Why the Ship Held Its Breath The Strange Science of Tuned Mass Dampers and Structures That Fight Vibration

Every ship floating in a swell has a rhythm of its own. Push a hull over to one side and release it, and it rolls back, overshoots, and rolls the other way, over and over, at a pace set by its shape and the way its weight is distributed. A heavy, wide ship rolls slowly. A narrow, top-heavy one snaps back quickly. Whatever the pace, it belongs to the ship.

The sea has rhythms too, in the spacing of its waves. When the two happen to match, each wave arrives just as the hull is ready to be pushed in the direction it is already moving. A modest swell can then produce a roll that makes sailors grip the rail and passengers lose their lunch.

Around the beginning of the twentieth century, a German engineer named Hermann Frahm looked at this problem and came up with an idea that seemed to defy common sense. He did not try to brace the ship against the sea. He put water inside it. Frahm built tanks on either side of the hull, linked them with a pipe, and let the water slosh between them. The tanks were sized so that the water moved at nearly the same rhythm as the ship's roll, but a little behind it. As the hull swung back from port, the water was still surging toward port, and its weight pulled against the ship's motion. The ship's energy went into stirring a tank of water, and the water disposed of it as turbulence and heat.

That patent, and a related one for a spring-mounted weight that could soak up vibration in a machine, planted the seed of an entire family of devices. They are called tuned mass dampers, and they hang inside skyscrapers, sit under footbridges, and swing from the ceilings of stadiums. The principle has never changed from Frahm's tanks.