Draft: Distance Between the Waterline and the Bottom of a Ship

Draft is the vertical distance between the water surface and the lowest point of a ship at rest. This does not necessarily mean the ship’s keel: what matters is the lowest point of the particular ship. The term describes a distance on the ship in relation to the water surface. The ship’s being at rest is explicitly part of this measurement reference.

Draft and water depth

Draft is measured from the ship to the water surface. Water depth, by contrast, is measured from the water surface to the bottom of the body of water. The two distances meet at the water surface, but have different endpoints and describe different things: draft is a measurement of a ship, while water depth describes the waterway. A draft value is therefore not also the water depth of a harbor or channel.

To compare them, the reference points must be clear in each case. On the waterway side, water depth and water level are different reference measurements. Draft itself refers to the stationary hull and ends at the ship’s lowest point.

Draft, freeboard, and depth of side

Assigned freeboard is another measurement, located above the water. The International Convention on Load Lines defines it as the vertical distance amidships from the upper edge of the deck line to the upper edge of the corresponding load line. It is therefore not measured from the water to the bottom of the hull. Freeboard and load lines are determined by precisely specified reference lines and markings on the hull.

Moulded depth must also be distinguished from these measurements. Under the Load Line Convention, moulded depth is measured vertically from the upper edge of the keel to the upper edge of the beam of the freeboard deck at the ship’s side. For wooden and composite ships, the definition specifies a different lower reference point at the keel. Moulded depth is therefore a construction measurement between specified points on the hull; it does not start at the water surface and is not the draft.

The same applies to depth of side and other ship measurements: technical specifications can only be compared when their measurement definitions are also known. Draft, moulded depth, and freeboard can all be described as vertical distances, but their starting and ending points differ. The relevant definition determines which distance is meant. The fact that all three are measured in the same general direction does not make them interchangeable.