The ‘Bank effect’ is a term that refers to the ability of the stern of a ship to swing towards the bank side when the ship is present in a river or around a constricted waterway. Stern of the ship is the area just opposite to the front of the ship i.e. the back portion of the ship.
The flow around a ship is usually asymmetric. This flow is induced by the presence of banks which in turn causes pressure differences (according to Bernoulli’s principle) between the port of the ship and along the starboard sides. As a result of this, a lateral force will eventually act on the ship which is mostly directed towards the nearest bank around the ship. This squat effect is increased owing to the decrease in the blockage. This phenomenon entirely depends on many known parameters, such as bank shape, water depth, the distance between ship and bank, properties of the ship, speed of the shop, and the propeller action. A reliable estimation of the bank effects is very important in determining the limiting conditions in which a ship can safely work and move in a waterway.
This phenomenon has several different names, some of the names are bank suction, bank cushion, stern suction, and ship-bank interaction. In a channel that is restricted, when the ship moves near the bank, the bow is then pushed away from the bank, this accounts for an effect which is known as the bow cushion, and the vessel is then attracted towards the bank, this effect is known as bank suction. The bank cushion effect results from high pressure that is build up between the bank and the bow of the ship.
The bank suction is generally caused by the loss of pressure and increased velocity of water in the space between the vessel and the bank. This space is generally restricted and there is an asymmetric flow around a ship that is caused by the vicinity of banks. This causes quite some pressure differences between the port and the starboard sides. As a result of this, there is a lateral force that acts on the ship, which is mostly directed towards the closest bank. However, the knowledge of the bank effects that are induced by bank geometry is very limited. Bank Effects generally offers researchers and other pilots the possibility to discuss the latest developments in the field of research and practice that is related to the ship behavior in the vicinity of banks.
Some institutions are involved in the research of ship maneuvering in lesser and restricted waters, ship to ship interactions, bank effects, etc. One of the institutions is the maritime division of Ghent university. When a fine bowed vessel slides through smooth blue water, the bow seems to cleave or to separate the water very smoothly by slicing a passageway through which it passes gracefully. Some boats push more water than the others, mainly depending on the size, hull shape, and speed. On an open ocean, it doesn’t matter where this water will go, but in waterways that are confined can interact with the banks, bulkheads, the bottom, and the hulls of passing vessels in both predictable and unpredictable ways.
Bank effect is one of those several related phenomena that we already know to be associated with pressure differences that form around any moving vessel in some confined waters. The principle of fluid dynamics is most closely associated with Bernoulli’s Law, which provides all the mathematical explanations for the formation of these asymmetrical zones of pressure differences. In the case of the bank effect, the water that is displaced while squeezing between the bank and the hull produces a difference in pressure between the port and starboard sides, and also in between the bow and the stern. In case of the right conditions, the pressure differences that are created can dramatically affect the handling of the vessel.
Bank effect has two main elements as we saw above, the bow cushion and the stern section. You might have seen how a vessel pushes the water forward, this push creates an area of high pressure that laps up on the stem. In case of open water, the bow wave dissipates easily causing no harm but in case of a narrow waterway, the bow wave often encounters the embankment from one side, forming a cushion that forces the bow away from the bank and towards the zone of low pressure on the other side of the bow.
To make everything go well and on time, the water must flow a bit faster on the side that is closest to the bank than on that side which is away from the bank. This usually triggers a drop in pressure which causes a lateral attraction towards the bank. The propeller pulls the water from underneath the hull can add to this effect also. The combinations of the bow pushing away and the stern sucking in often creates the efficiency for a vessel to sheer randomly towards the midchannel. This thing might not be of significant importance unless another vessel is present there. Most of the time, the bank effect does not account for much. In general, if we talk about the encounter with the bank effect, we can see that it depends mainly on three things which are hull shape, speed, and waterway configuration. To this effect, shallow water and prop action also can contribute.