Tag Archives: fluiddynamics

Squat effect


The MS Oasis of the Seas is the world’s largest cruise ship (holding the title jointly with its sister ship, MS Allure of the Seas) and stands seventy-two metres above the water line. During the delivery process, as it was sailing from the STX Shipyard in Finland where it was built to its first stop, it had to pass underneath the Great Belt Bridge that connects the Danish islands of  Zealand and Sprogø, a bridge that stands only sixty-five metres above the waterline. Oasis of the Seas is equipped with telescoping funnels and ballast tanks, but this would only create a tiny margin of safety between the ship and the bridge.


Luckily, the ship was able to rely on something called the squat effect to help it make it under the bridge. The squat effect occurs when a ship moves through shallow water at speed. The water is forced underneath the ship, increasing its speed, and this causes the water pressure to drop (by Bernoulli’s Principle), thereby pulling the ship deeper into the water.

ship-no-squatIn deep water no squat effect is present.

ship-squatIn shallow water the squat effect causes the ship to sink lower into the water.

Oasis of the Seas approached the bridge at twenty knots (10.3 m/s; 23 mph), close to its twenty-two knot top speed and this was enough to pull it an additional thirty centimetres into the water, allowing it to pass safely under the bridge.

How are mushroom clouds formed?

Mushroom clouds (perhaps more properly known as pyrocumulus clouds) are traditionally associated with nuclear explosions, but any sufficiently large explosion (for example, a volcanic eruption) will create a mushroom cloud.

The mushroom cloud resulting from the Priscilla test of Operation Plumbbob.

When a large explosion occurs a cloud of very hot gas is created. This hot gas, being less dense than the surrounding air, rises rapidly upwards. As this cloud of hot gas rises it pushes against the air above it and this air resistance causes the top layer to move sideways whilst the hotter gas below continues rising upwards, creating a swirling doughnut-shaped vortex (in the photograph above a very hot “filament” is visible at the centre of this vortex). As the “cap” rises this swirling vortex pulls in cooler air from ground level, creating the “stalk” on which the cap sits.

The formation of a mushroom cloud during the Tumbler-Snapper series of nuclear tests.

The shape of a mushroom cloud is the result of a Rayleigh-Taylor instability at the interface between the hot less-dense and cold more-dense air. These instabilities occur in a number of different situations, and can be easily demonstrated at home by dropping coloured oil into water, creating tiny upside-down mushroom clouds as shown below in photographs by James Riordon of AIP.

The simulated formation of a Rayleigh-Taylor instability.