wunder beta🛶 How a canal lock lifts a boat uphill
Nothing touches the hull. A lock just changes the height of the pond the boat is sitting in, and gravity does the rest. But every boat that goes up sends 123,000 litres down the hill — and somebody has to put it back.
What you’ll learn
- One lock, one cycleFollow a single boat through one chamber and name each move that gets it there.A lock is a walled-off piece of canal whose water level is changed by gravity through small sluices, so a floating boat is carried to the next level without ever being lifted.
- Gates the water shuts for youExplain why lock gates are shaped the way they are, and what they cost to keep.Mitre gates close as a V into the higher water so pressure seals them, which is also why the chamber must be equalised before they will move — and even so, a gate is a consumable 72-ton dam.
- A staircase up a hillDescribe how repeated chambers carry a canal up a slope, using Caen Hill and Fonseranes.One chamber is one step, so a hill is climbed by repetition: 29 locks and 237 feet at Caen Hill, and eight touching chambers over 21.18 metres at Fonseranes.
- Where the water comes fromAccount for the water a lock spends and trace where it is replaced from.Roughly one chamberful runs downhill per passage and never returns, so summits are kept alive by reservoirs, feeder streams and, where there is nothing to gravity-feed, pumps.
- Big locks, and the people who keep themConnect the small chamber to ship-sized locks and to the maintenance that keeps both working.A Panama lock is the same box of water with heavier hardware, and both ends of the scale depend on hand-measured gates, published operating rules and someone in charge of the chamber.
Questions this course answers
What actually raises the boat inside a lock?
The chamber is filled until it matches the higher canal, and buoyancy carries the boat up with the surface. Nothing touches the hull.
Why must the levels be equalised before a main gate opens?
A closed gate is holding back a difference in water height. The paddles take that difference away first, so the gate can then be moved with very little force.
Why do mitre gates close as a shallow V pointing upstream?
The Canal & River Trust puts it directly: the double mitre lock uses the force of the water to keep the gates tightly shut. Pressure on the back of the leaves seals them rather than forcing them open.
What makes a staircase lock different from an ordinary flight of locks?
Fonseranes has eight chambers and nine sets of gates — there is no separate stretch of canal between the steps, so emptying one chamber fills the next.
Roughly how much water leaves the upper pound each time a boat passes a lock?
Whether the boat is climbing or descending, the chamber ends up needing to be at the other level for the next passage. The Trust records over 4,000 lockfuls — some 600 million litres — passing down Watford Locks in a year.
How much water does a single lock at Caen Hill use each time it fills?
The Canal & River Trust gives approximately 123,000 litres per fill, and notes that each lock there fills and empties over 2,000 times a year.
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