Module 1 - Nautical Terms
Hull and Below the Waterline
What you will learn
Relate hull dimensions and underwater appendages to clearance, loading and pilotage decisions without treating draught as a complete safety margin.
Dimensions such as draught, freeboard, air draught, length and beam describe different limits. Safe planning depends on the stated condition and an allowance for uncertainty, not on one number alone.
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.
| 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 |
Turn dimensions into a clearance brief
A yacht drawing 2.0 metres approaches a charted shallow patch. The skipper still needs the chart datum, predicted and observed water level, required under-keel margin, sea state, squat and the vessel's loading condition before deciding whether or when to pass.
1. Confirm the boat
Use the current maximum draught and air draught for the loaded vessel, not a remembered brochure figure.
2. Confirm the water and overhead limits
Read the charted depth or clearance, height datum, tide information and applicable notes.
3. Protect a margin
Allow for uncertainty and dynamic effects appropriate to the vessel, speed and conditions.
Sense check: Predicted water depth minus static draught is only the start of an under-keel-clearance assessment, not the finished safety decision.
Common mistake or limitation
- Draught is not the same as the minimum safe water depth. A skipper must protect an appropriate under-keel margin and account for the conditions and uncertainty.
- Freeboard and draught change with loading and trim, while air draught concerns overhead clearance. Substituting one for another can create a serious clearance error.
- Keel and rudder arrangements vary. Do not infer the lowest or most vulnerable point from a generic yacht drawing.
Recap
- Draught, freeboard and air draught are different vertical measurements.
- Loading and dynamic effects can change the margin available.
- Safe clearance uses the vessel's current data, chart and tide information, uncertainty and an appropriate margin.
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 2 September 2026 - Compass Revision editorial review.
Continue studying Nautical Terms
This public lesson is part of Revision Module 1. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.