The might of the sea and its merciless ability to turn on human beings when they are most vulnerable fascinate John Flanagan. The Sydney author, who lives in the seaside suburb Manly with his wife, Leonie, and spent years sailing dinghies, says he is amazed that modern ships can still break up and sink.

"You would think they might have solved that problem by now," says Flanagan, 71, who is best known for his children's fantasy series Ranger's Apprentice.

The author's first experience of the sea was when he travelled to England in 1967 on the Australis, a 35,000-tonne passenger ship. "I was used to coming across to Manly on the ferry then to see my future wife but when I was on that ship and it instead turned right and went out the heads, it was an amazing feeling – this giant mass of steel, moving and rolling. There's this sense the sea can turn on you. The allure is the risk, and the concept of travel."

The sea is the setting for Flanagan's third adult novel, The Grey Raider, which is about a Confederate sloop-of-war, CSS Manassas, that hunts down Union merchant vessels during the American Civil War. At the heart of the novel is the fierce rivalry, and mutual hatred, of two former comrades, Pelle, the captain of the Manassas, and the man sent by Abraham Lincoln to pursue him, Stacy, the captain of the USS Oswego.

The novel is loosely based on the true story of the CSS Alabama, a commerce raider that attacked Union ships until it was sunk in 1864 by the USS Kearsarge outside the port of Cherbourg in France.

Flanagan's story also traces the exploits of a beautiful Confederate spy, Kitty Averill, who must dodge the agents of Allan Pinkerton, the real-life detective who ran Lincoln's security service during the war.

Patrick O'Brian's sea adventures inspired Flanagan's story but his biggest influence are the Horatio Hornblower novels, set during the Napoleonic wars, by C.S. Forester, whom he regards as a master storyteller. The Grey Raider has the sort of detail that will delight lovers of nautical adventure, including the sailing methods of 19th-century four-masted sloops that were also steam driven and armed with seven-inch rifles and 32-pounder cannons.

Flanagan, who grew up in Clovelly and was taught by the Christian Brothers at Waverley College, is a former advertising copywriter who turned to TV writing in the 1980s with a colleague, Gary Reilly. The admen created the TV series Hey Dad!, writing 293 episodes, as well as Hampton Court and My Two Wives.

"TV taught me story structure and character development," Flanagan says. "I did all the storylines for Hey Dad!. I had a white board and I'd draw a paradigm of the three-act structure. I still do it for my books."

But Flanagan also started writing children's fiction 25 years ago to entertain his youngest child, Michael (Flanagan also has two daughters: Penny, a singer-songwriter; and Katie, a comedian who appears on ABC TV's The Weekly under her stage name, Kitty, the name of Flanagan's mother and the character in his new novel). This is how Ranger's Apprentice was born, but the first book, The Ruins of Gorlan, was not published until 2004, when Flanagan was 60.

Along with his Brotherband series and two adult thrillers, his books have now sold almost 10 million copies worldwide, making him one of Australia's most successful authors.

More sea stories may be in the pipeline, says the author, who is putting the final touches to a prequel to The Ruins of Gorlan before its October release.

As to Flanagan's own sea adventures, he travelled last year on a luxurious four-masted barkentine through the Greek islands. "The fun thing about that was waking up one morning moored off  Santorini in a caldera eight kilometres across. I just went, 'Oh my god'. That is what is great about sea travel – finding the unexpected."