Canals and locks
Carry bulk goods by water where no river goes, and lift boats over hills.
Everything for this step
- Field guide
- Boats and water travel → 11 sections Engineering → 7 sections
- Builds
- None yet.
- Reference sheets
- 7 sheets in Boats and water travel
- Sciences it rests on
- Engineering · Earth sciences
- Track
- On water, level 3 of 6 · next: Seagoing sailing ships
Why this matters
On still water one horse tows about 30 tonnes; on a good road it pulls 1–2 tonnes in a cart, and a packhorse carries about 125 kg. Coal, stone, grain, lime and ore are heavy and cheap, so the cost of moving them decides whether they are worth producing at all: the Bridgewater Canal, opened in 1761, halved the price of coal in Manchester. The pound lock makes a canal possible over any ground a supply of water can reach.
The shape of it
- Survey the level first: a canal is a chain of dead-flat pounds joined by locks.
- Line the bed with puddled clay so it holds water.
- Build pound locks with mitre gates, which the water above presses shut.
- Count the water: every boat over the summit takes a lockful from the top.
- Tow from a towpath kept clear along one bank.
- Guard the locks — sheer, deep chambers drown people.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Survey the level and plan the pounds
A canal does not slope. Each pound — the stretch between two locks — is dead level, and the locks do all the climbing.
- Follow the contour wherever possible: every lock costs water, time and labour.
- Level with water. The Roman chorobates was a table about 6 m long with a water channel cut in its top; where the water stands level, the table is level. With instruments, use a surveyor's level and staff.
- Group the locks into flights where the ground climbs, so one keeper can work them.
- Put the summit where water can reach it — from streams, springs or a reservoir higher still.
2 · Make the bed hold water: puddled clay
Most ground leaks. The 18th-century answer, still used for repairs, is puddle:
- Use a clay sticky enough that a moist lump rolled into a 3 mm thread does not crumble.
- Mix it with water and a little sand, and work it — cut, trodden and kneaded, or trodden by cattle driven along the bed — until it is a dense, even paste with no lumps or air.
- Lay it in layers about 15–20 cm thick, each worked into the one below, to about 60–90 cm in all on the bed and sides.
- Keep it wet. Puddle that dries out cracks and leaks; it spends its life under water.
3 · Build a pound lock with mitre gates
A pound lock is a chamber with a gate at each end. The first were built on China's Grand Canal in 984. The mitre gate — two leaves meeting at an angle that points upstream, drawn by Leonardo da Vinci around 1500 — is what makes a lock tight: the water above pushes the leaves together, so the higher the water, the tighter the seal.
- Chamber: stone or brick in lime mortar, a little longer and wider than the largest boat. The lift of one lock is usually 2–4 m.
- Gates: timber leaves strapped with iron, swung on a heavy balance beam.
- Paddles: small sluices, in the gates or in culverts beside them, opened with a windlass to fill and empty the chamber.
- Level before opening. A gate is never opened against a head of water — let the chamber level with the pound first.
4 · Count the water: feed the summit
Every boat that crosses the summit takes a lockful of water from the top pound: chamber length × width × lift. A lock 22 m × 2.2 m with a 2.5 m lift uses about 120 m³ each time, and evaporation and seepage take more.
- Reservoirs above the summit, fed by streams, carry the canal through summer.
- Side ponds beside a lock store part of each lockful for re-use.
- Pumping water back up needs an engine — a later tier.
Do the sum: lockages a day × water per lockage, plus losses, against what the feeders supply in the driest month.
5 · Tow from a towpath
The towpath runs along one bank, level and firm, clear of trees and posts, and continuous under bridges so the horse need not be unhitched. At walking pace a horse keeps a loaded boat moving all day.
- Fix the tow rope well forward of the middle of the boat, so it tows straight rather than into the bank.
- Slow down long before a lock or a bridge: a loaded boat carries a long way.
- Keep the path clear of mooring ropes and obstacles; the horse cannot step over them in the dark.
6 · Guard the locks: people drown in them
A lock chamber is a deep, sheer-walled box of cold water with moving gates and strong currents from the paddles.
- Ladders or recessed steps in every chamber wall, and a throw line and pole at every lock.
- Nobody opens a paddle while anyone is in the water. If a boat catches on the sill or under a gate beam as the level drops, close the paddles at once and refill the chamber.
- Gate beams are walked on: keep them clear and gritted, with a handrail on long ones.
- Fence the chamber sides where children live nearby.