Why Did the Vasa Sink? The Ship Built With Two Rulers

On the afternoon of 10 August 1628, a crowd gathered along the Stockholm waterfront to watch the pride of the Swedish navy leave on her first voyage. Vasa was the most heavily armed ship in the Baltic: 64 guns, two full decks of bronze cannon, a stern covered in hundreds of painted and gilded sculptures. She fired a salute, set four of her ten sails, and moved off across the harbour. She got about 1,300 metres. A gust caught her, she leaned, water rushed in through the open gunports, and she went down in front of everyone, flags still flying.

The question of why has been asked ever since, first by a royal inquiry that same year and then, once the ship came back up in 1961, by the archaeologists who could finally measure her. Most of the answer is well known. One part of it is stranger, and it starts with four wooden sticks found in the wreck.

The ship was ordered in 1625 by King Gustav II Adolf, who was at war with Poland and wanted a navy to match his ambitions. The contract went to Henrik Hybertsson, a Dutch master shipwright working in Stockholm, and to the shipyard he ran with his business partner. Dutch shipbuilders were the best in Europe, and Stockholm’s yard was full of them, working side by side with Swedish carpenters.

While she was being built, the plans grew. The king, following the war from the other side of the Baltic, kept asking for more guns. Vasa ended up with two complete gun decks, unusual for a Swedish ship of her day, on a hull that had not been designed for them. Then, in 1627, Hybertsson fell ill and died. Others finished the ship from his plans and from whatever was in their heads.

The physics behind that test is simple to state. A ship floats upright because its weight pulls down through a centre of gravity while the water pushes up through the centre of the hull below the waterline. When she leans, that upward push moves sideways and levers her back. The higher the weight sits, the shorter that lever becomes. Vasa carried her heaviest cargo, two decks of guns, high above a hull that was narrow and shallow underwater, with only about 120 tonnes of stone ballast in the bottom.

Adding ballast was the obvious cure and it was not available. More stone would sink her deeper, and the lower gunports already sat only a little more than a metre above the water. Every bit of steadiness bought with ballast brought those openings closer to the sea. Try it:

The inquiry that followed questioned the captain, the surviving shipwright and the crew. No one was punished. The ship had been built to the dimensions the king approved, the ballast was what there was room for, and nobody had a way to calculate stability in 1628. That would take more than another century of mathematics. For more than 300 years that was where the story stopped.

In 1956 a private researcher, Anders Franzén, pulled up a plug of blackened oak from the bottom of the harbour with a coring tool. Divers confirmed it was Vasa, sitting upright in the mud. The cold, brackish water of the Baltic had kept out the shipworm that eats wrecks in saltier seas. Over the next years divers tunnelled under the hull and passed steel cables beneath it, and on 24 April 1961 she broke the surface again.

The rulers would be a curiosity if the hull were perfectly symmetrical. It isn’t. Careful surveys of Vasa show that her two halves are not mirror images: the port side is built more heavily than the starboard side, and timbers on one side do not match their partners on the other. Fred Hocker, the museum’s director of research, has suggested an explanation that fits both clues. A yard full of Dutch and Swedish carpenters may have split the work by side, each gang building its half with its own idea of what a foot was.

A hull heavier on one side sits with a slight list before anything else happens. On a stiff, well ballasted ship that would hardly matter. On Vasa, with almost no margin, it would have meant starting the voyage already leaning a little to port, the side she heeled over to. Hocker is careful about this, and so should we be: the asymmetry did not sink the ship on its own. The ship was top-heavy, and would have been dangerous with two perfect halves. The two rulers may simply have taken away the last few degrees she had to spare.

Nobody on the yard did anything wrong by the standards of his own ruler.

It is an old failure that keeps coming back in new materials. In 1999 NASA lost the Mars Climate Orbiter because one team’s software reported thrust in pound-seconds and another’s expected newton-seconds. Small, consistent differences compound, the way a fold doubles a stack of paper in Britney Gallivan’s twelve folds. And Vasa was built at the exact moment Europe was trying to make calculation trustworthy: only a decade earlier, John Napier had published the rods that became Napier’s bones, and a century before that Dürer had filled Melencolia I with the compasses and scales of a craftsman’s measuring world.

Today Vasa stands in a museum built around her on Djurgården, the most visited museum in Scandinavia. She is the best preserved ship of her century anywhere, precisely because she failed so quickly: she never fought, never wore out, never got broken up for timber. Walk around her and the asymmetry is invisible. You need a tape measure, and some idea of which foot it is counting in.