Squat Effect:
The Squat Effect: A Hydrodynamic Phenomenon in Shallow Waters
The squat effect is a hydrodynamic phenomenon that occurs when vessels navigate through shallow waters, causing them to sit lower in the water than expected. This effect is primarily caused by the acceleration of water flow between the ship’s hull and the seabed in confined areas, resulting in reduced pressure and increased draft.
Causes and Characteristics
Water Flow Dynamics:
As a ship moves through shallow water, the confined space between the hull and seabed causes water to accelerate, leading to a pressure reduction.
Vessel Design Impact:
The Block coefficient (Cb) of a vessel influences its squat behavior:
- Vessels with Cb < 0.7 tend to squat by the stern (Cb < 0.7 is finer ship designed for speed)
- Vessels with Cb > 0.7 typically squat by the head or bow (Cb > 0.7 is heavier ship designed for more cargo carrying capacity)
Speed Relationship:
The squat effect is approximately proportional to the square of the ship’s speed. Reducing speed by half decreases the squat effect by a factor of four.
Conditions Affecting Squat
- Most noticeable when the depth/draft ratio is less than four.
- More pronounced when sailing close to a bank
- Can lead to unexpected groundings and handling difficulties
Indicators of Squat
Mariners and ship pilots should be aware of the following signs:
- Vibration
- Poor helm response
- Shearing off course
- Change of trim
- Change in wash ( vessel wash: trailing effect of water churning left by propeller rotation)
Prevention and Management
To minimize the squat effect and ensure safe navigation, the following measures can be taken:
- Speed Control: Maintain slow speeds in shallow waters.
- Avoid Shallow Waters: When possible, steer clear of shallow areas and narrow channels
- Maintain Safe Distances: Keep a safe distance from banks and other structures
- Draft Management: Ensure the ship’s draft is appropriate for the water depth
- Continuous Monitoring: Closely monitor the ship’s speed and draft
