The Scottish tourist office has got another image to put into its montage of Caledonian wonders: the Falkirk Wheel, a spectacular rotating boat lift. And as well as being a marvel of engineering and a work of art, it links the country's two greatest cities
A massive steel and concrete structure that looks strangely like the skeleton of a giant bird is the unique form of Scotland's mechanical marvel – the world's first rotating boat lift. RMJM's 35 m high Falkirk Wheel is the almost-complete link between the Forth & Clyde and Union canals, and it marks the halfway point between Edinburgh and Glasgow. Due to open on the 24 May, the 1800-tonne structure dominates the countryside around the small town of Falkirk in the Scottish lowlands.

Designed to replace a flight of lock gates built in the 19th century, the wheel is the showpiece of the £78m Millennium Link project to restore the two long-derelict canals. The painted steel wheel will carry boats over the 25 m height difference between the canals in four minutes using less electricity than it takes to power an average saloon car (95 kW) – thus creating an imaginative solution to a long pondered problem for civil engineers working the waterways. Tony Kettle, the project's lead architect at RMJM, says he wanted the link between the canals to be an iconic, celebratory structure. "I thought of the canals as a spine linking Edinburgh and Glasgow with the wheel forming a sort of fish-like skull that celebrates the connection between the two cities," he says, varying the bird imagery.

Two boats will travel in two 35 m long water-filled "gondolas" mounted on the ends of a propeller-shaped rotating arm. Balanced water levels in the gondolas help stabilise the structure, while a system of synchronised hydraulic motors and gears housed in the box-girder arm allows the gondolas to remain level while the arm rotates through 180°.

Boats approaching from the higher Union Canal navigate an 80 m long concrete aqueduct, which passes like a thread through the eyes of a series of five giant concrete needles. Kettle says: "I wanted the aqueduct to look like it was floating and therefore emphasise its linear form – as though it were saying, 'Look, no hands'." When the boats reach the lower canal level, the gondola's gate is released, again using hydraulic technology, and the boat is free to float into the large basin of the Forth & Clyde.

The idea for a link to reconnect the two central Scottish canals was conceived more than five years ago by public agency British Waterways. But funding problems, followed by the wheel's redesign post tender, mean that its completion will come two years later than originally planned. After joint venture contractor Morrison-Bachy-Soletanche won the contract in 1999, it and British Waterways decided to make more of the opportunity to create a new tourist attraction. RMJM was hired to help come up with ideas, and Kettle came up with a concept for a rotating bridge using a Lego model to demonstrate the mechanism to British Waterways chairman George Greener. He was delighted by the idea and Ripon-based steel specialist Butterley Engineering was soon put to work refining the concept and building the structure in its factory.

Using what Kettle describes as "layers" of images taken from Celtic design, maritime objects, oil industry pumps and fish skeletons, the wheel's team has completed an object that is both machine and modern art.

How the wheel turns

After the arms of the Falkirk Wheel scoop up a canal boat and a fair dollop of the canal, they will rotate through 180° on their 3.5 m diameter axle, using 95 kW of power – about the amount needed to boil 12 domestic kettles. This is possible, of course, because the arm is counterweighted and geared. The system is driven by one hydraulic power pack that pumps oil through 10 hydraulic motors, each the size of a telephone. The motors use synchronised gears to turn the wheel at a controlled pace of one revolution in eight minutes. Julian Bonfield, technical director at Butterley Engineering and the wheel’s chief engineer, says controlling the tension between the gondolas was one of his prime challenges. “The tolerances had to be exactly right and were worked out while the wheel was being manufactured,” he explains.