Draft, also spelled draught, is the vertical distance between a ship’s waterline and the lowest point of its hull (the keel). It represents how much of the ship sits below the water’s surface.
Why Draft Matters
- Under-keel clearance — determines whether a vessel can safely pass through a channel, harbor, or over a shoal without grounding
- Air draft — the corresponding height above the waterline, critical for passing under bridges and overhead structures
- Cargo operations — draft changes as cargo is loaded or discharged, and draft readings are used to calculate cargo weight via draft surveys
- Port and canal restrictions — many ports and canals (including the Suez and Panama Canal) have maximum draft limits vessels must comply with
Types of Draft Readings
- Forward draft — measured at the bow
- Aft draft — measured at the stern
- Midships draft — measured at the vessel’s midpoint
- Comparing forward and aft draft readings reveals the vessel’s trim (whether it’s sitting deeper at the bow or stern)
How Draft Is Read
Draft marks — numerals painted on the hull at the bow, stern, and midships — allow draft to be read visually from a boat or the quayside. Modern vessels may also have electronic draft gauges.
Draft vs Depth
Draft is not the same as depth (the vertical distance from the keel to the main deck) — draft is specifically the portion below the waterline, while depth is a fixed structural measurement of the hull itself.
Exam Relevance
Draft reading, draft survey calculations, and the relationship between draft, trim, and stability are core topics in Ship Stability and Cargo Work sections of deck officer competency exams.