruiser|
|Succeeded by:||Cressy class armoured cruiser|
|Length:||435 ft (133 m)
(462 ft 6 in (140.97 m) o/a)
|Beam:||69 ft (21 m)|
|Draught:||25 ft 6 in (7.77 m)
27 ft 6 in (8.38 m)
|Propulsion:||2 shaft triple
(Ariadne, Spartiate, Amphitrite and Argonaut: 18,000 hp)
(Ariadne, Spartiate, Amphitrite and Argonaut: 20¾ knots)
|Range:||2,000 nmi (3,700 km) at 19 knots
(bunker capacity 1900 tons)
4.5 inch casemates and gun shields
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."
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 good 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 Diadem class was intended, like the preceding Powerful class, 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 25,000 ihp (22 knots) to 16,500 ihp (about 20 knots), the thickness of the protective deck reduced from 6 to 4 inches (100 mm), and the armament simplified by replacing the single 9.2-inch (234 mm) guns fore and aft with pairs of 6-inch (152 mm) 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.
Although the powerplant used Belleville boilers, which were of the then-new water tube type, most of the ships enjoyed the reputation of good steamers; the exception was Niobe, whose engines gave persistent trouble. The first four ships were troubled by vibration; to remedy this, the engine rooms of the last four was rearranged; the new configuration also provided a 1,500 hp (1,100 kW) increase in power. . All eight ships exceeded their design power on trials (for details, see below); the first four averaged 20.5 knots (38.0 km/h) in service, the later four 21 knots (39 km/h).
In general the Diadems served most of their careers in home waters, although there were some deployments to China Station, the Mediterranean and the Caribbean. Niobe was sold to Canada in 1910. After 1906, they were only occasionally in commission, and by 1914 the class had for the most part been relegated to training ship and depot ship duties. However, with the outbreak of the First World War, several of them were reactivated, initially for patrol duties with the 9th Cruiser Squadron in the Eastern Atlantic. In 1917 Amphitrite and Ariadne were converted into minelayers, for service with the Dover Patrol. Ariadne was the only war loss, being torpedoed by the German submarine UC-65 on 26 July 1917. By the end of the war the survivors had again been withdrawn from active service, and all of them had been sold for scrap by 1932, with the exception of Andromeda, which served as a boys’ training ship until 1956.
Standard British practice at that time was for building costs to exclude armament and stores. 
|Ship||Builder||Date of||Cost according to|
|Laid Down||Launch||Completion||(BNA 1904 & 1905)||(BNA 1906)|
|Diadem||Fairfield, Govan||23 Jan 1896||21 Oct 1896||19 July 1898||£582,662||£554,863|
|Amphitrite||Vickers, Barrow||8 Dec 1896||5 Jan 1898||17 Sep 1901||£575,300||£552,795|
|Andromeda||Pembroke Dockyard||2 Dec 1895||30 Apr 1897||5 Sep 1899||£601,356||£574,916|
|Argonaut||Fairfield, Govan||23 Nov 1896||24 Jan 1898||19 Apr 1900||£573,704||£545,756|
|Ariadne||J&G Thompson, Clydebank||29 Oct 1896||22 Apr 1898||5 Jun 1902||£565,464||£541,927|
|Europa||J&G Thompson, Clydebank||10 Jan 1896||20 Mar 1897||23 Nov 1899||£589,835||£564,690|
|Niobe||Vickers, Barrow||16 Dec 1895||20 Feb 1897||6 Dec 1898||£574,878||£548,283|
|Spartiate||Pembroke Dockyard||10 May 1897||27 Oct 1898||17 Mar 1903||£680,188||£654,661|
|Ship||Date||8 hours full power
|Diadem||1897||17,262 ihp (12,872 kW)||20.65 knots (38.24 km/h)||15,861 ihp (11,828 kW)||19.9 knots (36.9 km/h)||12,791 ihp (9,538 kW)||19.80 knots (36.67 km/h)||Jane's 1900|
|Amphitrite||1900||18,229 ihp (13,593 kW)||20.78 knots (38.48 km/h)||13,695 ihp (10,212 kW)||19.50 knots (36.11 km/h)||Jane's 1900|
|Andromeda||1898||16,751 ihp (12,491 kW)||20.41 knots (37.80 km/h)||12,621 ihp (9,411 kW)||19.30 knots (35.74 km/h)||Jane's 1900|
|Argonaut||1899||18,894 ihp (14,089 kW)||21.17 knots (39.21 km/h)||13,815 ihp (10,302 kW)||19.86 knots (36.78 km/h)||Jane's 1900|
|Ariadne||1899||19,156 ihp (14,285 kW)||21.50 knots (39.82 km/h)||14,046 ihp (10,474 kW)||20.10 knots (37.23 km/h)||Jane's 1900|
|Europa||1898||17,137 ihp (12,779 kW)||20.40 knots (37.78 km/h)||16,298 ihp (12,153 kW)||19.9 knots (36.9 km/h)||12,739 ihp (9,499 kW)||19.33 knots (35.80 km/h)||Jane's 1900|
|Niobe||1898||16,834 ihp (12,553 kW)||20.59 knots (38.13 km/h)||16,834 ihp (12,553 kW)||20.5 knots (38.0 km/h)||12,961 ihp (9,665 kW)||19.27 knots (35.69 km/h)||Jane's 1900|
The Diadem (Dec 1898) did a mean of 19.27 knots (35.69 km/h) in a 69 hour run from Gibraltar to the Nore with 14,268 horsepower (10.64 MW). For 60 hours with 24 boilers with 12,791 ihp (9.538 MW), she made 19.3 knots (35.7 km/h) on trial. Andromeda did a sea trip at 800 ihp (600 kW) = 16 knots (30 km/h).