Nautical Academy Lecture Notes: Under Keel Clearance (UKC)

Overview

• UKC refers to the vertical distance between the vessel's keel and the seabed.
• Always adhere to company policies and guidelines when calculating UKC.

Key Components in Calculating UKC

1. Depth of Water

   • The depth measurements at specific locations:
     • 23 m, 18.1 m, 12.3 m
   • Lowest sounding (12.3 m) will be used for calculations.
   • Dependent on the Lowest Astronomical Tide (LAT) which predicts the lowest level under average conditions.

2. Height of Tide

   • Increases water depth above LAT.
   • Can be found in tide tables.

3. Vessel's Draft

   • Static Draft: Draft when the ship is at rest.
   • Dynamic Draft: Draft when the ship is in motion, accounting for various factors.
   • Use static draft for calculations.
   • Example: Assume static draft of 7.50 m.

4. Ship Squat

   • Calculated based on ship speed and block coefficient.
   • Formula for Open Waters:
     • Squat = Block Coefficient * (Ship Speed^2) / 100
   • Formula for Confined Waters:
     • Squat = Block Coefficient * (Ship Speed^2) / 100 * 2
   • Example Calculation:
     • Speed: 11 knots, Block Coefficient: 0.845
     • Squat in Open Waters: 1.02 m
     • Squat in Confined Waters: 2.04 m (due to trim by the stern).

5. Swell

   • Value determined from weather forecasts (e.g., assume swell of 1 m).

6. Category of Zone of Confidence (CatSock)

   • Categories (A1 to U) define accuracy of depth measurements.
   • Example Calculation:
     • For 12.3 m depth in CatSock A2:
       • 2% of 12.3 m = 0.25 m + 1 m = 1.25 m.

Calculation of UKC

• Subtract the following from the depth of water:
  • Vessel's draft (7.50 m)
  • Ship squat (2.04 m)
  • Swell (1 m)
  • Zone of Confidence (1.25 m)
• Resulting UKC Example: 0.51 m.

Minimum UKC Requirements

• Company policy states minimum UKC must be greater than 10% of the ship's draft.
• Minimum UKC for 7.50 m draft = 0.75 m.
• Current UKC (0.51 m) does not meet this requirement.

Solutions to Meet UKC Requirements

1. Wait for High Tide

   • Example: High tide height = 2.5 m.
   • New UKC = 3.01 m (meets requirements).

2. Reduce Ship Speed

   • Speed Reduction by 50% reduces squat by 25%.
   • New squat = 1.53 m, resulting UKC = 1.02 m.
   • Allows passage without high tide.

Considerations for Water Density

• Changes in water density affect ship's draft during shallow water navigation.
• Important for calculating UKC, especially when moving from higher to lower density water.

Additional Resources

• Check links provided for detailed calculations on squat and changes in ship's draft due to water density.

Conclusion

• Understanding and calculating UKC is crucial for safe navigation.
• Always refer to company policies and guidelines.