Air draft facts for kids

The deck of this lift bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide enough space for the vessel to pass through.

Air draft (or air draught) is the distance from the surface of the water to the highest point on a vessel (like a ship or a boat). Think of it as the height of the ship above the water. Knowing a ship's air draft is very important for safe travel, especially when passing under bridges or power lines.

What is Air Draft?

Air draft is simply how tall a ship is from the water up. Every ship, from small boats to giant cargo vessels, has an air draft. This measurement helps captains know if their ship can fit under things like bridges or overhead cables without hitting them. It's a key piece of information for planning a safe journey.

Why is Air Draft Important for Ships?

Knowing the air draft is crucial for several reasons. It helps prevent accidents and makes sure ships can travel safely on different waterways.

Navigating Under Bridges

One of the main reasons air draft matters is because of bridges. Many bridges are built over rivers or canals where ships travel. If a ship's air draft is taller than the space under a bridge, it won't be able to pass. This is why some bridges, like the Allanburg Bridge in the picture, are designed to lift up or swing open. They let very tall ships go through safely.

Planning Safe Routes

Ship captains and navigators use air draft information to plan their routes. They check charts and maps that show the height of bridges and other overhead obstacles along a waterway. This way, they can choose a path where their ship will fit. If a route has a bridge that's too low, they might have to take a longer way around or wait for a bridge to open.

How is Air Draft Measured?

Air draft is measured from the water's surface to the very highest point of the ship. This highest point could be the top of a mast, an antenna, a radar dome, or even the ship's funnel (smokestack).

What Affects Air Draft?

The air draft of a ship can actually change slightly. Here's how:

• How much cargo a ship carries: When a ship loads a lot of heavy cargo, it sinks lower into the water. This means the water level gets closer to the ship's highest point, making its air draft seem smaller.
• Water level: The height of the water itself can change due to tides or floods. If the water level rises, the space under a bridge shrinks, and the ship's air draft effectively becomes larger in relation to the bridge.

Understanding air draft helps ensure that ships can travel safely and efficiently around the world, avoiding costly and dangerous collisions.

Images for kids

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  Bridge of the Americas

See also

In Spanish: Calado aéreo para niños