H.M.S. H 21 (1917)

H.M.S. H 21 (1917)
Pendant Number:	H.21^[1]
Builder:	Vickers^[2]
Ordered:	Jan-Feb, 1917^[3]
Launched:	20 Oct, 1917^[4]
Commissioned:	Jan, 1918^[5]
Sold:	Jul, 1926^[6]

H.M.S. H 21 was one of forty-four "H" class submarines completed for the Royal Navy in the 1910s.

Service

At the end of 1920, she was one of four "H" class boats in reserve submarine group "F" at Portland, under the collective command of Lieutenant Andrew L. Besant and tender to Vulcan.

At the end of 1922, she was one of three "F" group boats under the collective command of Lieutenant Harold E. Spragge.^[7]

Captains

Dates of appointment are provided when known.

Lieutenant-Commander Henry E. Smyth, August, 1917 – 15 August, 1918
Lieutenant & Commander Edward A. Aylmer, 17 August, 1918 – 31 January, 1920
Lieutenant Alexander McG. McCulloch, 8 March, 1920 – 14 April, 1920 (temporary)
Lieutenant Alexander McG. McCulloch, May, 1920^[8]
Lieutenant in Command Andrew L. Besant, 5 August, 1920^[9] – 3 January, 1921^[10] (and also 3 other "H" class submarines)
Lieutenant in Command John D. de M. Leathes, 5 May, 1922^[11] – 5 May, 1922^[12] (and 2-3 other "H" boats in Group "F" Reserve at Portland)