Home / Guides / Under Keel Clearance Guide

Under Keel Clearance (UKC): How Ship Draft Determines Safe Passage

In March 2021, Ever Given ran aground in the Suez Canal and blocked global trade for six days. While wind and loss of steerage were the official causes, the fundamental physics question was under keel clearance. At 15.7 metres draft in a 24-metre deep canal, the margin was about 8 metres on paper. Squat effect at 13 knots and bank suction reduced that margin dramatically. Understanding UKC is understanding the difference between a routine transit and a billion-dollar grounding.

What Is Under Keel Clearance?

Static UKC is the simple calculation: charted water depth plus tide height minus the ship's static draft. If the chart says 20 metres of water, the tide adds 2 metres, and the ship draws 15 metres, static UKC is 7 metres. This assumes the ship is stationary in flat water. It is a snapshot.

Dynamic UKC is what actually matters when the ship is underway. It takes static UKC and subtracts everything that reduces the gap: squat effect, wave-induced motions, heel during turns, and an allowance for seabed type. If static UKC is 7 metres and squat eats 1.2 metres, wave response consumes 0.5 metres, and the rock bottom requires a 1-metre safety margin, dynamic UKC drops to 4.3 metres. That is the real number.

Standard minimum UKC requirements vary by location:

• Open sea pilotage: typically 20% of draft. A ship drawing 15 metres needs 3 metres of clearance.
• Confined channels: typically 15% of draft. That same ship needs 2.25 metres.
• Alongside berths: typically 10% of draft. The ship needs 1.5 metres at berth.
• Soft bottom areas: some ports reduce minimums where touching mud is less catastrophic than hitting rock.

These are minimums. Many ports require more. The Panama Canal Authority sets UKC dynamically based on lake levels rather than a fixed percentage. The Suez Canal Authority manages UKC through speed limits because squat, not static depth, is the binding constraint.

The Squat Effect: Why Ships Sink Deeper at Speed

Three factors drive squat:

1. Speed through water. Squat increases with the square of speed. Double the speed, squat roughly quadruples.
2. Water depth relative to draft. The shallower the water, the more restricted the flow path, the greater the squat.
3. Block coefficient (Cb). Fuller hull forms push more water aside. A boxy bulker (Cb 0.85) squats much more than a fine-lined container ship (Cb 0.65).

The open-water approximation formula:

Example 1 -- Open water: A Panamax bulker (Cb 0.85) at 12 knots. Squat = 0.85 × 144 ÷ 100 = 1.22 metres.

Example 2 -- Confined channel: The same ship at 6 knots in the Panama Canal, where channel walls amplify the effect. Confined squat reaches about 0.5 metres. The 6-knot canal speed limit exists specifically to control squat and stay within available UKC margins.

Example 3 -- The dangerous case: A Suezmax tanker at 20 metres draft doing 13 knots in the Suez Canal. Open-water squat approximates 1.4 metres. Confined amplification can push this to 2.5-3.0 metres. Static UKC of 4 metres shrinks to 1.0-1.5 metres of dynamic clearance. Add bank suction near the canal wall, or a steering correction, and the margin evaporates.

Squat also changes trim. Most ships squat by the bow in shallow open water. In very confined channels, the stern may squat more because the propeller pulls water from under the hull aft. The deepest part of the ship shifts, and a pilot thinking about bow clearance might miss that the stern is now half a metre deeper.

For how draft is measured to feed the UKC equation, see our guide to reading draft marks.

UKC Requirements by Major Port and Canal

Panama Canal. A unique case because the canal operates with freshwater Gatun Lake at 26 metres above sea level. The 15.24-metre TFW maximum depends on the lake being full. During the 2022-2024 drought, the Panama Canal Authority reduced allowable draft in stages to as low as 13.56 metres. Laden Neopanamax ships could not transit. Cargo was offloaded for rail transport across Panama, or ships diverted around Cape Horn -- adding 12 days and roughly half a million dollars in fuel per voyage.

Suez Canal. At 24 metres deep, the canal accommodates vessels up to 20.1 metres draft. But the real constraint is squat at transit speed. The Ever Given at 15.7 metres draft had about 8 metres of static UKC; squat at 13 knots plus bank suction cut the dynamic margin severely. The 2015 canal deepening and new parallel section were partly about adding depth for UKC and partly about widening the effective cross-section to reduce confined-channel squat amplification.

Rotterdam. The Netherlands has a tidal range of only 1.5-2 metres, so tide does not create temporary depth. Rotterdam dredged and maintains a 24-metre channel. A 22-metre draft ore carrier always has 2 metres of static UKC at low tide. No tidal windows needed -- just engineering.

Port Hedland. The world's largest bulk export port operates on tidal windows. Valemax vessels at 23 metres draft can only transit during spring high tides, which add 5-7 metres to the 19-metre channel depth. A window opens once every two weeks. Miss it, and the ship waits at a cost of $70,000-$100,000 per day.

Tidal Windows and Draft Planning

Port Hedland is the extreme example. A Valemax at 23 metres draft needs roughly 26-27 metres of water for safe clearance. Channel depth is 19 metres. The shortfall of 7-8 metres must come from tide. Neap tides provide about 3 metres -- not enough. Spring tides provide up to 7 metres -- just enough, for a few hours. Operators coordinate loading rates at Ponta da Madeira in Brazil, the 35-day voyage across the Indian Ocean, and arrival timing to hit a window that opens for hours every two weeks. A 1% draft error of 23 centimetres can mean missing the window. Two weeks of waiting at $80,000 per day costs over a million dollars.

The economics shape entire trades. Iron ore from Australia to China runs on tidal windows at Hedland, Dampier, and Walcott. Container lines at Felixstowe spent 130 million pounds deepening the channel from 14.5 to 16 metres specifically to eliminate tidal dependency for ULCVs. What used to require a tidal window is now an any-tide arrival.

Real-time draft data is critical. The cargo-plan estimate that left the load port is an estimate. Actual draft on arrival can differ by 10-20 centimetres due to density changes, fuel consumption, and ballast adjustments. Knowing the actual number, not the planned one, determines whether the ship makes the window.

How to Calculate Minimum UKC

The dynamic UKC formula, based on PIANC (World Association for Waterborne Transport Infrastructure) guidelines:

Each component matters:

• Squat: Cb × V² ÷ 100 for open water. Multiply by 2-3 for confined channels.
• Wave response: Typically significant wave height ÷ 2 for head seas. A 1.5-metre swell eats 0.75 metres of clearance.
• Heel effect: A 2-degree heel on a 30-metre beam ship drops the bilge about 0.5 metres. Pilots manage this by limiting turn rates in shallow water.
• Bottom type allowance: Soft mud may allow 0.3 metres of net clearance. Rock or coral requires 1.0 metre or more.
• Net UKC reserve: PIANC recommends 0.5-1.0 metres depending on survey accuracy and consequences of grounding.

Worked example: A Panamax bulker drawing 12.0 metres approaches a 14.0-metre channel. Tide is 2.5 metres. Speed is 10 knots, Cb is 0.82. A 1.5-metre swell is running. Bottom is firm sand.

• Static UKC = 14.0 + 2.5 - 12.0 = 4.5 metres
• Squat = 0.82 × 100 ÷ 100 = 0.82 metres
• Wave response = 1.5 ÷ 2 = 0.75 metres
• Heel (turn) = 0.3 metres
• Bottom allowance (sand) = 0.5 metres
• Net reserve = 0.5 metres
• Dynamic UKC = 4.5 - 0.82 - 0.75 - 0.3 - 0.5 - 0.5 = 1.63 metres

At 1.63 metres dynamic UKC, the transit is safe but tight. At 14 knots instead of 10, squat jumps to 1.61 metres and dynamic UKC collapses to 0.84 metres. The pilot slows down or waits for conditions to improve.

Many ports use their own formulas. Port Hedland uses a probabilistic model. The Panama Canal manages UKC operationally. Rotterdam relies on fixed depth and pilot judgment. The important thing is using a formula at all, rather than guessing.

Real-Time Draft Monitoring for UKC Management

On a 200,000 DWT Capesize, a 1% draft error is roughly 17-20 centimetres. That represents about 2,000 tonnes of cargo. More critically for UKC, 20 centimetres can be the difference between making a tidal window and missing it.

Modern real-time draft monitoring solves this. Computer vision and AI read draft marks continuously at all six positions -- forward port and starboard, midship port and starboard, aft port and starboard. The data feeds directly into UKC calculation systems, providing dynamic clearance updates throughout the transit. When fuel consumption reduces draft, the system reflects it. When ballast transfers shift trim, the system reflects it. The UKC number stays current.

For port authorities, real-time monitoring changes operations: better scheduling of tidal windows using actual draft data, earlier warning when clearance margins tighten, data-driven dredging based on real transit records, and an objective record if a grounding does occur.

On a Valemax, every centimetre of draft represents about 100 tonnes of cargo. On a UKC calculation where the total margin is 2-3 metres, every centimetre counts. Real-time monitoring turns UKC management from pre-departure paperwork into a live operational tool.

Frequently Asked Questions

What is under keel clearance and why does it matter?

How is squat effect calculated?

Squat is the hydrodynamic sinkage from water accelerating under a ship's hull in shallow water. The approximation is Squat = Cb × V² ÷ 100, where Cb is block coefficient and V is speed in knots. A Panamax bulker (Cb 0.85) at 12 knots squats about 1.2 metres in open water. In confined channels, squat amplification can be 2-3 times greater. Professional pilots use PIANC models accounting for channel geometry, depth-to-draft ratio, and hull form. Squat also changes trim, shifting the deepest point of the hull forward or aft depending on conditions.

Why do tidal windows matter for UKC?

Tidal windows exist because channel depths are fixed but water levels change. A Valemax at 23 metres draft cannot transit Port Hedland's 19-metre channel without tidal help. Spring high tides add 5-7 metres of depth, but that window opens once every two weeks. Missing it costs $70,000-$100,000 per day. Operators plan loading rates in Brazil, sailing speeds across the Indian Ocean, and arrival timing in Australia around tide predictions published years in advance. A 1% draft error can mean missing the window entirely.

How can technology improve UKC management?

Traditional UKC relies on estimated draft from cargo plans, which can be off by 10-20 centimetres. AI-powered real-time draft monitoring provides continuous measured draft at all six reading positions. This feeds dynamic UKC calculations, giving bridge teams live clearance updates during transit. For ports, it enables better tidal window scheduling, earlier warning of tightening margins, and data-driven decisions about transit speeds. The shift from estimated to measured draft eliminates the largest source of uncertainty in UKC management. On a large bulker, the difference between estimated and measured draft can represent 2,000-4,000 tonnes of cargo value.