There are two ways to transport a boat between water at two different elevations. One method is to employ locks, often found on river and canal waterways. The other method is to physically lift the boat from one waterway and place it on to another. Boat lifts are not very common, and the Falkirk Wheel, near the town of Falkirk, is the only rotating boat lift of its kind in the world, and is regarded as an engineering landmark for Scotland.
The 35 meters tall Falkirk Wheel connects the Forth and Clyde Canal with the Union Canal, which were previously connected by a series of 11 locks that stepped down across a distance of 1.5km. But these has been dismantled in 1933, breaking the link. It was decided that a boat lift was necessary to connect the two waterways, but British Waterways were not keen on just any boat lift. They decided to create a dramatic 21st century landmark structure. The result - a perfectly balanced structure that is The Falkirk Wheel - the world's first and only rotating boat lift.
Boats enter one of the Wheel's gondolas and are lowered or raised, along with the water that they float in, to the basin below or above. This works on the Archimedes principle of displacement. That is, the mass of the boat sailing into the gondola will displace an exactly proportional volume of water so that the final combination of 'boat plus water' balances the original total mass. The equal weight allows the gondolas to remain perfectly balanced, so a very small amount of energy is actually required to turn the Wheel. The wheel rotates through 180° in five and a half minutes while using just 1.5kw, of electricity, the same amount as it would take to boil eight household kettles of water.
A similar design of boat lift has been suggested for a proposed new canal that would run along Marston Vale in Bedfordshire. The lift would link the Grand Union Canal at Milton Keynes with the River Great Ouse at Bedford. (Also see The Incredible Magdeburg Water Bridge in Germany)
Here is a video of the Falkirk Wheel in operation.
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