Explore in 3D · Canal model

Raise the boat in a canal lock

The land was not flat, so the canal needed locks. A lock helps a boat move between higher water and lower water. Be the lock keeper!

Goes with

The canal lock model

A model, not to scale

The boat waits in the lower canal. The water in the lock is low.

Try this

  1. Take the boat up to the higher canal.

  2. Now bring it back down again.

  3. Try to open a gate when the water is not level. What happens?

What did you notice?

  • Where did the water come from to lift the boat?
  • Where did the water go when the boat went down?
  • Why must the water be level before a gate opens?

Tell a grown-up, or draw it in your workbook.

For grown-ups: what this model shows, and what it changes
  • Not to scale. The boat, mule, house and lock are squeezed close together so they all fit on one page. A real lock chamber is long enough to hold a whole canal boat.
  • Water does the lifting. No pump is used. Opening the upper paddles lets water flow down from the upper canal into the lock, and the boat floats up. Opening the lower paddles lets water flow out to the lower canal, and the boat floats down.
  • Why the order matters. A gate opens only when the water is level on both sides. If one side is higher, the water pushes the gate shut. The model refuses a gate until the levels match, shows arrows for the push, and says why. Only one end can let water through at a time, so water never rushes straight through the lock.
  • How fast the water moves. The water flows through each paddle like water through a hole in a tank: faster when the paddles open wider, and faster when the height difference is bigger. As the levels get close, the flow slows down, and it stops when they are level. The model uses Q = C·a·√(2gh) with C = 0.6 and an assumed paddle opening of 0.5 m² at each end, a real C&O chamber of about 30.5 m × 4.6 m (100 × 15 ft), a lift of 2.4 m (8 ft) and canals 1.8 m (6 ft) deep. Fully open, the lock fills in about 5½ minutes of lock time. Lock time runs 30 times faster than real time, and only while something moves. Real locks also slosh a little; this model leaves that out.
  • The push on a gate. The "water push" meter is the difference in water pressure on the two sides of a closed gate, added up over the whole gate: F = ½ρgw(h₁² − h₂²) for a 4.6 m wide gate. With a full lock it is about 325 kN, roughly the weight of 22 cars (1.5 tonnes each). People cannot push against that, which is why keepers waited until the water was level. The model lets a gate open when the levels are within 3 cm.
  • Calm motion. With calm motion on, nothing moves by itself. Each press of "Wait 1 minute" moves lock time ahead by one minute and shows the new water level.
  • Mitre gates. The two halves of each gate meet in a point that faces the higher water, so the water pressure pushes them tighter.
  • The towline. Two mules walk in tandem, as on the C&O towpath, and stay hitched in this simplified scene. Real C&O crews unhitched the mules and guided boats into the lock with ropes.
  • What the pictures add. The spray, splash rings and ripples are drawn to show where water is moving; the lock level itself comes only from the flow formula above. On tablets and computers the water surfaces are mirrors that reflect the walls, gates and sky.
  • Real history first. Week 17's reading follows mules on a towpath and the lock keepers who worked the locks. Read it first, then use the model to act out a lock keeper's job.