Draught / Draft

What is draught?

Draught (draft) is the vertical distance from the waterline down to the lowest point of the keel. It is how much water a ship needs to float, and it sets which channels, berths and ports she can enter, expressed in metres.

Draught is the single dimension that decides whether a loaded bulk carrier can physically reach a berth, which is why it sits at the centre of every voyage and port-rotation calculation. It is not a fixed property of the ship the way length or beam are. Draught grows as cargo, bunkers and water come aboard and the hull sinks deeper, and it falls as the ship is discharged or lightened. The classification societies that survey and certify the fleet, Lloyd’s Register, DNV and ABS, record the summer and scantling draughts in the ship’s load-line and stability documentation, and the International Maritime Organization sets the load-line framework under which the maximum draught is assigned.

Because draught moves with loading, the figure quoted on a vessel’s particulars is the summer draught: the draught at the summer load line, the deepest she may legally float in salt water under normal conditions. Operationally, though, the master and the desk care about the draught the ship is actually at on a given day, because that is what must clear the depth of water under the keel at every point in the rotation. The rest of this page works through how draught is measured and read off the hull, how it differs from the adjacent measures it is routinely confused with, and how it scales across the mainstream bulk carrier classes.

How draught is measured and read

Draught is read directly off scales painted on the hull at the bow, stern and amidships. The terms a charterer and master use to describe and combine those readings are set out below.

Term	Definition	How read	Reference
Draught marks	Numbered scales painted at the bow, stern and amidships on each side	Read where the waterline cuts the scale; figures give the draught at that point in metres	Class load-line documentation (Lloyd's Register, DNV, ABS)
Forward (fore) draught	Draught read at the bow marks	Average of the two forward marks, port and starboard	Draught survey practice
Aft draught	Draught read at the stern marks	Average of the two aft marks, port and starboard	Draught survey practice
Amidships draught	Draught read at the midship marks	Used to correct for hull deflection (hog or sag)	Draught survey practice
Mean draught	The representative draught of the whole hull	Derived from fore, aft and amidships readings, corrected for trim and deflection	UN ECE draught survey procedure
Trim	The difference between forward and aft draught	Aft draught minus forward draught; positive is trim by the stern	Stability and loading practice
Even keel	Forward and aft draughts equal, zero trim	Forward draught equals aft draught	Stability and loading practice
Air draught	Height from the waterline to the highest fixed point (defined here for contrast, not part of draught)	Vessel particulars; varies with loading inversely to draught	Class / vessel particulars

Reading the marks is the foundation of a draught survey. The surveyor reads all six sets of marks (forward, amidships and aft, on each side), averages each pair, and corrects the result for trim and for any hog or sag in the hull. The corrected figure is the mean draught, which is what enters the hydrostatic tables to give displacement. Trim is the longitudinal attitude of the ship: a vessel trimmed by the stern sits deeper aft than forward, which is the normal loaded condition for a bulk carrier, while even keel means the forward and aft draughts are equal. Units are metres throughout the modern fleet, though some older marks and some US-built tonnage also carry draughts in feet.

Draught vs air draught vs freeboard

Draught, air draught and freeboard are three vertical measurements taken from the same waterline, and they are routinely confused because all three change together as the ship loads. They measure different things and bind different constraints.

Draught measures down to the keel and answers whether there is enough water beneath the ship. Air draught measures up to the highest fixed point of the structure (the masthead, radar scanner or, on a geared ship, the crane tips) and answers whether the ship will pass under a bridge, a loading gantry or an overhead power line. Because the ship sinks as she loads, draught and air draught move in opposite directions: a fully laden bulk carrier has her deepest draught and her lowest air draught at the same moment. Freeboard is the height of the freeboard deck above the waterline, the slice of hull standing clear of the sea, and it is the inverse of draught: the deeper the draught, the smaller the freeboard. The minimum permitted freeboard is what the load line fixes, which is the same thing as fixing the maximum draught. For the full set of principal dimensions and how they relate, see vessel dimensions.

Where draught is measured on the hull

Draught is not read from a single point. It is read from scales at both ends of the ship and corrected to a representative figure, and the scales are referenced to the keel.

Image Placeholder Bulk carrier bow and stern showing the draught-mark scales, the waterline and the keel datum Profile of a loaded bulk carrier with numbered draught-mark scales painted vertically at the bow and at the stern. The waterline crosses each scale to give the forward and aft draughts, and the figures are measured down to the keel datum, the lowest point of the hull. The vessel sits slightly deeper aft, the normal trim-by-the-stern condition for a laden bulk carrier.

The marks are placed close to the perpendiculars, near the bow and the stern, and on larger vessels a third set is painted amidships. Each scale runs vertically up the hull plating in metres, and the draught is read off where the surface of the water cuts the scale. The datum for the figures is the keel: a draught of 14.2 metres means the lowest point of the keel sits 14.2 metres below the surface. Because a loaded hull can hog (arch up in the middle) or sag (dip in the middle), the amidships reading is used to correct the mean of the fore and aft draughts so that the displacement read from the hydrostatic tables reflects the true immersed volume rather than a straight-line approximation.

Why draught is the binding port constraint

For a loaded bulk carrier, draught is almost always the limit that decides whether a port call is possible at all. Beam and length matter at the lock and the berth, but it is the depth of water that most often turns a cargo away.

• Channel and berth depth. The charted depth of the approach channel and the depth alongside the berth set a hard ceiling on the draught a ship can arrive at. A terminal that advertises a maximum arrival draught is stating the deepest a ship may be when she transits the channel, not a target.
• Under-keel clearance and squat. Ports require a minimum under-keel clearance (UKC), the gap between the keel and the seabed, as a safety margin. The effective clearance is eroded by squat, the bodily sinkage and trim change a ship experiences when moving through shallow water at speed, so the master must slow down to preserve UKC. Port-authority UKC guidance and the ship’s own squat tables govern the allowable speed and draught in the channel.
• Tidal-window arrivals and departures. Where the maximum draught exceeds what the channel offers at low water, the ship can only transit on a rising or high tide. This produces tidal-window sailing, in which the laden departure is timed to the tide and a missed window costs a full tidal cycle of delay. Tide-restricted iron ore and coal ports run their whole loaded-departure schedule this way.
• The draught survey to determine cargo quantity. Because draught fixes displacement through the hydrostatic tables, the difference in displacement between the light and loaded conditions weighs the cargo. This is the draught survey, the standard method for establishing bulk cargo quantity by displacement when there is no weighbridge, and it is what ties draught directly to the bill of lading figure and to deadweight tonnage.
• Salt versus fresh water sinkage. A ship floats deeper in fresh water than in salt because fresh water is less dense, so the same cargo gives a greater draught at a river berth than at a sea berth. A loadport in brackish or fresh water must allow for the ship rising to her marks as she reaches the open sea, which is the purpose of the fresh-water allowance built into the load line.

How draught varies by bulk carrier class

Draught scales with the size of the hull, which is precisely why the larger classes are shut out of the shallower ports. Each mainstream class occupies a representative summer-draught band that tracks its deadweight.

Reading down the table, summer draught rises with deadweight and steadily closes ports off. A Handysize at roughly 10 metres can enter most of the world’s cargo ports, which is the basis of the class’s flexibility. A Capesize at roughly 18 metres can only call at the small number of deepwater terminals dredged and built for her, which is why iron ore and long-haul coal moves on dedicated lanes between purpose-built ports. The Panamax and Supramax bands cover the middle ground, where draught is a live constraint at some receivers but not a structural exclusion. Treat every band here as representative: published class definitions vary at the edges, and the figure for any specific ship should be checked against her own particulars and the current Clarksons fleet data before use.

Common confusions about draught

Draught is mixed up with several adjacent terms, and getting the distinction wrong changes the answer to a real operational question.

• Draught versus depth. Moulded depth is a fixed structural dimension of the hull, the vertical distance from the keel to the freeboard deck, and it does not change with loading. Draught is the part of that depth that is below the waterline at any given moment, and it varies with how the ship is loaded. A ship’s depth is a number on her drawings; her draught is a number on her marks today.
• Summer draught versus scantling draught. The summer draught is the draught at the summer load line, the everyday maximum. The scantling (design) draught is the deeper draught the hull is structurally built to take, used on routes and at ports where a deeper mark applies. The two are different figures, and a fixture that assumes the scantling draught is achievable everywhere will run into port draught limits.
• Freshwater sinkage. A ship sits deeper in fresh water than in salt water for the same weight, because fresh water is less dense. Loading to the salt-water summer mark while lying in fresh or brackish water overloads the ship once she reaches the sea, which is why the load line carries a separate fresh-water mark above the summer mark.
• Draught versus air draught. These are opposite measurements from the same waterline: draught down to the keel, air draught up to the masthead. They move in opposite directions as the ship loads, and confusing them produces the wrong clearance answer at a bridge or under a loading gantry.

Scope and what this page does not cover

This page explains draught as a vessel attribute: what it means, how it is measured and read off the hull, how it differs from air draught and freeboard, why it is the binding constraint on port access, and how it scales across the mainstream bulk carrier classes. It does not perform or certify a draught survey for a named vessel, interpret a specific load-line certificate, calculate squat or under-keel clearance for an actual transit, or replace the approved hydrostatic tables, stability booklet and port-authority guidance that govern a real arrival. Those are matters for the master, the classification society, a qualified draught surveyor and the port authority, working from the vessel’s own documents and the local charted depths. The figures here are representative, not measurements of any specific ship.