How Does a Boat Climb a Hill? A Box That Fills and Empties

A lock is a watertight chamber built into a canal where the level changes, with a gate at each end. A boat going uphill enters through the lower gate into a chamber at the lower level, the gate closes behind it, and valves are opened allowing water from the upper level to flow in, which raises the boat with it. When the levels match, the upper gate opens and the boat proceeds. Going downhill reverses the sequence, draining the chamber into the lower level. No pumping is involved, since water flows downhill through the valves under its own weight, and the gates are held shut by the pressure of the higher water, which is why they are built as a shallow V pointing upstream so the pressure wedges them together.

The device has consistent limitations that shape how waterways are built:

• •Every passage sends a chamber full of water downhill, whichever way the boat is going
• •That water must be supplied at the summit, which requires reservoirs and feeders
• •A drought closes a canal, since without water at the top nothing can pass
• •Passage takes time, typically ten to twenty minutes per lock
• •A flight of many locks becomes a serious bottleneck
• •Side ponds and paired locks recover some water and add cost and complexity

The device developed in stages over a long period. Chinese engineers built a double gate arrangement on the Grand Canal in the tenth century, described in contemporary sources, which is the earliest clear pound lock. European canals used flash locks, a single gate released to flush a boat through on a surge of water, which was dangerous and wasteful. The modern arrangement with mitre gates appeared in Italy in the fifteenth century, with Leonardo da Vinci associated with work on the Milan canals. Britain built an extensive network from the mid eighteenth century, which carried the bulk goods of early industrialisation until railways displaced it within a century, and the surviving system is now used almost entirely for leisure.

Operating a lock by hand follows a sequence that has not changed much and that rewards doing in order. Approach slowly and check which way the lock is set, meaning whether the chamber is full or empty relative to where the boat is. If it is set against you, the chamber must be emptied or filled before the near gate will open, which is done by winding the paddles at the far end. Open the near gate, take the boat in, close the gate behind and close its paddles. Open the paddles at the far end gradually rather than all at once, since a rush of water throws a boat about inside the chamber, and keep the boat under control with ropes while the level changes. Open the far gate when the levels match, leave, and close everything behind you, which is both courtesy and water conservation.

Several devices avoid the water cost or the time, and each has been built. An inclined plane hauls boats up a slope on a cradle, either in water or dry, and several operated historically with a few surviving. A boat lift raises a caisson of water vertically, with counterweights or hydraulic rams balancing the load, and because the caisson displaces water equal to the boat's weight the load is the same whether a boat is present or not. A rotating lift built in Scotland in 2002 balances two caissons on a wheel and uses very little energy to turn. Tunnels and aqueducts avoid the level change entirely where the terrain allows, at considerable construction cost, and the great aqueducts of the canal age are among its most impressive structures.

A chamber with a gate at each end fills from above or drains below, raising or lowering the boat using gravity alone, with the gates set as a V so water pressure wedges them shut. Every passage sends a chamber of water downhill, so the summit must be supplied and a drought closes the route. Boat lifts avoid that by raising a caisson whose weight does not change.