What is Under Keel Clearance (UKC) in navigation?

Under Keel Clearance (UKC) is the vertical distance between the lowest point of the ship's keel and the seabed. Net UKC = Charted Depth + Tide Height − Ship's Draft − Squat − Safety Margin. A positive UKC is mandatory at all times. Most port authorities and company SMS policies specify a minimum UKC (e.g. 10% of draft or 0.5 m minimum), whichever is greater.

What is squat effect and how does it affect UKC?

Squat is the hydrodynamic sinkage and trim change that occurs when a ship moves through shallow water. As speed increases and water depth decreases, water accelerates under the hull creating a low-pressure zone that pulls the ship downward. Using the Barras (2004) formula: Maximum Squat = (Cb × V²) / 100 for open water, or (Cb × V²) / 50 for confined channels. A 200m vessel at 12 knots with Cb=0.80 can squat 0.115m in open water — directly reducing available UKC.

What is a safe UKC for harbour approaches?

Safe UKC requirements vary by port and company policy, but common standards are: minimum 0.5 m absolute UKC, or 10% of the ship's maximum draft, whichever is greater. OCIMF recommends 10% draft for tankers in open water and higher margins in confined channels. Always check the specific port's regulations, pilot orders, and company SMS — some require 15% draft for shallow draught vessels in confined waters.

How do you calculate net UKC step by step?

Step 1: Get charted depth at the shallowest point along the track. Step 2: Add predicted tide height at the time of passage. Step 3: Subtract the ship's maximum draft (including any list or trim corrections). Step 4: Subtract calculated squat at planned speed (Barras formula). Step 5: Subtract any other allowances (density correction, wave allowance). The result is the net UKC. If it is below the required minimum, reduce speed or wait for higher tide.

Does ship speed affect squat in shallow water?

Yes — squat increases with the square of speed. Doubling speed quadruples squat. This is why speed reduction is the primary tool to manage squat in shallow water. Reducing from 12 knots to 8 knots reduces squat by 56%. The critical speed (where squat accelerates dramatically) is typically reached when the depth-to-draft ratio falls below 1.5.

Mastering Under Keel Clearance & Squat Effect

Sailing into shallow waters, ports, or canals drastically alters how your vessel behaves. A ship that sails comfortably in the deep ocean can suddenly find itself scraping the bottom of a channel due to hydrodynamic forces. This is where Under Keel Clearance (UKC) and the Squat Effect become the most critical factors in your Passage Plan.

The Danger of the Squat Effect

The Squat Effect is a hydrodynamic phenomenon. As a ship moves forward through shallow water, it displaces the water ahead of it. This water must rush back under the Keel to fill the void left behind. According to Bernoulli's principle, an increase in fluid velocity results in a decrease in pressure. This creates a powerful suction effect that pulls the ship deeper into the water, effectively increasing your draft.

Mathematical Calculation of Squat

While dynamic squat changes based on hull form, the standard empirical formula utilized in maritime navigation relies on your Speed Over Ground (COG/SOG) and the vessel's Block Coefficient (Cb).

Maximum Squat (m) = (Cb × V²) / 100

(Where V is the speed in knots. For confined waters, the denominator changes to 50, doubling your squat).

Because squat increases exponentially with the square of your speed, reducing your speed by half reduces your squat effect to just one-quarter.

Calculating Dynamic Under Keel Clearance (UKC)

Your static UKC on the chart is dangerously misleading. To find your true, dynamic UKC, you must consider the following equation:

Dynamic UKC = (Charted Depth + Height of Tide) - (Static Draft + Maximum Squat + Heel/Roll Allowance + Wave Sinkage)

If your Bathymetry readings (soundings) show a UKC approaching your company's minimum safe limits (usually 10% of your maximum draft), immediate action to reduce speed is required.

Sources and verification

Use these references as the starting point for verification; always follow current flag-state, company, port, and approved shipboard documents for operational decisions.

Calculate Squat Instantly with CaptainCalc

Don't rely on paper charts and guesswork in confined waters. CaptainCalc features a built-in Squat & UKC calculator that deterministically applies your vessel's exact Block Coefficient and confined-water multipliers instantly, completely offline.