#FridayPhysicsFun – This week I visited the Vasa Museum with a friend, who remarked “Normally I would like for technical projects to spend a bit more on beautiful design, but this is a case where they should have spent less on sculpture and more on engineering.”
Conversation
The Vasa was a warship launched 1627 to be the new Swedish flagship... it promptly sank on its maiden voyage after just 1.5 km. It was recovered in the 1960s from the bottom of Stockholm harbour, and is now in a fantastic museum.
en.wikipedia.org/wiki/Vasa_(shi
vasamuseet.se
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Why did it sink? It was basically too top-heavy, making it tilt far more than intended when there was a surprise gust of wind. It also had lots of open cannon ports that easily let in water when it tilted. Once it took in water it got even more unstable.
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The technical term is that it had bad initial stability: it tilted a lot when the centre of gravity moved to the side. The broader the ship the more stable it is, but Vasa was narrow and tall.
en.wikipedia.org/wiki/Initial_s
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The shipwright had tried to widen the ship but too late in the process: the king had signed off on the design, changed it repeatedly, and wanted it done fast. During a stability test it was clear that it was so unstable the test had to be stopped early…
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Ship stability is a funny mechanics problem. The ship is pulled down by gravity as if there was a spring attached to the centre of gravity point. This is rigid body mechanics.
en.wikipedia.org/wiki/Ship_stab
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It is pushed up as if there is a spring pulling from a "centre of buoyancy" point, the centre of mass of the displaced water. This can move depending on tilt and ship shape: it is not rigid.
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What matters is the torque acting on the ship from these forces: does it turn the ship back towards vertical when tilted, or the opposite? This depends on where the projection of the centre of buoyancy is on the vertical axis.
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The projection point is the point the ship rocks around, and the distance to the centre of gravity is the "metacentric height". A big height makes things more stable and rolling slower.
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This is a standard calculation today (made even better with CAD). There was no real theory like this back in the 1600s, but practical experience. Still, this was one of the first two-deck ships Sweden built so experience was a bit lacking.
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There are plenty of good engineering lessons from the disaster. Most are more about project management than mechanics: don’t change specifications underway, document things, don’t hurry projects too much… and report failed tests!
simscale.com/blog/2017/12/v
faculty.up.edu/lulay/failure/
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