How canal locks work.

In short. A lock is a sealed section of canal with gates at both ends. You close the gates, let water in or out through paddles until the level inside matches the level you are heading for, then open the far gates and cruise on. It is a water lift with no moving machinery beyond gates and sluices, and the design has barely changed in five hundred years.

What a canal lock actually is

The Canal & River Trust puts it plainly: a lock is a stretch of canal blocked off at each end by solid gates, opened or closed to let water fill or drain from the chamber between them. That chamber is the lock. The stretch of canal between one lock and the next is a pound, and the water in it sits at one level.

Canals cannot run uphill, and Britain is not flat. Locks are how a canal climbs a hill in steps, each step the height of one chamber.

How a lock works, step by step

The Trust describes the sequence as three moves: the boat moves in, the gates close and the chamber fills, and once the water is level the far gates open and the boat moves out. In practice, going uphill:

• The chamber is emptied to the level of the low side, and the bottom gates opened.
• The boat goes in and the bottom gates are closed behind it.
• The paddles at the top end are wound open. Water flows in through the sluices and the boat rises with it.
• When the level inside matches the level above, the top gates can be opened and the boat cruises out.

Going downhill is the same in reverse: fill the chamber to the high side, take the boat in, close up behind, then wind the bottom paddles and let the water out. Nothing pumps. Every lock operation moves a chamber of water downhill by gravity, which is why the network needs reservoirs feeding the summits.

The parts, in the order you meet them

• Gates. Usually mitred — two leaves meeting at an angle that points upstream, so water pressure pushes them shut. Some locks have a single gate at the top end.
• Paddles. The sliding covers you wind up with a windlass. They uncover the sluices.
• Sluices. The openings water actually flows through, in the gate or in a culvert around it.
• The chamber. The lock itself, sized to the boats the canal was built for — on a narrow canal, about 7ft wide and 72ft long.
• The cill. A ledge at the top-gate end, marked on the lock side. Going down, a boat that drifts back onto the cill can catch its stern and tip. It is the one genuine hazard in ordinary lock work.

Pound locks, staircases and flights

• Pound lock. The standard type described above — a chamber with gates at both ends. It replaced the older flash lock, which was little more than a removable barrier that released a "flash" of water for a boat to ride.
• Lock flight. Several locks in a row, each with its own pound between them. You work them one at a time.
• Staircase. Locks sharing gates, so the top gate of one is the bottom gate of the next, with no pound between. They must be worked in the right order and are usually signed, sometimes staffed in summer.

The mitre gate is not a modern invention. The Trust credits the double mitre lock to Leonardo da Vinci in the 15th century, and notes the first lock with gates at both ends was built in Holland in 1373. The gates you wind open on a Tuesday afternoon are a Renaissance design.

How long does a lock take?

The Inland Waterways Association's planning rule is three miles an hour plus ten minutes for each lock — six locks an hour. That is the number to plan a day around, and it is why boaters count locks rather than miles. A fifteen-mile day with two locks is a gentle cruise; a six-mile day through a flight of eighteen is a day's work.

With a good crew, locks close together and nobody coming the other way, it is quicker. Single-handed, or queueing at a popular flight in August, considerably slower.

Can you work a lock on your own?

Yes, and plenty do. Single-handing means securing the boat while you work the gates and paddles, which is why solo boaters develop a routine and a long centre line. It is slower and it is harder in wind. The Trust's Boaters' Handbook is the standard reference for doing it safely, and worth reading before your first flight rather than during it.

Locks are free to use and, on most of the network, self-operated — you work them yourself with a windlass. Some heavily-used or complex sites have lock keepers on duty in season, and staircases sometimes do too.

Are canal locks still used?

Constantly. The freight traffic the canals were built for has gone, but the locks are worked every day by boaters, and the navigation authority maintains them as a live network rather than a museum.

Lock gates last about 25 years

This is the detail that surprises people. Lock gates are wooden and sit in water permanently, so they wear out and are replaced on a cycle — the Canal & River Trust puts the working life of a gate at around 25 years. Winter stoppages are largely gate replacement, done in the closed season with the pound drained.

Which means the flight you learned to boat on, or the gates you were photographed leaning against, are on a clock. The balance beam somebody's shoulder has pushed a hundred times gets lifted out and burnt eventually, and a new one goes in.

That is the honest reason people frame a lock rather than a boat: it is a specific place on a specific day, and it will not look exactly like that indefinitely. Send us the photograph and we will make it as a print.

Where to go next

• What to take on a canal boat holiday — the practical list, including the lock kit people forget.
• Life on a narrowboat — how the locks fit into the rhythm of a year.
• Buying a narrowboat — if the flight has given you ideas.

Written by Craig Fearn, Narrowboat Gifts.