Module 1 - Nautical Terms
Hull and Below the Waterline
The hull is the vessel's main body. Draught is the vertical distance from the waterline to the lowest underwater point in the stated loading condition. Freeboard is measured upward from the waterline to a defined deck edge or freeboard deck. Air draught is the height from the waterline to the vessel's highest relevant point.
Loading, fuel, water and crew can alter displacement, trim, draught and freeboard. Heel, waves, squat and uncertainty in predicted or charted water levels can reduce the margin available in shallow water. Draught therefore contributes to an under-keel-clearance decision; it does not by itself state a safe minimum depth.
Keels, centreboards, rudders, propellers, shafts and drives are underwater appendages whose form and lowest point vary between vessels. A keel can contribute lateral resistance and, on many sailing yachts, ballast stability, but not every keel is a universal 'structural backbone'. Learn the actual boat's arrangement and vulnerable points.
- Hull — the body/shell of the boat
- Waterline — where the hull meets the water
- Draught — depth of hull below waterline
- Keel — underwater structure whose form and function vary by vessel
- Rudder — underwater steering surface; arrangement varies
- Freeboard — waterline-to-deck height under a stated convention
- LOA — Length Overall of the vessel
| Term | What it measures | Decision it informs |
|---|---|---|
| Draught | Waterline to the lowest underwater point in a stated condition | Under-keel clearance |
| Freeboard | Waterline to a defined deck edge or freeboard deck | Loading, reserve height and exposure |
| Air draught | Waterline to the highest relevant point | Bridge, cable and overhead clearance |
| LOA / beam | Overall length / maximum breadth under the stated convention | Berth, lock and manoeuvring space |
Optional quick check
Section 3 of 9
A yacht draws 2.0 m and predicted water depth is 2.4 m. What is the sound conclusion?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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