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⚖️ How a canal boat became a scale

Follow a loaded canal boat into a weighlock, where buoyancy, a drained cradle, and a clerk's rod turn a waterline into a cargo toll.

3
lessons
~20 min
to learn
🏛️ History
subject
Adults
level
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What you’ll learn

  1. The boat becomes a measuring instrumentUse buoyancy, displacement, freeboard, and tare weight to explain how a floating boat can indicate its own mass.A loaded hull sinks lower, and calibrated waterline readings separate total boat weight from cargo weight.
  2. Inside a weighlockTrace the operation of a weighlock from displacement, then drainage, to cradle, lever, reading, and refill.Special canal locks isolated a boat and transferred its load from water to a mechanical scale.
  3. From measurement to moneyConnect gauging readings and weighlock results to cargo subtraction, distance, and canal tolls.A physical waterline became a recorded cargo weight and then a charge in a canal's tariff system.

Questions this course answers

Why does a heavier floating canal boat sit lower in the water?

A heavier floating boat needs a larger upward buoyant force, which comes from displacing more water.

What is subtracted from a loaded boat's measured weight to find cargo weight?

The loaded measurement includes both hull and cargo, so the established empty-boat weight is removed.

What happened in a later dry weighlock?

Later weighlocks drained the chamber so the boat's load could pass directly into a cradle connected to a scale.

Why take gauging-rod readings at several points?

Cargo placement can change the boat's trim, so several readings give a fairer estimate of its overall loading.

A boat weighs 25 tons empty and 30 tons loaded. What cargo weight does the toll clerk use?

The cargo is the loaded total minus the empty boat: 30 minus 25 equals 5 tons.

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