A new book shows how, to house its massive crew, Henry VIII’s favourite warship had one more deck than was originally thought

It was Henry VIII’s favourite warship, and notched up several victories before sinking in battle with the loss of hundreds of lives. Now a maritime historian has found that the Mary Rose was even more formidable than previously thought, with three decks to accommodate its vast crew rather than two.

The finding is among several significant discoveries made by Peter Marsden, who studied historical records, contemporary images and the wreck itself. He told the Observer: “The vessel was already absolutely vast. Put another deck in there and it is much bigger than we thought.”

The Mary Rose, a “floating castle” built in Portsmouth, sank in a battle against the French in the Solent in 1545. More than 450 sailors died. The wreck was raised off the seabed in 1982 and housed in an award-winning museum in Portsmouth, which opened in 1984.

The vessel was first thought to have had only one deck in the “castles”, then this was revised to two. Now, in a forthcoming book, Marsden will present new evidence for three decks. “I came at it from an entirely different angle. That is, where on earth did all these people sleep, rest and eat? There just wasn’t enough room with two decks. I know half the crew is on duty when the other half is resting but, still, an awful lot of space is needed. This hasn’t been considered previously.”

He added: “It is clear that previous reconstructions do not allow enough room for everyone. There is no doubt that she had an extra deck in her sterncastle and her forecastle – so there were three castle decks in all.”

The discovery is awkward for the Portsmouth museum, whose design was based entirely around a warship with two castle decks. “It cannot be changed,” Marsden said. “You’d have to take the roof off the existing museum.”

Marsden writes in his book, 1545: Who Sank the Mary Rose?: “The story of the Mary Rose is primarily of a personal vendetta between the kings of England and France … Henry VIII had used the ship as a weapon that … over 33 years, had brought terror to ordinary folk living quietly on the coast, mostly in Brittany, and in the year before her loss when Henry seized the French port of Boulogne. Understandably, the angry King Francis had had enough and sent a huge invasion force of French warships and troop carriers to attack England in 1545. It was bigger than the famous Spanish Armada of 1588.”

Facebook Twitter Pinterest An illustration of the Mary Rose from about 1546. Photograph: by permission of the Pepys Library, Magdalene College Cambridge

The book reveals new details about the battle in which the Mary Rose was sunk on 19 July, as well as analysis of its contents and the men who sailed in her.

Reconstructing the full story of the battle for the first time, he has established exact timings of when the French fleet arrived off the Isle of Wight: “Previously, dates have varied from 17 to 19 July, based on records written years after the event.”

By studying the tides and currents on the fateful day, Marsden has pinned down the arrival to about 3pm, and the sinking of the Mary Rose to about 7pm. That was about an hour before sunset so there wasn’t much time to rescue people, he said. “This is all new.”

Why she sank has been the subject of much debate, he said: “Gun ports were left open while she was in battle, and … a gust of wind unexpectedly heeled her over and she flooded and sank.

“But why it occurred has been the problem. At the time, the English court blamed the crew, the French believed that gunfire from their galley had caused the disaster and, since the ship was raised, other possible reasons, including the pilot possibly being a Frenchman or that the hired Spanish mercenaries did not understand orders in English have entered the possibilities.

“A third castle deck would further explain why she heeled over.”