Canal & River Locks
Page 2: Basic construction and operation
All locks have three elements:
- A watertight chamber connecting the upper and lower canals, and large enough to enclose one or more boats. The position of the chamber is fixed, but its water level can vary.
- A gate (often a pair of "pointing" half-gates) at either end of the chamber. A gate is opened to allow a boat to enter or leave the chamber; when closed, the gate is watertight.
- A set of lock gear to empty or fill the chamber as required. This is usually a simple valve (traditionally, a flat panel lifted by manually winding a rack and pinion mechanism) which allows water to drain into or out of the chamber; larger locks may use pumps.
The principle of operating a lock is simple. For instance, if a boat travelling downstream finds the lock already full of water:
- The entrance gates are opened and the boat sails in.
- The entrance gates are closed.
- A valve is opened, this lowers the boat by draining water from the chamber.
- The exit gates are opened and the boat sails out.
- If the lock was empty, the boat would have had to wait 5-10 minutes while the lock was filled.
- For a boat travelling upstream, the process is reversed: for instance, the chamber is filled by opening a different valve which allows water to enter the chamber from the upper level.
- The whole operation will usually take between 10 and 20 minutes, depending on the size of the lock, and whether it was originally set "for" the boat.
- Boaters approaching a lock are usually pleased to meet another boat coming towards them, because this boat will have just exited the lock on their level and therefore set the lock in their favour — saving some work and some 5-10 minutes. (This is not true for staircase locks, where it is quicker for boats to go through in convoy.)
A plan and side view of a generic, empty canal lock. A lock chamber separated from the rest of the canal by an upper pair and a lower pair of mitre gates. The gates in each pair close against each other at an 18° angle to approximate an arch against the water pressure on the "upstream" side of the gates when the water level on the "downstream" side is lower.
Page 1 - Introduction & use of locks...Go
Page 2 - Construction and operation (Current page)
Page 3 - Details & Terminology...Go
Page 4 - Variations...Go
Page 5 - Illustrations...Go
Page 6 - History & Development...Go
Page 7 - Use of water...Go
Page 8 - Alternatives...Go