Under-Keel Clearance (UKC)

Under-keel clearance (UKC) is the vertical distance between the lowest point of a ship’s hull (the keel) and the seabed. It is the safety margin that prevents a ship from touching or grounding on the bottom.

Answer in Brief

UKC = Water Depth − Ship’s Draft

A UKC of zero means the ship is aground. Maintaining adequate UKC is a fundamental navigational safety requirement.

Minimum UKC Requirements

UKC requirements are not universally standardised — they vary by:

• Port authority regulations
• Pilotage authority rules
• Company SMS (Safety Management System) requirements
• Ship type (tankers often have stricter requirements due to grounding/pollution risk)

Typical minimum UKC values:

• Open sea (deep water): Not practically relevant — depths are thousands of metres
• Coastal passages: Many company SMSs require minimum 10% of draft or a fixed value (e.g., 1.0 metre)
• Port approaches and channels: Often 10–15% of draft, or as specified by port authority
• Alongside berth: Varies — some berths have very small UKC at low tide by design

Example: A loaded bulk carrier with a draft of 13.5 metres entering a channel with 10% UKC requirement needs: 13.5 × 0.10 = 1.35m UKC minimum. Channel depth must be at least 14.85 metres.

Factors Affecting UKC

UKC is not static — it changes constantly:

Tidal height: Water depth changes with tide. A channel passable at high water may be impassable at low water.

Ship’s draft: Varies with cargo loaded, fuel/water consumption, and density of water.

Density of water: Salt water (1.025 t/m³) vs fresh water (1.000 t/m³). A ship floats deeper in fresh water. The Fresh Water Allowance (FWA) accounts for this.

Squat: In shallow water, a moving ship sinks deeper due to the Bernoulli effect. Squat increases with speed and in shallow water. A ship doing 12 knots in shallow water may squat 0.5–1.5 metres depending on hull form. Speed must be reduced to control squat.

Heel and trim: A listed or trimmed ship will have one part of the keel closer to the bottom. Maximum draft must be calculated for the actual trimmed/heeled condition.

Swell: In swell, the ship rises and falls. At the trough of a swell, the ship is lower — effective draft increases.

UKC Calculation in Passage Planning

Passage planning (APEM — Appraisal, Planning, Execution, Monitoring) requires UKC assessment for every shallow water section:

Step 1: Obtain chart depths along planned track (use largest scale chart available) Step 2: Apply tidal corrections for the time of passage Step 3: Calculate maximum draft for the ship’s loading condition (including trim) Step 4: Add allowance for squat at planned speed Step 5: Add allowance for heel during manoeuvring Step 6: Add company-required safety margin Step 7: Verify result meets minimum UKC requirement

If minimum UKC cannot be achieved: adjust time of passage (for tidal benefit), reduce cargo, reduce speed, or reject the berth as unsafe.

Static vs Dynamic UKC

Static UKC: UKC when the ship is stopped. Simply water depth minus static draft.

Dynamic UKC: UKC when the ship is moving. Reduced by squat, heel, and wave motion. Dynamic UKC is what matters for passage planning.

Frequently Asked Questions

Q: Who is responsible for UKC calculations? The Master and Chief Officer for passage planning. The Pilot during pilotage — but the Master retains ultimate responsibility. The Master can refuse to proceed if UKC is inadequate regardless of pilot’s recommendation.

Q: What is the consequence of insufficient UKC? At best: contact with the bottom (touching), which triggers a sea protest and inspection. At worst: structural damage, grounding, and in worst cases, hull breach and flooding.

Q: Is there a universal minimum UKC in international law? No single international standard. SOLAS requires adequate UKC as part of passage planning, but the specific value is left to the operator and port authority.

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