Three virtually unknown short stories by Patrick O’Brian of derring-do on the high seas, featuring everything from shark fishing on the Great Barrier Reef to encounters with pirates in the fog-bound northern Pacific, are due to be released next month to mark the centenary of the Master and Commander author’s birth.

The stories, Noughts and Crosses, Two’s Company and No Pirates Nowadays, were published in the Oxford Annual for Boys in the late 1930s under a pseudonym, but have been out of print since then. They feature an Irishman, Sullivan, and a Scot, Ross. O’Brian’s publisher HarperCollins says these two characters can “clearly be seen as precursors to Jack and Stephen”, or Aubrey and Maturin, the ship’s surgeon and intelligence agent at the heart of the 21-book series for which he is best known.

In his biography of O’Brian, Dean King calls 1936’s publication of Noughts and Crosses a “watershed” moment. “In the past, he had written about animals, about men and animals, and about men on unequal terms,” writes the biographer. “For the first time, in Noughts and Crosses, he introduced two characters on relatively even footing. Sullivan, a tall, thoughtful Irishman, and Ross, a tough, red-bearded Scot, are, despite their personality differences, bound by friendship, though at first it is not apparent.”

Noughts and Crosses sees Sullivan and Ross shark fishing on the Great Barrier Reef, only to be driven to an atoll for shelter “when the very devil of a storm turns the whole of the sea white”, said HarperCollins. When they set out again, the sea is full of sharks “driven to a bloodlust”, and the pair become becalmed. In Two’s Company, the friends take on a job as lighthouse keepers on a reef in the northern seas – “long months of isolation and boredom can test even the stoutest of friendships” – and in No Pirates Nowadays, they are searching for the island of Sakhelien in the northern Pacific, and stumble across pirates.

HarperCollins will publish the stories individually, as ebooks and downloadable audio, on 4 December, with actor Robert Hardy – Cornelius Fudge in Harry Potter and the voice of the Aubrey-Maturin books – voicing the audiobooks. Sullivan and Ross also appear in a novel by O’Brian, The Road to Samarcand, which HarperCollins will reissue in February in a new edition complete with the three short stories.

Chris Smith, O’Brian’s editor at HarperCollins, said that the author’s “portrayal of life aboard ship, of the landscape, and of the sea, is faultless, and the sense of period and power of characterisation throughout is masterly”, with the same qualities to be found in these early stories.

O’Brian died in 2000, having sold more than 14m copies of his books and won fans ranging from George RR Martin to Iris Murdoch. Hardy, who met him before his death, said that the author and his invention Stephen Maturin “were all but one and the same creation”, and described his 21 Aubrey-Maturin novels as “magnificent”. “The discovery of three earlier stories, which in subtle ways foreshadow the later partnership, has given me a new and enigmatic challenge and the joy of recording O’Brian’s mastery of character, tension and danger,” said the actor.