What is Bank Effect?
What is Bank Effect?
Forces and movements influencing the vessel while the vessel is navigating near the bank such as Bow Cushion and Bank Suction Effect is called Bank Effect.
Bow Cushion: -
When a vessel is steaming in the centre of the river or canal where both sides are with permanent solid wall structure, the flow of the water on the port and starboard side would be equal. When the same vessel navigates near the bank, the water flow is forced to pass between the narrow gap of the ship’s bow and the bank. This water tends to pile up on the starboard side of the ship causing the ship bow to sheer away from the bank or you can say the bow is pushed away from the bank by the flowing water.
Bank Suction: -
As the bow pushes away from the bank the stern draws near to the bank which is called bank suction. In other words, as the bow cushion happens the vessel moves bodily towards to the side of the bank which appears stern which is sucked by the bank. The bank suction is caused by the decreased water pressure at the stern.
How to control these effects?
To control the effect of the bank and bow cushion when a vessel is navigating near to the edge of the bank, the vessel had to apply controlled helm towards the bank and reduce speed. By doing the above manoeuvre’s it is possible to maintain the vessel parallel to the bank.
Factors that contribute to these effects: -
1. 1.Distance between Ship and Bank ,as the vessel Closer the distance between the bank more the effect since the flow of water at the edge of the bank are more.
2. Ships Speed when the vessel moves faster, she will develop a larger bow wave compared to the same vessel proceeding slower. As more water is displaced it may not get out of the way easily. Thus, it piles up between the bow and the bank, increasing the bow cushion.
3. Depth of the canal or depth near to the bank ie., reduced UKC would contribute to these effects
4. Bank Geometry
5. RPM ,higher Revolution will generate more water beneath the hull helping to sustain the low-pressure at the stern which will boost for the bank suction.
Why Bank Effect happens?
Bank Suction: -
To understand the bank effect, let us consider a box shape (i.e.) container vessel steaming closer to the bank or canal where both sides are with permanent solid wall structure as she is moving forward, she will displace/push water forward. The displaced water will flow either side of the vessel (i.e.) port and starboard side from forward to aft of the vessel to fill the void space generated by the passing vessels hull.
Since the gap between starboard side of the hull and the bank is very less the flow of water will be faster to fill the void space created by hull as compared with the flow of water on the port side.
In addition to the Bernoulli’s Law ,the above principle can be understood from the equation in Physics that is known as “ Continuity Equation” This causes a drop in pressure or negative pressure in the starboard side of the hull which in turn creates suction effect causing bodily shift of the vessel or we can say suction of the vessel to the side of the bank which appears the stern to be sucked by the bank.
The above principle will not affect much in the open ocean in deeper depth since the area is surrounded by ample of water without any permanent solid wall structure. The water pushed by the vessel will be replaced immediately and the flow is not restricted. Hence the “Continuity Equation” in physics will not exist in open water.
Bow Cushion: -
The same phenomenon is causing high pressure (Positive Pressure) build-up between the bank and bow of the ship causing the bow of the ship on the other hand to reject from the bank on a moving ship which is close proximity of the bank.,
let us see what is meant by Continuity Equation in Physics.The picture below describes about Continuity Equation in Physics ,A gardener uses the hose pipe to water the plants. In order to water the plants at a longer distance, he would control the stream of water flowing from the tap by compressing the hose pipe with his thumb.
As you can see the diameter of the hose pipe as marked in Region A compared with the diameter of the compressed hose pipe tip is smaller. Even though the compressed hose pipe tip is smaller, the amount water coming out from the tap tip will be the same as region A.
In order to flow the same amount of water in the compressed tap tip the water flow will be faster in the compressed tap tip causing the water to reach at the longer distance in the garden this is called Continuity Equation.
The Region A in the above example can be compared when a vessel is steaming in the centre of the river or canal where both sides are with permanent solid wall structure, the flow of the water on the port and starboard side would be equal.
The compressed tap tip can be compared with the same vessel steaming closer to the bank or canal where both sides are with permanent solid wall structure, the gap between starboard side of the hull and the bank is very less the flow of water will be faster creating a negative pressure near the hull thus causing the vessel to suck bodily towards the bank .