Squats in Ships: Understanding the Phenomenon

Squat, a hydrodynamic phenomenon, refers to the decrease in the under-keel clearance experienced by a ship while navigating in shallow waters. This occurrence causes the ship to descend deeper vertically, accompanied by a slight alteration in trim. This phenomenon is also known as the shallow water effect.

Failure to monitor squats diligently can result in serious consequences such as grounding, loss of steering control, and collisions. Grounding occurs when the ship's bottom hull makes contact with the seabed, potentially leading to damage to the vessel or a significant oil spill from fuel oil tanks.

PHYSICS BEHIND THE SQUAT

The science behind squat is rooted in Bernoulli's theory.

Consider a pipe with an inlet (U1) and outlet (U2). The area of the outlet (U2) is half the area of the inlet (U1), indicating that the inlet has a greater area.

A stream of water having volume V enters the pipe. Let V1, P1, and V2, P2 are velocities and pressures from the inlet (U1) and outlet (U2) respectively.

To maintain volume equilibrium, the rate of flow of the water must increase. That is,

A1 > A2 : V1 < V2 : P1 > P2

Decreasing area = increasing speed = decreasing pressure

Similar to the flow of water through a pipe with varying areas at the inlet and outlet, when a vessel moves through shallow water, the reduced area between the ship's keel and the seabed increases water velocity and decreases pressure, pulling the ship downward. 

FACTORS THAT INFLUENCE SQUAT

• Squat grows proportionally with the square of a ship's speed. As the vessel accelerates, the increased water displacement beneath the hull results in a greater lower pressure area, intensifying the squat effect.

Squat α V².

• Squat exhibits an inverse relationship with water depth. As the depth decreases the effect squat increases.

• The block coefficient influences squat proportionally, with a more pronounced effect in confined waters compared to open waters.

• The blockage factor (S), represents the ratio of the vessel's transverse section to that of the canal. Squat increases as the blockage factor rises. The formula for the blockage factor is:

S = (b*d)/(B*D),

where B and D denote canal width and depth, while b and d represent the ship's breadth and submerged depth (draft).

When ships pass each other in rivers or canals, squats can double due to combined blockage factors.

In open water, determining the channel width is challenging, and an artificial measure known as the width of influence is used. This width, representing riverbanks, increases with vessel speed. For slower vessels, it is approximately 8 times the beam, while for faster vessels, it is around 12 times.

Squats can affect vessels in water depths up to 15 times the draft, but significant effects only occur when the water depth is less than 2.5 times the draft.

SQUAT AND TRIM

The squat effect exhibits variations based on the characteristics of the vessel. Fuller hull forms experience bow grounding while fine hull forms tend to ground at the stern.

Changes in trim depend upon the characteristics of the vessels are outlined as follows:

1. Vessel on an even keel and has Cb = 0.7 - The ship squats and there is no alteration in trim.

2. Vessel on an even keel and has Cb>0.7

The vessel has a fuller hull form and the LCB of such vessels usually lies ahead of the LCF. As a result the ship squats and trims forward. Forward trimming is due to the loss of buoyancy from the forward portion.

3. Vessel on an even keel and has Cb<0.7

The vessel has a fine hull form and the LCB of such vessels typically lies behind the LCF. As a result the ship squats and trims aft. Aft trimming is due to the loss of buoyancy from the aft portion.

4. Vessels with existing aft trim - The ship squats and trims to the aft.

5. Vessels with existing forward trim - The ship squats and trims to the forward.

SQUAT AND HEEL

A ship navigating through the water at an inclined angle will experience an increase in the heel due to squat concentrating in the area with the least under-keel clearance.

Likewise, a vessel in an upright position will suffer squat accompanied by a heel if it moves over a seabed that slopes gradually shallower beneath on one side of the ship.

FORMULA TO CALCULATE SQUAT

In the computation of squat on a ship, squat information diagrams, typically provided alongside the vessel's maneuvering information, serve as essential tools. In the absence of these diagrams, empirical formulae, derived from the analysis of approximately 300 ships, are employed. It's important to note that these formulae may have a slight overestimation, incorporating a safety margin. Currently, an exact prediction of anticipated squat remains elusive.

Various formulas are employed for squat calculations, with Dr. Barrass's formula being widely acknowledged and utilized. This formula exists in several versions, spanning from more intricate expressions to simpler ones. The most straightforward iteration is outlined below:

In open waters, the squat can be estimated using the formula

Squat = 0.01 * Cb * V² meters,

while in restricted waters, the formula becomes

Squat = 0.02 * Cb * V² meters.

where V represents the vessel's speed in knots, and Cb is the block coefficient of the ship.

The restricted waters calculations assume that the vessel's under-keel clearance is approximately 20% of its draft.

A more comprehensive formula for squat determination is expressed as

where V represents the vessel's speed in knots, Cb is the block coefficient of the ship, and S is the blockage factor.

PREVENTIVE MEASURES TO MINIMIZE THE RISKS ASSOCIATED WITH SQUAT

- Preemptively calculating squat,

- Factoring in squat when determining under-keel clearance,

- Implementing speed reduction.

INDICATIONS THAT A SHIP HAS ENTERED SHALLOW WATERS

- Noticeable increase in ship squat,

- Increased sinking of the vessel's body,

- Changes in trim,

- Elevated wave-making,

- Decreased maneuverability,

- Alterations in draught indicators,

- Reduction in propeller RPM,

- Diminished speed,

- Sudden vibrations,

- Decreased rolling, pitching, and heaving movements,

- Presence of mud in the water around the hull,

- Expanded turning circle diameter,

- Extended stopping distances and times,

- Impaired rudder effectiveness,

- The broader appearance of the wake.

ADVANTAGE OF SQUAT

While squat has the potential to cause a vessel to capsize, it is not universally disadvantageous. In certain scenarios, vessels strategically leverage the squat effect to navigate under bridges. When a vessel lacks the necessary air draft to clear a bridge safely, the controlled sinking induced by squat enables the vessel to pass underneath the structure securely.

CONCLUSION

In conclusion, understanding and managing the phenomenon of squats in ships is crucial for safe and efficient maritime operations. As we've explored, squat can have significant implications on vessel performance, particularly in terms of maneuverability, trim, and overall stability. The precautions outlined, such as calculating squat beforehand, factoring it into under-keel clearance considerations, and adjusting speed, are crucial for mitigating potential risks associated with this hydrodynamic effect.

Recognizing the signs that a ship has entered shallower waters is essential for proactive navigation, allowing crews to respond promptly to changing conditions. Whether it's the increase in squat, changes in trim, or alterations in various vessel dynamics, these indicators serve as valuable cues for mariners to adapt their approach and ensure the safety of the ship, cargo, and crew.

In the absence of precise predictions for squat, the reliance on empirical formulae becomes imperative. The maritime community continues to evolve its understanding of squats through practical trials and experiences, emphasizing a conservative approach to ensure reliability and safety in navigation.

As technology and research progress, staying informed about the latest developments in squat prediction methods and safety measures will undoubtedly contribute to enhanced navigation practices, fostering a more secure and efficient future for maritime transportation.