Science Puzzle
How Does a Canal Lock Lift a Boat?
Canals have to be nearly flat, or their water would all run away downhill. So when a canal meets a hill, it climbs in steps called locks. A narrowboat can weigh 15 tonnes or more, yet a lock lifts it a couple of metres without a pump or a crane.
How does a lock lift a boat?
The Answer
It lets water from the upper canal flow down into the lock, and the rising water floats the boat up.
The boat sails into the lock chamber from the lower canal and the bottom gates are shut behind it. Then the crew open small sluices, called paddles, in the top gates. Water from the upper canal pours into the chamber and keeps flowing until the water inside is level with the canal above. The boat rises with it. Once the levels match, there's no longer any water pushing against the top gates, so they swing open easily and the boat carries on at the higher level.
Going down works the same way in reverse: close the top gates, open the paddles in the bottom gates and let the chamber drain into the lower canal. Nothing is pumped either way. Gravity does all the lifting, because water always flows from high to low until both surfaces are level.
There's a cost, though. Every boat that passes through sends a whole lockful of water downhill, roughly 100,000 litres for a typical narrow lock, so canals need reservoirs near their highest point to keep the top topped up. The idea is old. The Chinese engineer Qiao Weiyue built a lock with two sets of gates on the Grand Canal in 984.
The principle: Water finds its own level. Connected water flows from higher to lower until both surfaces are at the same height, so a lock can lift a boat using nothing but gravity and water from the canal above.