These were Elswick Pattern W guns developed for the Brazilian Navy for installation on the battleship Rio de Janeiro. They were an improved version of the Elswick 12"/45 (30.5 cm) Pattern L guns used to arm the Brazilian battleships Minas Gerais and Sao Paulo. The design of the Mark XIII guns was very similar to that for the 12"/45 (30.5 cm) Mark X guns used on HMS Dreadnought, but they were not interchangeable with them.

Rio de Janeiro was a very unusual design in that she had seven twin turrets fitted, the most of any dreadnought ever built. The turrets themselves were developed by Elswick as a more or less private venture and their design was then carried over to the 14" (35 cm) twin turrets developed for HMS Canada (ex-Almirante Latorre).

Rio de Janeiro was taken over by the British at the start of World War I and renamed HMS Agincourt.

It is noted that when she was first taken over, Agincourt lacked sub-caliber guns and as a result firing practices were restricted and her battle efficiency was reduced. There were many problems with the newly designed hydraulic loading gear during her first firing trials and the electric igniters failed to function at the first salvo, requiring use of the back-up percussion method. There were also problems with the shells themselves, which showed an alarming tendency to break up in flight. This was attributed to the age of the shells, some of which were stenciled "Repaired in 1892." There were also design problems with the guns, such as the butt strap joining A and B tube which was thought to impede the projectiles as they passed and to a poorly designed forcing cone, which was thought to cause the shells to not seat properly.

At Jutland (Skagerrak) she was in action for a short period and where it was reported that the sheet of flame generated when she fired her guns "was big enough to create the impression that a battle cruiser had blown up; it was awe inspiring." Agincourt fired a total of 144 rounds from her 12" (30.5 cm) guns during this battle, her only combat engagement.

HMS Agincourt is notable for having had Dr. Oscar Parkes, author of "British Battleships: 1860 - 1950," as a crewmember.

Constructed of a heavy inner A tube in three parts, full length wire, B tube and jacket along with breech bush, screwed on ring and Welin breech block. A total of fourteen guns were made and a further seven to a modified design which had a one-piece A tube and tapered inner A tube.

• ^The Rate of Fire figure given above is found in references for British guns of this caliber, but "Warrior to Dreadnought: Warship Development 1860-1905" quotes Jellicoe's 1906 figures for rates of fire for these guns in gunlayers' tests and in battle practice and notes that the latter figures corresponded well to those actually attained by the Japanese at Tsushima:

  Gunlayers Test: 2 rounds per minute
  Battle Practice: 1 round per minute

• ^Projectile weights from ADM 186/169. The sources below disagree as to muzzle velocity, propellant charge weights and maximum ranges. I have chosen to use those values given in "Jutland: An Analysis of the Fighting."
• ^As Agincourt was not designed by the Royal Navy, she was often on the bottom of the priority list when it came to new equipment. For example, it is noted that Agincourt received very old shells for her initial outfit, some dating back to the 1890s. For this reason, I believe it to be unlikely that she was ever given "Greenboy" APC projectiles. She was probably given 4crh projectiles sometime during the war, perhaps after depleting her outfit during Jutland (Skagerrak).
• ^3.13.2AP Cap weights for Hadfield shells were 73.0 lbs. (33.1 kg) for the Mark VI and 34.75 lbs. (15.7 kg) for the Mark VIa and 99 lbs. (44.9 kg) for the Mark VIIa. Cap weights for other manufacturers could differ by as much as 9 lbs. (4 kg).
• ^"British Naval Guns 1880-1945 No 4" says that the propellant weight was "285 lbs. (129.3 kg) MD45." However, I believe this to be a misprint, as the chamber volume, muzzle velocity and ballistic properties are noted in this same article as being identical to those for the 12"/45 (30.5 cm) Mark X. For this reason, the propellant weight shown above is the same as that for the Mark X.

• ^These mountings were reworked during World War I to increase their maximum elevations to +16 degrees, a rather costly operation for the small increase in range thus achieved.
• ^Superfiring turrets could not fire within 30 degrees of the axis because the blast effects would have penetrated into the lower turrets through the sighting hoods.
• Turrets were named after the days of the week (Monday, Tuesday, etc.).
• During construction special care was taken to buttress the ship against the hull stress caused by broadside firing and turret weight. Longitudinal girders were fitted beneath the quarterdeck to support the weight of the three turrets in this area.

"Warrior to Dreadnought: Warship Development 1860-1905" and "The Grand Fleet: Warship Design and Development 1906-1922" both by D.K. Brown
"British Battleships of World War One" by R.A. Burt
"Jutland: An Analysis of the Fighting" and "British Naval Guns 1880-1945 No 4" article in "Warship Volume V" both by John Campbell
"The Big Gun: Battleship Main Armament 1860-1945" by Peter Hodges
"Battleships of World War I" by Peter Hore
"The Big Battleship" by Richard Hough
"British Battleships: 1860 - 1950" by Oscar Parkes
"British Battleships of World War Two" by Alan Raven and John Roberts
"A Concentrated Effort: Royal Navy Gunnery Exercises at the End of the Great War" article by William Schleihauf in "Warship International" No. 2, 1998
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ADM 186/169 "Shell Committee: Reports 1917 and 1918"
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Special help by Neil Stirling

05 December 2007 - Benchmark
20 February 2009 - Fixed typographical errors
04 February 2014 - Added note regarding quarter deck strengthening
23 September 2016 - Converted to HTML 5 format
29 February 2020 - Reorganized notes