Ship Squat & Shallow Water UKC Calculator
Determine dynamic squat sinkage in meters and feet, dynamic trim direction, and net Under-Keel Clearance (UKC) to prevent vessel grounding in shallow fairways.
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Ship Hydrodynamic Squat & Shallow Water Dynamics
When a large commercial vessel navigates in shallow or restricted waters (canals, dredged entrance channels, rivers), the water displaced forward must rush beneath the bottom of the hull and along the sides to refill the void left aft. By Bernoulli's principle, this accelerated fluid flow creates a localized drop in hydrodynamic water pressure beneath the keel. The ship is sucked downward into the water column, a phenomenon known in maritime navigation as Ship Squat.
Barrass Empirical Squat Governing Formulations
Dr. C.B. Barrass established widely accepted empirical equations correlating vessel block coefficient, speed through water, and channel confinement:
- Confined Fairway / Canal:
Where channel cross-sectional area restricts return flow:Squat_max (meters) = (Cb × V_knots²) / 50 - Open Shallow Water:
Where water can disperse laterally:Squat_max (meters) = (Cb × V_knots²) / 100 - Dynamic Trim Direction Rules:
- Cb ≥ 0.70 (Full-form ships: Tankers, Bulk Carriers): Squat occurs predominantly at the BOW. The bow dips lower than the stern.
- Cb < 0.70 (Fine-form ships: Container ships, Cruise liners): Squat occurs predominantly at the STERN. The propeller and rudder dip deeper.
- Even-keel intermediate: If Cb ≈ 0.70, bodily sinkage occurs with minimal change in trim.
- Net Under-Keel Clearance (UKC):
Dynamic UKC = Water Depth - (Static Draft + Squat_max)
Frequently Asked Questions
Why does reducing speed from 12 to 6 knots reduce squat by 75%?
Squat increases with the square of ship speed (V²). Halving vessel speed from 12 knots to 6 knots reduces hydrodynamic suction to (6/12)² = 1/4 of its former value, immediately restoring 75% of lost under-keel clearance.
What is the Block Coefficient (Cb)?
The block coefficient is the ratio of the underwater volume of a ship to the volume of a rectangular block having the same length, breadth, and draft. Boxy bulk carriers have high Cb values (∼0.80 to 0.86), while sleek container ships have lower Cb values (∼0.60 to 0.68).
What is the minimum safe Under Keel Clearance in port approaches?
Most international port authorities and oil company marine vetting policies (SIRE / OCIMF) mandate a minimum dynamic UKC of at least 1.0 meter (3.3 feet) or 10% of vessel maximum static draft, whichever is greater.
How does channel width affect squat?
In a narrow dredged trench or canal, return currents are forced directly under the ship at high speed, doubling effective squat compared to broad open water where displaced water easily flows sideways.