The Physics of Shipping in Constricted Waters

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After days of arduous labor, the Ever-Given container ship was finally released allowing for the international commerce movement across the Suez Canal to resume. The giant Japanese-owned and Taiwanese-operated cargo ship, Ever Given, got stuck in the internationally significant waterway, the Suez Canal, on 23 March 2021. With a length of 400 meters, which is the Canal’s maximum capacity, the result was a complete block, bringing international commerce to a halt. Now, the ship is freed and focus is directed towards investigating the causes of the incident. In light of this incident, we picked three physical phenomena that affect vessels passing through constricted waterways to tackle in this article: the squat effect, the bank effect, and the bank cushion effect.

Some basics first, how do ships float in water? There are different forces that act on bodies, and the force resulting from their interaction is known as the net force. When the forces are inequal or unbalanced, the net force affects the movement of the body. There are two forces acting on the ship: the gravitational force, which tends to sink the ship, and the buoyancy force, which tends to float it. These two forces are equal in magnitude and opposite in direction; they cancel the effect of one another and the net force equals zero. As a result, the ship can maintain its optimum position while sailing.

Figure 1: A net force that equals zero allows the ship to float. Source.

The squat effect happens due to pressure drop under the ship in the shallow waters; but what causes this drop? The answer lies in the continuity equation. In simple terms, the mass of a flowing liquid is conserved; hence, the decrease in the liquid’s flow velocity causes an increase in its pressure and vice versa. This is Bernoulli’s theorem; the continuity equation means that a liquid’s velocity increases when passing through narrower vessels. Now, how does this apply to ships? As the ship moves forward, it pushes water; this pushed water is replaced by the flow of water surrounding the ship. In constricted waterways, this flow becomes restricted, which causes higher velocities as per the continuity equation. Higher velocity in turn means lower pressure, as seen in Figure 2. This lowered pressure disrupts the balance between gravitational and buoyancy forces creating a net force downwards, making the ship sink.

Figure 2: Bernoulli’s theorem and creating lower pressures under ships in restricted waterways. Source.

What about the bank and the bank cushion effect? If you are not familiar with the ship parts, kindly check (Figure 3); particularly the stern and the bow.

Figure 3: Parts of a ship or a boat. Source.

This phenomenon has been well-identified since 2009; when sailing near the bank, the water between the ship and the bank speeds up. Recall Bernoulli’s theorem; this means reduced pressure on the side facing the bank. The difference in pressure creates a suction force that attracts the ship’s stern towards the bank. However, the situation around the bow is the exact opposite; the pressure increases resulting in a rejection force instead of a suction force. This is the bank cushion (also called bow cushion) effect. Both effects are illustrated in (Figure 4).

Figure 4: Suction force on the stern and rejection force on the bow. Source.

The effects we tackled increase with speed. So, ship captains have to monitor their speed when sailing through restricted waters to minimize these effects, and guarantee the safety of their voyage. As the world is holding its breath awaiting the findings of Ever Given crisis investigations, you dear reader have now become familiar of some of the basic scientific principles involved in shipping through constricted water paths, like the Suez Canal.

You can watch this video on the bank effect:

References

ft.com
myseatime.com
washingtonpost.com
aceboater.com
yadda.icm.edu.pl

Banner image reference: edition.cnn.com

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