Infrastructure
The short and long tail of Water Transport
Water holds weight up for free. Follow that one fact outward and you reach canals that run on rain, a boat lift powered like eight kettles, and aircraft the law treats as ships.
Put a horse in front of a cart on an eighteenth-century road and it can haul a modest load. Walk the same horse along a canal towpath with a rope tied to a boat, and it can move almost fifty times as much weight. Same horse, same legs, same oats. The only thing that changed is that the load is floating.
Two and a half centuries later the advantage is still enormous. Moving one tonne of cargo one kilometre on an average bulk carrier puts about 3.5 grams of carbon dioxide into the air; on a long-haul truck it's about 105 grams, thirty times as much. Carbon dioxide comes from burning fuel, so that is roughly the fuel gap too, and it's why shipping moves over 80 per cent of the world's merchandise trade.
Water transport rests on one gift: water holds things up for nothing. The short tail is the machine the world has built around that gift, meaning the ships, the box and the rules. The long tail is what happens at the edges, where a canal runs short of rain, a Scottish boat lift turns on the energy of eight kettles, and an aircraft that stays low enough counts, under international rules, as a ship.
Part oneThe short tail
What actually carries the world's cargo?
Three kinds of ship do most of it. Bulk carriers take loose dry cargo such as iron ore, coal and grain, poured straight into their holds. Tankers take liquids: crude oil, fuels, chemicals and liquefied gas. Container ships take nearly everything else, sealed inside identical steel boxes.
By carrying capacity, the first two dominate. At the start of 2024 bulk carriers were 42.7 per cent of the world fleet's capacity and oil tankers 28.3 per cent, and at the start of 2026 the two together still made up 69 per cent. Container ships, the ones in every photo of a port, are a smaller slice by weight, largely because what they carry is light for its value. A hold full of trainers weighs far less than a hold full of iron ore.
The boxes still move in enormous numbers. Ports handled 920 million TEU in 2024, where one TEU is one twenty-foot box's worth. That figure counts port handlings rather than journeys, so a box loaded in Shanghai and unloaded in Rotterdam turns up in both ports' totals.
People travel by water in numbers that surprise most of us. The IMO puts the ferry industry on a par with commercial aviation, carrying about 4.27 billion passengers and 373 million vehicles a year. Each trip is small; the total is not.
How much of world trade goes by sea, and who is counting?
The bookkeeper is UN Trade and Development (UNCTAD), which builds its fleet statistics with data from Clarksons Research. Its headline is that shipping moves over 80 per cent of world merchandise trade. In 2024 that came to 12,720 million tonnes, roughly a tonne and a half for each of the eight billion-plus people alive.
The unit matters. Tonnes loaded tells you how much stuff moved; tonne-miles also counts how far it went. In 2024 seaborne volume grew 2.2 per cent while tonne-miles grew about 6 per cent, because ships avoiding the Red Sea took the long way around Africa. The same cargo used up more ocean.
Ships get two measures as well. Deadweight tonnage (dwt) is how much a ship can carry, while gross tonnage (GT) describes how big it is. At the start of 2026 the world merchant fleet had about 116,000 ships of at least 100 GT, some 62,000 of them over 1,000 GT, with a combined 2.5 billion tonnes of carrying capacity.
Why does a steel ship float?
Because it pushes aside its own weight in water before it runs out of hull. Archimedes' principle says the upward force on a submerged object equals the weight of the fluid it displaces. That force comes from pressure: water pushes harder the deeper you go, so it presses up on a hull's bottom harder than it presses down anywhere above.
A steel bar sinks because it can't displace its own weight of water. Shape the same steel into a hollow shell and it encloses a huge volume of air, so it settles only until the water it has pushed aside weighs as much as the ship and everything in it. Load cargo and it sinks deeper, displacing more water, until the forces balance again.
Here's a small calculation with a practical sting. For load-line purposes, seawater is taken to weigh 1,025 kilograms per cubic metre and fresh water 1,000. Each tonne of ship needs 0.976 cubic metres of seawater to hold it up but a full cubic metre of fresh water, about 2.5 per cent more volume. A boxy ship drawing 12 metres at sea will sit roughly 30 centimetres deeper when it sails up a river.
That's why the marks painted on a ship's side come as a set. Alongside the summer, winter and tropical lines there are fresh-water and tropical fresh-water lines that allow a deeper draft, because a ship loaded to its fresh-water mark in a river port floats at its summer mark once it reaches the sea. The Panama Canal quotes its maximum drafts in "tropical fresh water" for the same reason: Gatun Lake is fresh.
Those lines exist because people drowned without them. In the nineteenth century, overloaded and badly repaired "coffin ships" sank often enough that Samuel Plimsoll, an MP and coal merchant, campaigned for a legal limit. Britain's Merchant Shipping Act of 1876 made load lines compulsory, though the line's position wasn't fixed by law until 1894. International conventions followed in 1930 and 1966.
Then why doesn't it tip over?
Floating is about how much water a ship displaces. Staying upright is about where the forces act. Weight pulls down through the centre of gravity, and buoyancy pushes up through the centre of the underwater volume. When a wave rolls the ship, the underwater shape changes and that centre of buoyancy slides toward the low side; if it slides far enough, the two forces make a twist that rolls the ship back.
Naval architects measure that tendency as the metacentric height, GM, and the IMO's Intact Stability Code sets minimum criteria for GM and for the righting lever that follows from it. More isn't always better. A ship with a large GM is "stiff": it snaps back with a short, violent roll that is hard on people and cargo. A small GM makes a ship "tender", rolling slowly and comfortably with a thinner margin before capsize.
The quiet killer is liquid that can move. Water sloshing in a half-full tank runs to the low side exactly when the ship is heeling, shifting weight the wrong way. The penalty grows with the cube of the tank's width, which gives a neat piece of arithmetic: split a tank lengthwise into two halves and each half carries an eighth of the original penalty, so together they carry a quarter. Split it into thirds and the penalty falls to a ninth.
Now picture the widest tank a ship can have: a car deck running the full width of a ferry. On 6 March 1987 the Herald of Free Enterprise left Zeebrugge with her bow doors open. Water poured onto the vehicle deck and "destroyed her stability", the government told Parliament; she went over in less than two minutes and 193 people died. The inquiry asked for research into bulkheads on vehicle decks, and after the Herald and the Estonia, which sank in 1994 with more than 850 lives, the IMO tightened the stability rules for roll-on, roll-off ferries.
Why is moving things by water so cheap?
Because water holds the weight up for free, an engine only pays for pushing water aside, and that bill falls steeply as a ship slows down. On land every tonne pays a toll at any speed. Heavy-truck tyres cost roughly 3.4 to 7.4 kilograms of drag for every tonne on the axle just to keep rolling, whether the truck is crawling or cruising, and air drag comes on top. A floating hull has no such floor.
| One tonne of cargo moved one kilometre | Grams of CO₂ equivalent |
|---|---|
| Bulk carrier (average) | 3.5 |
| Container ship (average) | 16 |
| Long-haul heavy truck | 105 |
| Long-haul air freight | about 900 |
From New Zealand's official emissions guide, which uses the UK government's 2025 factors for ships and aircraft and New Zealand fleet data for trucks. The air figure includes an allowance for the extra warming caused at altitude.
Speed is the lever. To go twice as fast, a hull has to shove about twice as much water aside each second and shove each bit about twice as hard, so resistance rises roughly with the square of speed. Power is force times speed, so it rises roughly with the cube.
Published fuel figures show the rule working. In numbers from a 2009 study of container lines, an 8,000-TEU container ship burns about 225 tonnes of fuel a day at 24 knots and about 150 tonnes at 21 knots. Check the cube: (21 ÷ 24)³ = 0.67, and 225 × 0.67 = 151, right beside the 150 quoted.
Now count per mile, which is what a shipowner pays for. At 24 knots the ship covers 576 nautical miles a day, burning 0.39 tonnes a mile; at 21 knots it covers 504 miles, burning 0.30. Sailing 12.5 per cent slower cuts fuel per mile by about 24 per cent. Over a 10,000-mile voyage the slower ship arrives about two and a half days later, having burned roughly 3,000 tonnes of fuel instead of 3,900.
When trade collapsed in 2008–09 just as new ships kept arriving, container lines did exactly this, a practice called slow steaming. It's also the horse's secret. At walking pace water barely resists, so nearly all of the animal's effort goes into moving the load.
Why is every shipping container the same size?
Because the box is an interface, and an interface only works if everyone agrees on it. Before containers, cargo travelled as "break-bulk": sacks, barrels and crates, each lifted, stowed and unloaded piece by piece.
On 26 April 1956 a converted tanker called the Ideal X left Port Newark, New Jersey, with 58 metal containers, each 35 feet long to match the truck trailers of the day, and reached Houston five days later. By the 1956 reckoning of Malcolm McLean, the trucking magnate behind the voyage, loading a medium-sized ship the conventional way cost $5.83 a ton; loading containers would cost under $0.16. That's a 36-fold difference before counting the days saved at every port.
Early boxes came in whatever sizes their owners liked, which defeated the point. Within about ten years the industry had agreed base dimensions and a latching system through ISO 668, first issued in 1968, and the TEU was first used as a unit in 1969. Port turnaround fell from an average of three weeks in the 1960s to less than a day.
The standard hides some lovely details. A "twenty-foot" container is slightly shorter than twenty feet, so that two of them joined by twistlocks come out as long as one forty-footer and stack cleanly with it. Every force the box carries passes through its corner fittings, which have a standard of their own.
A standard shape isn't enough without honest numbers. Since 1 July 2016 the SOLAS convention has required a verified gross mass for every packed container before loading, because a box heavier or lighter than declared leads to bad stowage plans, collapsed stacks and containers lost overboard. The stability arithmetic from two sections back only works if you know where the weight is.
Who makes the rules at sea?
A ship spends its life crossing jurisdictions, so the rules come in layers. At the top is the International Maritime Organization, the UN agency that sets global standards for the safety, security and environmental performance of shipping. Its founding convention was adopted in Geneva in 1948 and came into force in 1958, and it has 176 member states.
The IMO writes rules but doesn't police them. Under SOLAS, the core safety treaty first adopted in 1914 in response to the Titanic, each ship's flag state is responsible for making sure it complies, and port states can inspect foreign ships they suspect fall short. So the flag matters, and owners choose. In 2026 the three biggest registries were Liberia, Panama and the Marshall Islands, while the country whose companies owned the most capacity was Greece.
Put UNCTAD's figures together and a big ship's life is a trade route of its own. It was probably built in China, South Korea or Japan, which completed 91 per cent of new tonnage in 2025. It may well be owned in Greece, China or Japan and flagged in Liberia, Panama or the Marshall Islands, which between them register more than 40 per cent of world capacity. When it's done, it will probably be broken up in India, Bangladesh or Türkiye, which handled 80 per cent of ship recycling that year.
The third layer is private. In 1760, customers of Edward Lloyd's coffee house in London formed a society whose register rated, or "classed", ships on surveys of their hulls and equipment, a risk assessment for anyone with money on a voyage. Classification societies still do that job. The twelve members of the International Association of Classification Societies set design and construction rules covering more than 90 per cent of the world's cargo-carrying tonnage.
Part twoThe long tail
Why does the Panama Canal run on rain?
Because every ship that crosses Panama is lifted over the isthmus by fresh water, and that water ends up in the sea. The canal's locks work as water lifts, raising ships 26 metres from the ocean to Gatun Lake and lowering them on the far side. The water that fills each chamber flows down from the lake by gravity, and nothing pumps it back.
In a 2002 statement the Panama Canal Authority put the cost at 52 million gallons of fresh water per transit. Let's make that number physical. Fifty-two million US gallons is about 197,000 cubic metres. An Olympic pool holds about 2,500 cubic metres, the size the canal itself uses when it compares water to pools, so one ship's crossing spends about 79 pools.
Now fit it to the machine. An original lock chamber is 304.8 metres long and 33.53 metres wide, a floor of about 10,200 square metres. Spread 197,000 cubic metres over that floor and you get a column 19 metres deep, about three-quarters of the full 26-metre climb. In the rainy season the canal typically offers 36 transits a day; if every one cost that much, the canal would pour about 7 million cubic metres of lake water into the oceans daily.
Compare that with the lake's income. In early October 2023, during an El Niño drought, rain and rivers were bringing about 7 million cubic metres a day into Gatun Lake, while canal operations, evaporation, cities and industry took out about 10 million. In a normal year the inflow averages about 15 million a day. The lake fell seven feet below where it should have been, its lowest rainy-season level on record, and that October was the driest since records began in 1950.
So the canal rationed. It planned to cut daily reservation slots from 25 in early November to 22 in December and as few as 18 by February, and limited ships in the newer locks to a 44-foot draft, which in Archimedes' terms means less cargo per ship. Rain eased things, and by late March 2024 it was back up to 27 slots a day. The stakes aren't only commercial: the same lakes supply water to more than half of Panama's population.
There are engineering fixes. The Neopanamax locks, opened in 2016, have chambers 427 metres long and 55 metres wide, with basins that reuse up to 60 per cent of the water in each lockage. Their floor is 2.3 times the size of an old chamber, so for a similar lift per step, without the basins they'd use about 2.3 times the water. With them, 40 per cent of 2.3 is about 0.92, so roughly the same water as an old-lock transit, for ships that carry more than 14,000 TEU instead of 5,000. In the old locks, passing water from one chamber to its neighbour, called cross-filling, saves the equivalent of six transits a day.
The fixes only stretch the water so far. On 20 August 2026 the canal announced it would cut daily slots again, to nine in the Neopanamax locks and 25, then 23, in the old ones from September, citing El Niño and below-expected rain. In September a Smithsonian scientist at a canal forum described a particularly intense El Niño that continues to strengthen. The long-term plan is a new reservoir on the Río Indio, a $1.6 billion project that would allow about 15 more transits a day. One of the world's great shipping lanes turns out to be a water utility.
Can you lift a boat without spending water?
Yes, if you let Archimedes do the balancing. The Falkirk Wheel in Scotland, opened in 2002, lifts boats 35 metres between the Forth & Clyde Canal and the Union Canal. It replaced a flight of 11 locks that took a day's heavy work and 44 lock gates to get through.
The wheel carries two water-filled gondolas on opposite arms. A boat that floats into one pushes out exactly its own weight of water, so a gondola with a boat in it weighs the same as a gondola without one. The two arms always balance, and turning the 1,800-tonne structure takes about 1.5 kilowatt-hours per rotation, roughly the energy needed to boil eight kettles.
Check it against a straight lift. Raising a 20-tonne boat 35 metres takes mass × gravity × height: 20,000 × 9.81 × 35 ≈ 6.9 million joules, or about 1.9 kWh. The wheel moves the boat and the water it floats in through that height on less energy than the boat alone would need, because it isn't lifting anything net. It swaps two equal weights and pays only for friction and getting them moving.
Why can't a small boat just go faster?
Because it gets trapped in its own wave. A hull pushing through water makes a wave that travels with it, and a deep-water wave's speed depends on its length, since longer waves travel faster. Set the wave's speed equal to the boat's and its length comes out as 2π × speed² ÷ g.
At low speed that wave is short, and several crests fit along the hull. As the boat speeds up the wave stretches, until its length equals the boat's waterline. Now the hull sits between a crest at the bow and the next one at the stern; push harder and the stern drops into the trough, and the boat is in effect trying to climb its own bow wave. That speed is the hull speed: in knots, about 1.34 times the square root of the waterline length in feet.
Run the numbers for two hulls, treating their full length as waterline. A 10-metre yacht (about 33 feet) has a hull speed of 1.34 × √33 ≈ 7.7 knots. A 400-metre container ship (about 1,312 feet) has one of 1.34 × √1,312 ≈ 48.5 knots. At 20 knots the big ship makes a wave only about 68 metres long, so six crests line its side, and it's cruising at around 40 per cent of its hull speed, where wave-making is cheap. Length buys speed cheaply.
It's a rule of thumb for ordinary hulls, and it has exceptions that teach. Very long, narrow hulls make small waves, and racing kayaks can beat their "hull speed" by more than 100 per cent without planing. Speedboats get past it the other way, by rising up and skimming over their own wave.
Big ships attack the wave itself. The rounded bulge under the waterline at the front of most large merchant ships is a bulbous bow, shaped so that the trough of the wave it makes lines up with the crest of the bow wave and the two cancel. The cancellation is tuned to one operating speed, which suits ships that cruise steadily between ports.
What happens when a boat stops touching the water?
It escapes water's drag and loses water's gift. Everything in the short tail depends on buoyancy holding the weight up for free. Lift the hull clear and something else has to hold it up, continuously, using energy.
A hovercraft holds itself up with air. Large fans blow air into a space under the craft that a flexible skirt keeps in, and the craft rises until cushion pressure times cushion area equals its weight. The pressure needed is surprisingly small: a 10-tonne craft on a 100-square-metre cushion needs about 1,000 pascals, around one per cent of the air pressure you're sitting in. The catch is that the air keeps escaping under the skirt, so the fans never get to stop, and with nothing solid to push against, the craft moves on air propellers.
Christopher Cockerell's SR.N1 crossed the English Channel in 1959, and for a while hovercraft carried cars across it; the SR.N4 The Princess Anne, now in a museum, is the largest commercial hovercraft ever built. Today the survivor is niche. Hovertravel has flown between Southsea and Ryde since 1965, a crossing of just under 10 minutes and Europe's only scheduled passenger hovercraft service. It exists because at low tide the sea at Ryde retreats so far that no ship can berth near the town, and a hovercraft simply crosses the exposed shore.
Hydrofoils take another route: underwater wings lift the hull clear once the boat is going fast enough, so only the slim foils stay in the water. Because water is several hundred times denser than air, small foils give a lot of lift. The IMO groups hovercraft and hydrofoils together as high-speed craft, with their own safety code since 1994.
The strangest option is to fly, just barely. A wing close to a surface works better because the ground blocks part of the air it pushes down and weakens the vortices at its tips. At a height of a tenth of the wingspan, induced drag is cut in half, and a ground-effect craft could in theory reach lift-to-drag ratios of 25 to 30, against 15 to 20 for ordinary subsonic aircraft. The Soviet Union built the giants, which it called ekranoplans: the KM of the mid-1960s, with ten jet engines and up to 540 tons, and the Lun, which began trials on the Caspian Sea in 1987.
Do the arithmetic and you find the catch. For a wing 40 metres across, as the KM's was at its widest, a tenth of the span is 4 metres, so the craft gets its best efficiency only while flying lower than a two-storey house over open water, which is exactly where the waves are. The Lun, with six missile launchers along its back, ended up being towed to Derbent to go on display in a "patriotic park".
The law has an answer for these machines. By agreement between the IMO and its aviation counterpart, ICAO, a wing-in-ground craft that can't sustain flight outside ground effect falls only under the maritime regime; if it can climb away and fly, aviation rules apply too. The international collision rules at sea were amended to include them, in force since 2003. Fly low enough and you're a ship.
How did one ship shut the Suez Canal for six days?
By being longer than the canal is wide. On 23 March 2021 the Ever Given, a 400-metre container ship with room for 20,124 TEU, lost the ability to steer in high winds, according to the canal authority, and ran aground diagonally across the channel. The Suez Canal's width at 11 metres of depth is 205 to 225 metres. A ship nearly twice that long, turned sideways, wedges bank to bank.
Freeing her on 29 March took about 30,000 cubic metres of dredged sand, a fleet of tugs and the help of a high spring tide. By then about 370 ships were waiting, and the canal was losing an estimated $14–15 million in fees a day. Wind was the first explanation offered, and the ship's technical managers said early checks ruled out mechanical or engine failure.
Narrow, shallow water changes how a ship behaves. Water squeezed between hull and bed speeds up, its pressure drops and the ship squats lower; the effect grows roughly with the square of speed and is felt more close to a bank. A canal is the one place where a 400-metre ship is short of room in every direction at once.
The deeper story is scale. The Ideal X carried the equivalent of just over 100 TEU; the Ever Given carries about two hundred times as much. And according to the canal authority's own table, when the canal opened in 1869 it was 8 metres deep, with a cross-section of 304 square metres, and could take loaded ships of 5,000 deadweight tonnes. By 2010, and still after the 2015 expansion, it was 24 metres deep, with a cross-section of 4,800 to 5,200 square metres, taking loaded ships of 240,000 tonnes. The ships it admits grew 48-fold while the channel's cross-section grew about 16-fold. Everything in the short tail pushes ships to be longer and fuller, and that pushes them toward the edges of the channels they depend on.
The blockage lasted six days. The lesson came back for longer: with navigation in the Red Sea uncertain, Suez transits were about 70 per cent below 2023 levels as of May 2025, and ships sailing the long way around Africa are largely why 2024's tonne-miles grew nearly three times as fast as its tonnes.
What the edges say about the middle
Every strange case in the long tail is one of the short tail's two facts pushed until it shows. Buoyancy holds weight up for free: the Falkirk Wheel uses it as a perfect balance, Panama shows what it costs when the route goes uphill, and hovercraft and ekranoplans show what you pay once you give it up. Moving water costs more the faster you go: hull speed explains why big ships are long and slow, and the Ever Given shows what happens when long meets narrow.
Put those together and you get the container ship: as long as the canals allow, as slow as the schedule tolerates, loaded as deep as the paint on its side permits. The horse on the towpath was working to the same rules.
Links in this essay
- almost fifty times as much weight canalrivertrust.org.uk
- thirty times as much measuringemissionsguide.environment.govt.nz
- over 80 per cent of the world's merchandise trade media.un.org
- 42.7 per cent of the world fleet's capacity and oil tankers 28.3 per cent safety4sea.com
- still made up 69 per cent unctadstat.unctad.org
- 920 million TEU in 2024 unctadstat.unctad.org
- counts port handlings rather than journeys data.worldbank.org
- about 4.27 billion passengers and 373 million vehicles a year imo.org
- 12,720 million tonnes iims.org.uk
- eight billion-plus people un.org
- the volume of a ship's enclosed spaces en.wikipedia.org
- "220,000-tonne vessel" aljazeera.com
- gross tonnage of 220,940 en.wikipedia.org
- Archimedes' principle grc.nasa.gov
- 1,025 kilograms per cubic metre and fresh water 1,000 en.wikipedia.org
- fresh-water and tropical fresh-water lines heritage.lrfoundation.org.uk
- "tropical fresh water" pancanal.com
- "coffin ships" rmg.co.uk
- in 1930 and 1966 imo.org
- Intact Stability Code imo.org
- "stiff" en.wikipedia.org
- with her bow doors open api.parliament.uk
- less than two minutes assets.digital.cabinet-office.gov.uk
- 193 people died gov.uk
- roughly 3.4 to 7.4 kilograms of drag for every tonne on the axle adv-vehicle-sim.sourceforge.net
- an 8,000-TEU container ship transportgeography.org
- 58 metal containers porthouston.com
- $5.83 a ton; loading containers would cost under $0.16 transportgeography.org
- agreed base dimensions and a latching system through ISO 668 transportgeography.org
- in 1968 en.wikipedia.org
- a standard of their own bic-code.org
- verified gross mass imo.org
- International Maritime Organization imo.org
- adopted in Geneva in 1948 and came into force in 1958 imo.org
- 176 member states imo.org
- first adopted in 1914 in response to the Titanic imo.org
- 1760, customers of Edward Lloyd's coffee house heritage.lrfoundation.org.uk
- more than 90 per cent of the world's cargo-carrying tonnage iacs.org.uk
- 52 million gallons of fresh water per transit pancanal.com
- compares water to pools pancanal.com
- 36 transits a day pancanal.com
- the driest since records began in 1950 pancanal.com
- a 44-foot draft pancanal.com
- 427 metres long and 55 metres wide, with basins that reuse up to 60 per cent of the water pancanal.com
- announced it would cut daily slots again pancanal.com
- a particularly intense El Niño that continues to strengthen pancanal.com
- a $1.6 billion project pancanal.com
- Falkirk Wheel scottishcanals.co.uk
- hull speed en.wikipedia.org
- bulbous bow en.wikipedia.org
- flexible skirt keeps in hovercraft-museum.org
- Hovertravel hovertravel.co.uk
- high-speed craft imo.org
- induced drag is cut in half aerospaceweb.org
- six missile launchers rferl.org
- only under the maritime regime imo.org
- 205 to 225 metres suezcanal.gov.eg
- the help of a high spring tide science.nasa.gov
- squats lower en.wikipedia.org