ship structureDepth is 9.6 metres and aft draught 7.0, yet the display flags insufficient clearance
DRAUGHT LEDGER · STATIC READING TO DYNAMIC CLEARANCE
Draught is a condition, load line an assigned limit, and UKC a passage calculation
The numbers demonstrate reference-consistent arithmetic, not a safety standard. Squat and allowances must come from the vessel, channel and conditions in question.| Item | Value | Basis |
|---|---|---|
| Charted depth | +8.40 m CD | Seabed below chart datum |
| Target tide | +1.20 m CD | Same vertical zero as charted depth |
| Nominal water depth | 9.60 m | 8.40 + 1.20 |
| Static extreme draught | −7.00 m | Lowest fixed point, not mean draught |
| Estimated squat | −0.45 m | Applicable vessel/channel data |
| Heel, pitch and wave allowance | −0.25 m | Planning assumption |
| Depth, tide and survey allowance | −0.30 m | Planning assumption |
| Residual UKC | 1.60 m | Still subject to port and company criteria |
UKC = 8.40 + 1.20 − 7.00 − 0.45 − 0.25 − 0.30 = 1.60 mMean draught 6.60 m describes longitudinal flotation, but shallow-water clearance uses the lowest fixed point and dynamic increases.Read the present waterline at fore, aft or other stations; verify marking standard and units.
An assigned freeboard/loading boundary amidships, differentiated by zone, season and water density.
Draught describes vertical immersion below the present waterline
Draft and draught are the common American and British spellings for the vertical distance from the waterline to a stated bottom reference. FAO small-vessel stability guidance illustrates draught from the underside of the keel to the waterline and relates it to the water depth needed to float freely. Technical documents can further distinguish moulded, design, scantling and extreme draught. Their endpoints may be moulded lines, keel references or the lowest fixed appendage, so a quoted number should retain its original definition.
Draught is a loading condition rather than one immutable construction dimension. Cargo, fuel, fresh water, ballast, people and stores change total weight and centre of gravity, causing the vessel to displace another volume. The same weight also needs a different volume when water density changes. Denser seawater normally supports it at a smaller immersed volume than fresh water. The exact change comes from the vessel's hydrostatics, tonnes per centimetre immersion or freshwater allowance—not a universal number of centimetres.
Forward, aft and mean draught describe longitudinal flotation together
When a vessel trims, its waterline is not parallel to the keel reference and forward draught TF differs from aft draught TA. Illustrative readings of 6.20 metres forward and 7.00 metres aft give trim of 0.80 metre by the stern, with a simple mean of 6.60 metres. That mean helps describe displacement condition, but it does not identify the deepest fixed point or automatically correct for draught marks that lie away from the forward and after perpendiculars.
Large vessels may carry numbers forward, aft and amidships on both sides. A harbor reading is affected by waves, surge, parallax, obstruction and units. Historical U.S. foot marks used the bottom of a six-inch-high numeral for the whole-foot level, with another six-inch space before the next numeral, and estimated intermediate inches by eye; metric systems use different dimensions. Before interpreting a photograph, establish whether marks are metric or imperial and what edge defines the value, then compare port and starboard readings for heel or observation error.
A load line limits permitted immersion rather than measuring current draught
The amidships disc and lines of a load line or Plimsoll mark solve a different problem from forward and aft draught scales. The IMO International Convention on Load Lines assigns freeboard and thereby limits how deeply a ship may be loaded, with reserve buoyancy, hull strength, weathertight integrity and related stability considerations. Assigned marks appear amidships on both sides. They provide legal boundaries, not direct readings of the vessel's present forward or aft draught.
Lines labeled TF, F, T, S, W and WNA distinguish tropical fresh water, fresh water, tropical seawater, summer, winter and winter North Atlantic conditions. Permitted freeboard changes with water density, season and zone, subject to the certificate and vessel particulars. The actual waterline must remain within the applicable assigned boundary, but compliance does not prove that a particular harbor basin, channel or berth is deep enough for that ship.
Loading and water density change static draught through displaced volume
A floating vessel's weight equals the weight of water displaced. Adding cargo increases displacement mass and moves the waterline up the hull; discharging, consuming fuel or removing ballast normally reduces immersion. Tonnes per centimetre immersion states the weight corresponding to one centimetre of mean-draught change near a given waterline. It changes with waterplane area. Dividing cargo mass by a generic TPC is therefore wrong: the value must come from that vessel's hydrostatic data around the relevant draught.
Transition between fresh and salt water also requires measured or accepted density. IMO port-data work places depth, water density, tidal height and vessel draught together because they jointly constrain cargo capacity and UKC. Lower-density water requires more displaced volume for the same mass, increasing draught. A ship legal at departure may experience another waterline and clearance at a destination of different density. A draft survey likewise combines multiple draught readings, density and hydrostatics, plus corrections for trim, consumables and constant weights, to estimate displacement and cargo.
Dynamic draught under way can exceed the still-water reading
At speed in shallow or confined water, altered pressure and wave patterns can produce bodily sinkage and trim known together as squat. UK Maritime and Coastguard Agency guidance calls for manoeuvring information with maximum squat plotted against speed at different depth-to-draught ratios and confinement conditions. Squat depends on hull form, speed, channel cross-section and clearance; no single shortcut formula can represent every ship and waterway.
Heel immerses the lowest point on one side, while pitch, roll and heave create transient extremes in waves. UK technical guidance on under-keel clearance includes squat, heel, tide, meteorological departure from prediction and sea state among a master's considerations. The Celtica Hav grounding also demonstrates that tide-gauge calibration and minimum-clearance policy directly affect the result. A static aft draught of 7.00 metres is one input, not a statement that exactly seven metres of water is enough.
An under-keel ledger aligns vertical references and preserves each allowance
Suppose charted depth is 8.40 metres and target tide height is plus 1.20 metres, both relative to chart datum, giving nominal depth of 9.60 metres. Subtract a 7.00-metre static extreme draught, applicable squat of 0.45 metre, a 0.25-metre planning allowance for heel, pitch and waves, and 0.30 metre for depth, tide and survey uncertainty. The residual UKC is 1.60 metres, with every sign and reference traceable.
That 1.60 metres is not automatic clearance to proceed. Ports, flag administrations, operators and pilotage authorities may prescribe an absolute or percentage criterion, and real-time level and current can change. The record should retain draught station and time, density, loading condition, applicable load line, chart and survey date, tidal datum, squat-data version and every allowance. Air draught runs upward from the waterline to the highest point for overhead clearance; the similar word describes the opposite direction and does not belong in under-keel clearance.
Questions
Continue exploring this subject
Is there a difference between draft and draught?
For ship immersion, they are mainly American and British spellings. The reference point used for the measurement still has to be taken from the vessel's documentation.
How is mean draught calculated from forward and aft draught?
A simple state uses (TF + TA) / 2. Precise displacement and trim corrections also account for mark positions, perpendiculars and the vessel's hydrostatic data.
Are draught marks the same as the load line?
No. Draught marks show the current waterline at reading stations. The amidships load line is an assigned immersion limit for applicable zone, season and water density.
Why does the same vessel draw more in fresh water?
Fresh water is normally less dense, so the vessel must displace more volume to balance the same weight. The exact change comes from its freshwater allowance and actual density.
Is UKC simply water depth minus static draught?
That is only the start. References must align, then squat, heel, ship motion, water-level and survey uncertainty, and the applicable minimum-clearance policy must be included.
Sources
Continue the research
- Safety practices related to small fishing vessel stabilityFood and Agriculture Organization
- International Convention on Load LinesInternational Maritime Organization
- IMO Train-the-Trainer Course: International load line conventionInternational Maritime Organization
- Under Keel ClearanceUK Nautical and Offshore Renewables Energy Liaison
- MGN 301 Amendment 1: Manoeuvring information onboard shipsUK Maritime and Coastguard Agency
- Grounding of Celtica Hav on a gas pipelineUK Marine Accident Investigation Branch
- How are draft marks read?U.S. Coast Guard Proceedings
- Enable ship deadweight optimization through improved port master dataIMO GreenVoyage2050