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Merchant Navy officer

Shackleton's restlessness at school was such that he was allowed to leave at sixteen and go to sea. One option was a Royal Navy officer cadetship in the Britannia at Dartmouth, but this was too expensive, and Shackleton passed the upper age limit of fourteen and a half in 1888. Alternatives were the mercantile marine cadet ships Worcester and Conway, or an apprenticeship "before the mast" on a sailing vessel. This third option was chosen. His father was able to secure him a berth with the North Western Shipping Company, aboard the square-rigged sailing ship Hoghton Tower.

Over the next four years at sea, Shackleton learned his trade and visited many parts of the world, forming a variety of acquaintances and learning to associate with people from many different walks of life. In August 1894, he passed his examination for second mate and accepted a post as third officer on a tramp steamer of the Welsh Shire Line. Two years later, he had obtained his first mate's ticket, and in 1898, he was certified as a master mariner, qualifying him to command a British ship anywhere in the world.

In 1898, he joined Union-Castle Line, the regular mail and passenger carrier between Britain and South Africa. One of his shipmates recorded that Shackleton was "a departure from our usual type of young officer", content with his own company though not aloof, "spouting lines from Keats or Browning ", a mixture of sensitivity and aggression but not unsympathetic. Following the outbreak of the Boer War in 1899, Shackleton transferred to the troopship Tintagel Castle where, in March 1900, he met Cedric Longstaff, an army lieutenant whose father Llewellyn W. Longstaff was the main financial backer of the British National Antarctic Expedition then being organised in London.

Shackleton used his acquaintance with the son to obtain an interview with Longstaff senior, with a view to obtaining a place on the expedition. Impressed by Shackleton's keenness, Longstaff recommended him to Sir Clements Markham, the expedition's overlord, making it clear that he wanted Shackleton accepted. On 17 February 1901, his appointment as third officer to the expedition's ship Discovery was confirmed; on 4 June he was commissioned into the Royal Navy, with the rank of sub-lieutenant in the Royal Naval Reserve. Although officially on leave from Union-Castle, this was in fact the end of Shackleton's Merchant Navy service.