Under-Keel Clearance & Squat Calculator
Maritime CalculatorsCalculate hydrodynamic hull sinkage squat, remaining seabed clearance, and vessel trim behavior in canals or shallow waters
Vessel static immersion read at the forward draft marks.
Vessel static immersion read at the aft propeller/rudder marks.
Charted soundings plus current harmonic tide elevation.
Speed through water; squat effect escalates squarely with velocity (V²).
Tankers/Bulkers ≈ 0.80 (dips bow), Containers ≈ 0.65 (dips stern).
Caution: Reduced Under-Keel Clearance Margin
Dynamic clearance is below minimum safety target (1.10 m). Reduce transit speed immediately to decrease squat!
How to Solve for Under-Keel Clearance (UKC) & Squat
Standard operational nautical calculations for merchant ships entering harbors, channels, and coastal waters.
Determine stationary seabed clearance by subtracting the ship's stationary keel draft from total charted water depth (including current harmonic tide height).
As a vessel accelerates, Bernoulli water velocity underneath the bottom creates suction toward the seafloor. In narrow canals or dredged rivers, confining banks double this downward sinkage dip!
Subtract the hydrodynamic squat dip from Static UKC to determine actual operating clearance underway. Safe harbor transit requires this dynamic margin to exceed 10% of maximum draft (or regional port authority minimums).
The Hydrodynamics of Restricted Waterways and Channels:
- Bernoulli's Principle in Canals: In restricted channels, water displaced by a large commercial merchant hull cannot freely dissipate to the sides. It must stream at elevated velocity underneath the ship bottom, generating extreme low-pressure suction toward the riverbed.
- Velocity Squared Escalation: Squat effect increases directly with the square of vessel speed (V²). A minor speed increase from 10 knots to 14 knots nearly doubles the downward hull suction and keel grounding danger!
How Block Coefficient (Cb) dictates forward vs. aft sinkage on commercial vessels:
- Tankers & Bulk Carriers (Cb > 0.70): Full, boxy forward hull lines push a massive bow wave, causing greatest water velocity and lowest suction pressure directly beneath the forward keel. These vessels inevitably squat by the bow!
- Container Vessels (Cb < 0.70): Streamlined, slender bow forms allow clean front flow, but large aft propeller suction and stern overhang generate maximum sinkage aft, causing fast container ships to squat by the stern!