Under keel clearance

Under keel clearance (UKC) is the term and calculations used in seafaring to ensure sufficient navigable water is available for ships at sea. It is most commonly described as the height and width of available water below the keel (hull of a ship). More simply, it can be described as how much water is left between the bottom of the ship and the seabed.

Master mariners should ensure there is sufficient minimum UKC for their ships and ports should ensure sufficient minimum UKC for the type and draft of ships due to arrive in the port. Ships typically calculate their UKC to meet criteria for ports to minimise the risk of maritime incidents. A minimum UKC value is determined, which includes a safety margin.

Methods of calculation
At a basic level, it is typically calculated in metres using the formula: Charted Depth − Draft -/+ Height of Tide = UKC. Ship masters and deck officers can obtain the depth of water from Electronic navigational charts. More dynamic or advanced calculations include safety margins for manoeuvring effects and squat.

Computer systems and software can be used to manage and calculate UKC for ships and ports. These include systems that dynamically manage UKC using models, forecasting and calculations.

The International Hydrographic Organization (IHO) sets a Standard for UKC Management for software and systems: S-129 Under Keel Clearance Management.

Uses and requirements
For US waters, the US Code of Federal Regulations require ships and their masters to calculate UKC based on the ship's deepest navigational draft. The regulations require the master to discuss the UKC calculation with the maritime pilot as the ship approaches US ports/waters.

The Australian Maritime Safety Authority state that maintaining adequate UKC is important for safe navigation in the Torres Strait.

Inadequate UKC, caused by shallow water and squat may lead to ship groundings. In 2019, the UK Marine Accident Investigation Branch found that indequate UKC contributed towards the grounding of the ro/ro freight ferry MS Stena Performer.

Changes in water level may cause issues with adequate UKC for ships. For example, recent water level reductions in the Panama Canal have reduced UKC for ships meaning some ships are not able to transit the canal.