The Falkirk Wheel

The boat lift that runs on about eight kettles of energy.

2 · The question

The Falkirk Wheel lifts boats 24 metres between two canals. Why does it need so little energy?

3 · Play first

Load boats into either gondola, then turn the wheel. Can you make it unbalanced?

Gondola A

Gondola B

What if the gondolas were sealed?

Both gondolas weigh the same, so the wheel is balanced. Add boats, then turn it.

The heavy side is already at the bottom: load the top gondola to watch it go.

4 · What's happening

1
Each end of the wheel has a gondola, a big tank full of canal water. A boat floats in through a gate.
2
A floating boat pushes aside exactly its own weight in water, and that water spills back into the canal. So the gondola weighs the same as before.
3
Both gondolas always carry about 500 tonnes, boat or no boat. The wheel just swaps two equal weights.
4
So the motors only fight friction. A half turn takes about five minutes and about 1.5 kWh, roughly enough to boil eight kettles.

5 · The maths

Archimedes' principle: a floating object pushes aside water equal to its own weight. So whatever boats come in, the gondola's total stays the same.

m_{\text{water left}} + m_{\text{boats}} = 500\ \text{t}

8 \times \big(1.7\ \text{kg} \times 4186\ \tfrac{\text{J}}{\text{kg}\,^\circ\text{C}} \times 90\ ^\circ\text{C}\big) \approx 1.4\ \text{kWh} \approx 1.5\ \text{kWh}

500 t water + 0 t boats=500 t

Gondola A, with the boats you loaded above.

6 · Under the hood

Inside the real wheel (for grown-ups)

Two canals, two heights

Since 2002 it has joined the Forth & Clyde Canal and the Union Canal in Scotland. The wheel lifts boats 24 m; two locks do the rest of the 35 m climb.

Its size

The wheel is 35 m across, and a half turn takes about five minutes.

Staying level

As the arms turn, gears keep each gondola upright, so the water and boats never tip.

Not perpetual motion

It does need energy, about 1.5 kWh per half turn, to beat friction and drive losses. It's just very little for what it lifts, because it never lifts the boats' weight against an empty side.

7 · Try it yourself

Weigh a floating boat

Fill a bowl to the brim, stand it on a kitchen scale, and note the reading. Float a toy boat in it and let the overflow run off (catch it in a tray, not on the scale). The reading comes back the same. Tap each time you try a different boat.

Build it

A tabletop Falkirk Wheel kit is on the list: two water gondolas you can load with little boats and turn by hand.

Tell me when it's ready

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