The crane’s innovation was in the use of a curved jib, made of riveted wrought iron platework to form a square-section box girder. This curved jib could reach further into the hold of a ship, clear of the deep gunwales alongside the quay.
Designing a strong curved jib required Fairbairn’s advanced theoretical understanding of the mechanics of a box girder. The tension forces were carried by the outer, convex surface of the girder which was made of back plates being chain-riveted together. The inner surface carried a compressive load. To avoid plate crumpling, it was made as a cellular structure: an inner plate and webs formed three rectangular cells, effectively box girders in their own right. The character of a box girder is to resist torsional twisting, so a composite face built up of them is also good at resisting crumpling.
The first of these cranes were a batch of six built for the Admiralty at Keyham and Devonport. These were hand-operated and could lift 12 tons to a height of 30 feet (9.1 m) and a radius of 32 feet (9.8 m) .The size of the crane jibs was determined by ships of the period, and their lifting capacity by men’s ability to raise the load. Experiments at Keyham with loads of up to 20 tons showed the jib design to be sound, and that the jib at least was capable of handling loads of up to 60 tons.