Module 1 - Nautical Terms
Hull and Below the Waterline
What you will learn
Explain hull, waterline, draught, freeboard, keel and rudder, and identify which measurement constrains entry into shallow water.
The hull carries the vessel, while draught describes how far its lowest point extends below the waterline and freeboard describes height above it.
The hull is the body of the boat. The waterline is where the hull meets the water surface. The draught is the depth of the hull below the waterline — critical for knowing what depth of water you need.
A sailing yacht's keel may be a long keel, fin keel or another arrangement. It provides lateral resistance and often carries ballast for stability. The rudder is an underwater steering surface, commonly near the stern, controlled through a tiller or wheel.
Check whether the yacht can use a shallow berth
A berth is shown with limited depth. Before approaching, compare the yacht's current draught with the expected water depth and the safety margin required for the conditions.
1. Find draught
Use the vessel's current loaded draught, not a remembered figure for an empty boat.
2. Find water depth
Account for chart datum, predicted tidal height and any local or observed differences.
3. Keep a margin
Allow under-keel clearance for uncertainty, waves, squat and the consequences of touching bottom.
Sense check: Available depth minus draught is only a starting clearance; it is not automatically a safe margin.
Common mistake or limitation
- Draught is not freeboard: draught is below the waterline, while freeboard is above it.
- A catalogue draught can change with loading and heel, and keel construction varies between vessels; use the actual boat's data and conditions.
Recap
- Draught is the vertical distance from the waterline to the vessel's lowest point.
- Freeboard is the height from the waterline to the deck edge or gunwale.
- Keel and rudder have different functions: the keel resists sideways motion and contributes to stability; the rudder steers when water flows across it.
Optional quick check
Section 3 of 8
Which measurement must be compared with available depth before entering shallow water?
Continue studying Nautical Terms
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