SHALLOW WATER EFFECTS

Shallow water:

When the depth of water is less comparing to the draft of the ship. The hydrodynamic forces affect the ship handlings in different ways. The effects become evident when the depth of water is less than 1.5 times of the draft of the ship.

In shallow waters, following effects may be evident:

Sluggish movement

Vibration

Erratic steering, slow response.

Smelling the ground

Squat

Bow cushion and bank suction effect

Canal effect

Sluggish movement:

	As the hull moves along the water, the water which is displaced is not instantly replaced by surrounding water.
	A partial vacuum is created.
	The vessel takes longer to answer helm.
	Response to engine movement becomes sluggish.
	Speed reduces.

Vibration:

	In shallow water vibrations set up.
	It becomes very difficult to correct a yaw or sheer with any degree of rapidity.

Steering:

	Steering becomes erratic.
	Rate of turning is reduced.
	Turning circle becomes larger.
	Loss of speed due to turning is less in shallow water.

Smelling the ground:

	Occurs when a ship is nearing an extremely shallow depth of water, such as a shoal.
	The ship likely to take a sudden sheer.
	The sheer is first towards the shallow, then violently away from it.
	The movements of a sluggish ship may suddenly become astonishingly lively.
	These effects are called smelling the ground.

Squat:

	Water displaced by the hull is not easily replaced.
	Bow wave and stern wave increase in height.
	Trough becomes deeper and after part is drawn downwards.
	Under keel clearance decreases.
	This effect is called squat.

Factors governing squat:

Squat varies on the following factors:

Ship's speed: Squat is directly proportional to the square of speed.

Squat µ V²(V=speed in knots)

Block co-efficient: Squat directly varies with C_B.

Squat µ C_B

Blockage factor (S): It is the ratio between cross section of the vessel and cross section of the canal or river. Squat varies with blockage factor as.

Squat µ S^0.81

So, in confined water, squat is more than in open water.

Squat may be calculated by the following simplified formulae:

Squat = (C_B X V²) / 100 (In open waters)

Squat = 2 X (C_B X V²) / 100 (In confined waters)

Precaution

	Squat may cause grounding in spite of enough UKC.
	Squat to be calculated beforehand.
	Speed to be reduced to reduce squat.
	While determining UKC, squat for the speed to be taken into consideration.

View more about squat

Bow cushion and bank suction effect:

	Occurs in narrow channels near proximities of banks.
	There is a tendency for the bow of a ship to be pushed away from the bank, called bow cushion.
	The ship moves bodily towards the bank, which appears at the stern, called bank suction.
	Caused by the restricted flow of water on the bank's side.
	Velocity of water to the bank increases and pressure reduces.
	Results in drop of water level towards the bank.
	As a result, a thrust is set up towards bank.
	A vessel approaching to the bank will have to apply helm to the bank and reduce speed to prevent the sheer from developing.

Canal effect:

	Water level drops towards a bank.
	Vessel heels towards bank to displace constant volume.
	Varies as the square of speed.
	Corrective helm to be applied.