Under Keel Clearance (UKC): How to Calculate Margin Below the Keel

At first glance, calculating Under Keel Clearance may seem relatively straightforward. However, audit findings, inspection deficiencies, and—far worse—grounding casualties continue to reveal persistent knowledge gaps in this critical aspect of passage planning.

UKC (Under Keel Clearance) is defined as the distance between the lowest point of the ship’s hull and the seabed—or, put simply, the vertical margin of water below the keel.

In theory, the premise is simple: check the sounding on the nautical chart, take the ship’s draft, apply the basic equation:

\text{Depth} - \text{Draft} = \text{UKC}

and navigate using the resulting figure.

In practical ship operations, however, reality is far more complex, and this elementary equation requires comprehensive refinement. We rely on two fundamental operational variables—the vessel’s draft and the depth of the waterway—both of which are influenced by numerous dynamic hydrodynamic and environmental factors.

A fundamental rule of navigation to keep in mind when calculating Under Keel Clearance: all corrections must be applied toward the side of safety (the conservative worst-case scenario). That is, allowances are always added to the draft and subtracted from the available depth.

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Factors Affecting Vessel Draft

Initially, the vessel’s draft must be understood as the Static Draft ( $D_s$ ) observed while motionless in calm water. Once dynamic forces act upon the moving hull, the actual immersion becomes the Dynamic Draft ( $D_d$ ).

Hydrodynamic Sinkage (Squat Effect)

When a ship moves forward, water displacement around the hull generates pressure variations and hydrodynamic resistance. This phenomenon becomes particularly pronounced in shallow waters and confined channels, causing bodily sinkage and a change of trim (increasing draft either at the bow or the stern).

The Squat Effect is an extensive technical subject that warrants dedicated evaluation. Every navigator must remember that squat has a quantifiable magnitude, and controlling it is the direct responsibility of the Officer of the Watch (OOW) for each specific vessel:

D_d = D_s + \text{Squat}

Heel Allowance

From basic ship stability theory, when a vessel heels, it rotates transversely around its metacentre. During this transverse inclination, the lower turn of the bilge immerses deeper than the keel line, projecting the hull into greater depths.

Data concerning draft increases due to heel can generally be found on the Wheelhouse Poster posted on every bridge. It specifies the added immersion in metres for each degree of list. Factoring in heel, our equation expands:

D_d = D_s + \text{Squat} + \text{Heel}

Under Keel Clearance - Расчет глубины под килем

Draft Observation Quality (Draught Quality — $D_q$ )

While we often assume our recorded draft is absolute, practical uncertainty frequently exists. Chop, swell, or passing traffic may prevent accurate reading of draft marks, or vessel motion alongside an exposed berth may skew observations. In some terminals, shoreside structures obstruct visual access to the bow or stern marks.

Furthermore, deep-sea transits often rely purely on theoretical draft values. For example, after a month-long Pacific crossing, visual verification is impossible while underway. Over that duration, the vessel consumed bunker fuel, generated bilge and sludge, and shifted ballast and domestic water reserves. While loading computers calculate updated draft figures based on tank soundings, visual confirmation is impractical. Therefore, a prudent navigator introduces an allowance for measurement uncertainty ( $D_q$ ):

D_d = D_s + \text{Squat} + \text{Heel} + D_q

Water Density (Dock Water Allowance — DWA)

Archimedes’ principle dictates that buoyant force depends directly on fluid density. Standard sea water has an average density of $1.025 \text{ t/m}^3$ , whereas fresh water has a density of $1.000 \text{ t/m}^3$ . Actual water bodies exhibit variable salinity influenced by river discharge, currents, tidal cycles, and seasonal shifts.

Salinity data for specific ports can be found in Sailing Directions (Pilots) and the Guide to Port Entry. It is standard procedure to request the current dock water density from the ship’s agent prior to arrival.

Actual density may deviate from reported figures and can only be verified alongside using a hydrometer. Every vessel, according to its hull geometry and displacement, undergoes draft variations when transitioning between salt and fresh water. This value is derived from the Deadweight Scale via the Fresh Water Allowance (FWA) and corrected as Dock Water Allowance (DWA). Because sea water is denser than fresh water, its buoyant support is greater. When a departure draft is observed in salt water, the density allowance ( $W_a$ ) must be added to project arrival immersion in brackish or fresh water:

D_d = D_s + \text{Squat} + \text{Heel} + D_q + W_a

Worked Example: Dynamic Draft

Observed Static Draft alongside: $9.80\text{ m}$

Wind chop / Reading tolerance ( $D_q$ ): + $\pm 0.10\text{ m}$

Calculated Full-Speed Squat on shallow passage: $1.10\text{ m}$ by the stern

Expected dynamic heel (up to $2^\circ$ under weather): added draft of $0.80\text{ m}$

Port water density: open coastal sea water ( $1.025 \text{ t/m}^3$ , hence $W_a = 0.00\text{ m}$ )

$9.80 + 1.10 + 0.80 + 0.10 + 0.00 = 11.80\text{ m}$

Estimated Dynamic Draft ( $D_d$ ) for this leg: $11.80\text{ m}$

Factors Affecting Available Water Depth

When evaluating a passage track, identify the shallowest sounding along the planned route—the Charted Depth. Depths shown on navigational charts are measured relative to Chart Datum (typically Lowest Astronomical Tide — LAT).

Astronomical Tide (Height of Tide)

Nothing impacts available water depth as significantly as tidal rise and fall. Proficient application of Admiralty Tide Tables (ATT) and co-tidal charts is a fundamental duty of the navigator:

\text{Depth} = \text{Charted Depth} \pm \text{Tide}

Survey Accuracy (CATZOC)

Depth soundings taken from navigational charts are subject to survey limitations. Modern Electronic Navigational Charts (ENCs) categorize bathymetric data quality through CATZOC (Category Zone of Confidence). atrzocstars Under Keel Clearance calculation

When activating the chart quality layer in your ECDIS, the system overlays data quality symbols (patterns of stars). CATZOC classifies bathymetric confidence into six categories: A1, A2, B, C, D, and U (Unassessed). Each category prescribes a numerical depth uncertainty deduction that must be accounted for in UKC calculations:

A1 =0.50 + 1%d

A2 = 1.00 + 2%d

B = 1.00 + 2%d

C = 2.00 + 5%d

CATZOC D: Greater uncertainty than Category C

CATZOC U (Unassessed): Systematic survey quality assessment has not yet been conducted. This does not necessarily denote incorrect data, but good seamanship dictates applying a deduction equal to or greater than Category C.

\text{Depth} = \text{Charted Depth} \pm \text{Tide} - \text{CATZOC Deduction}

Seabed Morphology and Sedimentation

Hydrographic surveys conduct depth soundings along spaced survey lines. Complete 100% seafloor coverage cannot be assumed across all chart areas, meaning sand waves, isolated rocks, and natural ridges may exist between sounding lines. In shallow waters, seabed composition is critical: over a rocky bottom, pinnacles or boulders may present localized hazards.

Subsea Obstructions

Subsea pipelines, telecommunication cables, abandoned oilfield platforms, fish farm structures, concrete blocks, and maritime debris represent hazards that may or may not be precisely charted. When sailing through areas with recent marine construction or decommissioned infrastructure, prudently account for potential seabed debris:

\text{Depth} = \text{Charted Depth} \pm \text{Tide} - \text{CATZOC} - (\text{Seabed / Obstruction Allowance})

Sea Swell and Wave Action

Surface wave disturbance causes bodily heave, pitch, and roll. Following meteorological analysis, the anticipated wave profile along the route must be incorporated as a precautionary deduction in the UKC calculation:

\text{Depth} = \text{Charted Depth} \pm \text{Tide} - \text{CATZOC} - \text{Obstruction} - \text{Wave Allowance}

Wind-Induced Recession & Atmospheric Pressure

Meteorological factors can lower surface water levels. Sustained offshore winds cause wind setup/drawdown, pushing water away from coasts and shallow bays. Furthermore, barometric pressure shifts alter local sea levels: meteorologists estimate that an atmospheric pressure increase of $1\text{ hPa}$ $1\text{ mbar}$ ) depresses local water surfaces by approximately $1\text{ cm}$ . Factoring an allowance of $0.20\text{ m}$ to $0.30\text{ m}$ into your Passage Plan for barometric and wind recession demonstrates rigorous, professional passage appraisal:

\text{Depth} = \text{Charted Depth} \pm \text{Tide} - \text{CATZOC} - \text{Obstruction} - \text{Wave} - \text{Wind/Pressure Allowance}

Worked Example: Available Depth Calculation

Least Charted Depth selected on transit leg: $25.0\text{ m}$
ECDIS CATZOC classification: Category B (deduction of $1.40\text{ m}$ )
Predicted height of tide at transit time: $+0.50\text{ m}$
Subsea cable / safety clearance margin: $1.00\text{ m}$
Forecasted swell and wave action: $1.50\text{ m}$
Anticipated barometric surge depression: $0.30\text{ m}$

25.0 + 0.50 - 1.40 - 1.00 - 1.50 - 0.30 = 21.30\text{ m}

Effective Available Depth ( $H_A$ ): $21.30\text{ m}$

Final Under Keel Clearance Assessment

Now, apply the fundamental equation using our dynamically corrected figures:

\text{Depth} - \text{Draft} = \text{UKC}

21.30\text{ m} - 11.80\text{ m} = 9.50\text{ m}

Calculated Under Keel Clearance: $9.50\text{ m}$

The final operational step is verifying whether this margin of $9.50\text{ m}$ meets company safety policies and local port limits.

Company UKC Policies and Statutory Limits

Every shipping company maintains a Safety Management System (SMS) and a Ship Operational Manual (SOM) defining minimum under keel clearance parameters. Violating these criteria constitutes a breach of professional navigational duty. Port authorities likewise enforce statutory UKC limits within their pilotage areas.

Standard commercial policies define minimum margins based on the operational area:

Ocean Passage: $50.0\text{ m}$ minimum depth
Open Coastal Waters: minimum $20\%$ of maximum dynamic draft
Harbor Waters & Fairways: minimum $10\%$ of maximum dynamic draft
Alongside Berth: fixed minimum of $0.50\text{ m}$

Evaluating our calculated coastal leg against company policy:

Company requirement (20% of Dynamic Draft $11.80\text{ m}$ ): $2.36\text{ m}$
Calculated UKC: $9.50\text{ m}$

Verdict: $9.50\text{ m} > 2.36\text{ m}$ — The passage leg is verified safe and fully compliant.

Practical Bridge Guidelines

Under commercial constraints, bridge teams may face tight margins when planning port arrivals. When safe navigation requires optimizing calculated clearance without compromising safety standards, consider the following practical approaches:

Verify Authoritative Non-Chart Soundings: CATZOC deductions apply when depths are sourced from general hydrographic charts. If an official, recent port hydrographic survey, port authority circular, or licensed pilot advice confirms maintained channel depths, document that reference in your Passage Plan. Citing official local data satisfies inspectors without requiring generic chart uncertainty deductions.
Dredged Channels: Where a fairway is officially designated on the chart as a maintained Dredged Area, separate allowances for seabed morphology and unknown obstructions can generally be omitted. These channels undergo regular maintenance dredging and bathymetric monitoring, making sudden obstacles unlikely.
Reduce Transit Speed: Hydrodynamic squat is proportional to the square of ship speed. Reducing transit speed lowers squat sinkage, directly increasing net UKC. Record speed restrictions in the Passage Plan, mark them on the ECDIS, and present them on the Pilot Card as a mandatory speed limit (e.g., Max Speed 5 knots).

Under Keel Clearance

Under Keel Clearance (UKC): How to Calculate Margin Below the Keel