Her Majesty's Army Vessel

His or Her Majesty's Army Vessel (HMAV) is the prefix used for a ship which is an operational unit of the British Army, commanded by British Army officers and crewed by army personnel in uniform. These ships fly the Army ensign.{{cite web , url=https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/433769/QR_Army.pdf , title=The Queens Regulations for the Army, publisher=British Government

Prior to the 20th century it was a ship prefix for His (or Her) Majesty's Armed Vessel.

References

British Army
Ship prefixes