https://archive.is/pmUEm
Brendan Greeley is FT Alphaville’s part-time boat correspondent.
The
hardest thing about teaching someone how to drive a boat is that it’s
not at all like driving a car. To steer a car, you turn the wheel until
your nose is pointing where you want to go, then you straighten out and
go there. This works because the car is attached to the road. It’s when
the car itself is no longer attached to the road that things get weird.
When you turn too hard, for example, the rubber in your tires loses
purchase on the street, and you are “in drift.” The normal rules no
longer apply.
When
you drive a boat, you are always in drift. You are attached to nothing.
Stuff happens in the water beneath you that does not make any intuitive
sense. Sometimes your stern (your tail) moves faster than your bow
(your nose), and in a different direction. Sometimes both stern and bow
are moving in the same direction at the same speed, but it’s not the
direction the bow is pointed. On a boat, you don’t always go where
you’re pointed.
On Wednesday, the Golden-Class container ship Ever Given made an unplanned berth
in the sand on both sides of the Suez Canal, stopping trade between
Europe and Asia. Evergreen Marine, which operates the Ever Given under a
Panamanian flag, told the Financial Times
in an email that the ship was “suspected to have met with a sudden gust
of strong wind, which caused the ship’s body to veer from its course
and accidentally run aground.” At press the ship was still where it came
to rest, tended by several tugs. It may be there for a while; you can check for yourself on VesselFinder.
It’s hard to describe what happened as a “grounding,” though. Container ship groundings are not unheard of in the Suez Canal. Sand comes up from the canal floor at a 4:1 ratio;
if a ship drifts out of the fairway, it’s most likely to dig a shoulder
into that sand and wait for a tug. Last March the OOCL Japan, a
container ship about the size of the Ever Given, had a mechanical failure in the Suez Canal, lost steering, took the ground, was refloated in several hours and continued on its way.
That’s
not what happened on Wednesday. The Ever Given had been part of a
northbound convoy, still at the southern end of the canal. That section
is lined with riprap — stacked boulders on the side of a waterway.
Riprap is supposed to catch waves and protect sand and mud from erosion.
It is not supposed to catch boats.
But
the Ever Given has punched through the riprap at a steep angle, and
wedged its bow bulb in the soft sand beyond it. This was not a
grounding. It was a walling.
It certainly was windy along the Suez Canal. According to Meteoblue, which provides weather data to apps and corporate clients, winds peaked above 30mph
at the Suez Protectorate on Wednesday, not far from the Ever Given.
Most harbours would fly a small craft advisory at that speed. But wasn’t
unprecedented. Wind peaked above 30mph twice in 2020 at the same location, in March and again in May.
A
gust of wind is by definition an accident, an act of God we all
understand. It makes sense, in a car-like way: you think you’re pointed
one way. Then something hits you, and you’re pointed another. But the
initial explanation offered by Evergreen steps gingerly around another
possible reason: Ever Given is a very large boat. And very large boats
in confined channels do not move in car-like ways.
On Wednesday afternoon Alphaville spoke to Evert Lataire, head of the Maritime Technology Division at Ghent University in Belgium. He had spent his day looking at a VesselFinder video
of Ever Given’s track through the canal for the same reason Alphaville
went digging: honestly, what is more exciting than a major shipping
disaster with no reported injuries or oil spilled? Lataire studies
hydrodynamics, the science of how liquids exert forces as they move.
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.
Most
of the research and design on ship hulls goes into efficiency and
stability at sea. But at sea is not where the Ever Given got stuck. And
ships have gotten big, fast, which means the consequences of
shallow-water hydrodynamics are changing by the year. In 2007, Lataire
points out, the biggest container ships carried 8,000 containers. Some
ships are now close to 25,000 containers. The Ever Given, finished by
Imabari Shipbuilding in Japan in 2018, carries just over 20,000
containers.
By
any historical standards, the Ever Given is a monster. But it’s a
monster in a specific way: it’s fat. The more containers you can stack
on a single ship, the cheaper the marginal cost of each new container.
But the specific engineering of container ships mean that they can’t get
longer; they have to get wider. An
oil tanker is a shoe box with a lid: hull on the bottom, oil in the
middle, deck on top. But a container ship is a shoebox without a lid:
hull on the bottom, then containers all the way up. It’s not as strong
without the lid.
There
are definitely hydrodynamic forces in the open ocean, it’s just that
the ocean is usually in charge of them. And the biggest stress on a
ship’s hull in heavy weather happens along the longitudinal bending
moment — lengthwise, between the bow and the stern. The longer a ship
gets, the worse the stress gets when a wave pushes up in the wrong
place. As far as length goes for container ships, “we are at the
limitations of welding and steel quality,” says Lataire. “I will not say
that it is impossible to weld thicker plates, but in a way this is the
economic limit.”
So
container ships can’t get longer, and they can’t stack any more
containers fore and aft. Instead, they stack them taller. And wider.
Container ships haven’t become monster long; they’ve become monster
beamy. Ever Given, for example, is too beamy for the Panama Canal. This
is why we need big towing tanks in Belgium dragging tiny models of
container ships through the water to figure out what happens: we keep
making bigger ships, but we’re still learning how big ships work.
So now you understand why Evert Lataire spent the morning looking at a YouTube video
of Ever Given’s location on VesselFinder. The trouble starts around
0:10. The ship is moving north, with westerly winds — they are coming
from the ship’s left, pushing it to the right. To compensate, the ship
has adjusted its heading to the left, into the wind, to make sure the
combination of screw and wind continue to push it at the correct
bearing, towards the Mediterranean. Sometimes in a boat, if you are
getting pushed right, you need to head left to go straight.
Then, around 0:14, the ship lurches left, into the wind. Lataire
thinks there might have been not a gust, but a temporary lull, meaning
the Ever Given was overadjusted to its left, moved to the left, and its
beamy hull began to hug the windward bank. Then everything happens
quickly, in a way that looks a lot like the bank effect. Bow shoots away
from the bank. Stern continues to hug the bank and move north. Ship
spins. Bow bulb punches through the riprap.
Trade comes to a halt.
Wind
definitely played a role, but there was probably something else
happening, too. The ships keep getting bigger. But everything on Earth
stays the same size.
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