The squat effect happens when a ship moving through shallow water sits lower in the water than it would at rest or in deep water. It matters because the seabed may be much closer to the hull than the crew expects. A ship that seems to have enough depth can suddenly lose under-keel clearance and risk grounding.
This is especially important in harbors, canals, rivers, and coastal channels where large ships travel through restricted water.
Key Facts
- Squat is the extra downward sinkage and trim change of a moving ship in shallow or restricted water.
- Faster flow under the hull causes lower pressure, which pulls the ship downward.
- Bernoulli idea: higher fluid speed usually means lower fluid pressure along a streamline.
- Under-keel clearance = water depth - ship draft.
- Effective clearance while moving = water depth - ship draft - squat.
- Squat increases strongly with speed, so a small speed reduction can greatly reduce the risk.
Vocabulary
- Squat effect
- The downward sinking and trimming of a moving ship caused by pressure changes in shallow or restricted water.
- Draft
- The vertical distance from the waterline to the lowest part of a ship's hull.
- Under-keel clearance
- The vertical distance between the lowest part of a ship and the seabed.
- Bernoulli principle
- A fluid principle stating that faster-moving fluid often has lower pressure than slower-moving fluid.
- Trim
- The difference between how deeply the bow and stern of a ship sit in the water.
Common Mistakes to Avoid
- Ignoring ship speed, because squat is much larger at higher speeds and may become dangerous even when the charted depth looks safe.
- Using only the ship's resting draft, because a moving ship can sink lower and reduce its actual under-keel clearance.
- Assuming shallow water only affects the bottom of the ship, because restricted water also changes pressure and flow around the hull.
- Forgetting trim change, because squat can make the bow or stern sink more than the rest of the ship and that end may ground first.
Practice Questions
- 1 A ship has a draft of 9.0 m in a channel that is 11.5 m deep. If squat at its current speed is 1.2 m, what is the effective under-keel clearance?
- 2 A cargo ship travels in shallow water with a resting draft of 7.5 m. The water depth is 9.0 m. If the minimum safe under-keel clearance is 0.8 m, what is the maximum allowed squat?
- 3 A captain reduces speed before entering a shallow channel. Explain why this reduces squat and lowers the risk of grounding.