"Hydrodynamics is what a five-year old would do, if a five-year old had a PhD."

 Brendan Greeley writing for FT Alphaville (which you should definitely sign up for) explains the physics behind the recent traffic jam at the Suez Canal. 

Sailors talk about hydrodynamics the way CEOs talk about macroeconomics: they either treat it with mystical reverence, or they claim to understand it and are wrong. Unlike with macroeconomics, though, if you know what you’re doing you can test the propositions of hydrodynamics on actual, physical models in a lab. As in: you build little boats and then you drag them through the water, in a towing tank. Hydrodynamics is what a five-year old would do, if a five-year old had a PhD.

Lataire works with Flanders Hydraulics Research at what he calls the world’s most accurately constructed shallow-bottom tow tank. He’s currently helping build an even bigger tank, to generate more data for a ship simulator to certify pilots. The tanks are shallow-bottomed, because hydrodynamics in shallow water are different. When a boat moves through the water, it pushes the water out of the way — it displaces it. “Where the water needs to be displaced, in a deep ocean it can go under the ship and that’s not a problem,” says Lataire. “But if it needs to go into shallow water, like the Suez, the water simply cannot go under and around.” 

The Suez Canal is basically just a 24m-deep ditch dug in the ground to let the ocean in. When a ship comes by and displaces the water, the water has nowhere to go; it gets squeezed in between the ship’s hull and the floor and the sides of the ditch. A ship in a canal can squat, for example — it can dig its stern into the water. When water gets squeezed between a ship’s hull and a sand floor, it speeds up. As water flow speeds up, its pressure drops, pulling the hull down to fill the vacuum. The effect is more pronounced at the stern, and so the ship settles into a squat: bow up, stern down. 

Lataire wrote his dissertation on a similar phenomenon as a ship passes close to a bank: the bank effect. The water speeds up, the pressure drops, the stern pulls into the bank and, particularly in shallow water, the bow gets pushed away. Stern one way, bow the other. A boat that had been steaming is suddenly spinning. It’s a well-identified phenomenon; in 2009 Ghent University’s Shallow Water Knowledge Centre put together a whole conference about it. Clever pilots on the Elbe, according to Lataire, will use it to shoot around a bend. 

However: the more water a ship displaces, the stronger the effect. And the closer the side of the hull is to the shore, the stronger the effect. The bigger the ship, the faster the bow shoots away from the bank.