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SHALLOW WATER EFFECTS ON SHIPS- BOW CUSHION AND SUCTION

 

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:

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

 

 

Vibration:

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

 

 

Steering:

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

 

 

Smelling the ground:

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

 

 

Squat:

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

Factors governing squat:

Squat varies on the following factors:

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

            Squat µ  V2    (V=speed in knots)

bullet Block co-efficient: Squat directly varies with CB.

            Squat  µ  CB

bullet 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  S0.81

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

bullet Squat may be calculated by the following simplified formulae:

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

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

 

Precaution

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

 

 

 

 

 

 

 

 

Bow cushion and bank suction effect:

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

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

 

 
  


 

 

 

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