Definition
The draft (or draught) of a hull is the vertical distance measured from the waterline to the lowest point of a vessel’s hull, typically the keel. It indicates how deep the ship sits in the water and determines the minimum depth of water required for safe navigation.
Overview
The draft is a fundamental parameter in naval architecture and marine operations. It varies with the vessel’s loading condition, distribution of weight (trim), and the density of the water (freshwater versus seawater). A ship’s design draft influences its suitability for particular ports, channels, and routes, as well as its stability and hydrodynamic performance. Draft readings are routinely recorded at the bow and stern to assess trim, and they are essential for compliance with international load‑line regulations, which prescribe maximum permissible drafts for safety.
Etymology/Origin
The term “draft” (also spelled “draught” in British English) derives from the Old English drægan, meaning “to draw.” In Middle English, draught came to denote “the act of pulling or drawing,” later extending to the depth of water needed to “draw” a vessel through it. The spelling “draft” has become standard in American English, while “draught” remains common in many Commonwealth nations.
Characteristics
| Aspect | Description |
|---|---|
| Measurement Units | Typically expressed in meters (m) or feet (ft). |
| Static Draft | The draft measured when the vessel is at rest, fully loaded, and in calm water. |
| Dynamic Draft | Changes in draft caused by vessel motion, waves, or variations in water density. |
| Trim | Difference between forward (bow) and aft (stern) drafts; affects handling and fuel efficiency. |
| Freeboard | The vertical distance from the waterline to the deck edge; inversely related to draft. |
| Load Line (Plimsoll Mark) | A marked line on the hull indicating the maximum permissible draft under specified conditions. |
| Influencing Factors | Cargo weight, ballast water, fuel consumption, freshwater vs. seawater density, hull fouling, and structural modifications. |
Related Topics
- Buoyancy – The upward force that counters the weight of the ship, directly related to displacement and draft.
- Displacement – The weight of the volume of water displaced by the hull; equal to the vessel’s weight at a given draft.
- Freeboard – The distance from the waterline to the top deck, used in safety assessments alongside draft.
- Load Line (Plimsoll Mark) – Internationally regulated markings indicating maximum safe drafts.
- Trim – Longitudinal inclination of the vessel, defined by the difference between bow and stern drafts.
- Ballast – Water or other material added to adjust draft and improve stability.
- Ship Stability – The vessel’s ability to maintain or regain an upright position, influenced by draft and center of gravity.
- Naval Architecture – The engineering discipline concerned with designing hull forms and predicting draft characteristics.