Bernoulli’s law and continuity law
In order to understand the effect of a solid bank or wall on the behavior of moving ships along it, it is necessary to study pressure distribution around ship's hull and relevant basic laws governing flow phenomena.
Continuity law: Velocity x cross section = const
V1 x S1 = V2 x S2 = const.
Consequence: if cross-section decreases, velocity increases and vice versa
Bernoulli’s law: static pressure + dynamic pressure = const.
Static pressure = atmospheric pressure + head of water
Dynamic pressure = C x velocity squared
Consequence: if velocity increases, dynamic pressure increases, and static pressure and head of water decreases and vice versa.
When the ship is moving close to a solid wall or bank suction force is created drawing the ship closer to the bank. This is because of reduced cross-section, accelerated flow, and reduced pressure in the space between the ship and the bank.
The suction force is proportional to the speed of the ship squared and inversely proportional to the distance from the bank.
Suction forces calculated for example ship are shown below:
RESTRICTED WATER EFFECT
Suction force together with bow cushion effect make stern to move closer to the bank. Rudder is to be used to counter this effect.
Because of the proximity of the bank ship takes a sheer and suction force moves close to the stern.
USING SUCTION FORCE TO THE ADVANTAGE
PASSING THROUGH NARROW PASSAGE
Entering the passage closer to the bank helps to turn to starboard as needed. If the ship is entering closer to the island, suction is in the wrong quarters and opposes turning to starboard.
ENTERING OR LEAVING SHALLOW BANK
When the ship is entering a shallow bank then due to restricted cross-section and reduced pressure under the bow portion of the ship trim to bow may occur and the ship may hit the bottom with the bow.
When the ship is leaving the shallow bank and entering the deep-water area, the opposite may occur and the ship may hit the bottom with the stem.