Скачать или смотреть How Squat, Bank & Bank Cushion Effects Influence Ships in Restricted Waters?

Any ship (regardless of its size) navigating through restricted waterways is heavily affected by hydrodynamic effects. In this video I will show three of the most common effects experienced by ships – Squat, Bank and Bank cushion effects.
Squat effect: When a ship moves through the shallow water, some of the water displaced rushes under the vessel to rise again at the stern. This decreases the upward pressure on the hull, making the ship sink deeper in the water than normal and slowing the vessel. This is known as squat effect, which increases with the speed of the vessel.
Bank effect: Bank effect refers to the tendency of the ship’s stern to swing towards the near bank when the ship is operating in a river or restricted waterway.
Bank Cushion effect: When the ship is near to the bank, the water is forced between the narrowing gap between the ship’s bow and the bank. This water tends to pile up on the starboard side of the ship, causing the ship to sheer away from the bank.
Before I move ahead, let’s understand some of the basic principles of physics we will be using to understand these effects.
What do you notice? – Can you see the increase of speed along the dock?
You will notice that the liquid flows at a faster pace in the constricted area in.
Why does this happen?
There is an equation in physics that is known as “ Continuity Equation” which states that when a fluid is in motion, it must move in such a way so that the mass is conserved. This means that in a continuous flow of fluid , the mass of fluid passing through point A is equal to that at B and also at C, in unit time.
Clearly, the mass of the fluid passing through the cross section (shaded area) at point A will be equal to that passing through point C or B.
To make this happen, the rate of flow of water at constricted point C must be more than that at A or B. This also means that the velocity of flow at a constricted point is always greater so as to satisfy the Continuity Equation.
Theoretically, if A is the cross section of the cylinder and v is the velocity of an inviscid (non sticky) fluid, then A is inversely proportional to v, i.e., Av = constant.
Now one may ask why we are delving into all these when the topic is Squat effect and Bank effects on ships passing through restricted waters? Simple, to clearly understand these effects we will have to realize a few basic principles.
The next is Bernoulli’s Principle, which states that for an inviscid flow (flow of an ideal fluid that has no viscosity), an increase in the speed of the fluid (i.e. its Kinetic Energy) occurs simultaneously with the decrease in pressure or decrease in its Potential Energy and vice-versa. Actually this principle is a corollary of the Law of Conservation of Energy which says the sum total of all the energies in an isolated system (roughly speaking no energy can be transferred into or out of this system) always remains the same.
Thus in my video, the increased speed at constricted point C caused a decrease in pressure at that point. If you can see it in the video!
The video is taken from my morning trip with the ferry to the Danish Island Ærø! i take this ferry every morning the whole year arround in every weather condition to come to my work at the simulator at Marstal Navigationsskole - www.marnav.dk