Draft and depth are both vertical ship measurements, but they measure fundamentally different things — one changes constantly, the other never does.
Draft: Changes With Loading
Draft is the distance from the waterline down to the bottom of the keel — essentially, how much of the ship is submerged. Draft changes continuously as cargo, fuel, and ballast are loaded or consumed — a vessel’s draft increases as it’s loaded and decreases as cargo is discharged or fuel is burned.
Depth: A Fixed Structural Dimension
Depth is the fixed vertical distance from the keel to the main deck — a structural dimension of the hull itself, specified in the ship’s design and construction, and identical whether the vessel is empty or fully loaded. Depth never changes; it’s a permanent characteristic of that specific vessel, like length or beam.
Why the Confusion Matters Practically
- Draft directly determines under-keel clearance and whether a vessel can safely enter a given depth of water — a real, safety-critical figure checked before every passage through shallow water
- Depth relates to freeboard (depth minus draft equals freeboard, roughly) and the vessel’s overall structural size, relevant to stability and load line calculations, not directly to available water clearance
Confusing “depth” (a fixed hull dimension) with “draft” (a variable, loading-dependent figure) in a stability or under-keel clearance calculation would produce a genuinely wrong, potentially dangerous result.
Quick Way to Keep Them Straight
- Draft — “how deep is the ship in the water” — changes with loading
- Depth — “how tall is the ship’s hull, keel to deck” — fixed, structural
Exam Relevance
Correctly distinguishing draft from depth is a foundational Ship Stability and Ship Knowledge topic tested in deck officer competency exams — precisely because the confusion is common enough among students that examiners specifically check for it.