Sunday, 28 January 2018

Shallow Water Effects

Any shipbuilding process starts with the owner signing the contract with a shipyard after giving the details of his requirements. Of these, the displacement and speed of the vessel are of utmost importance to the owner. If these requirements do not match with the owner’s specifications, then fine is levied on the shipyard. The shipyard builds the ship. Before the yard delivers the ship to the owner, it performs a lot of tests. One such test is the speed test. The objective of the test is to substantiate that the vessel has met the speed requirements of the owner. However, during the speed trial, the vessel speed is found to be less than the specified speed.
                So, is the shipyard at fault?
   Is there fault in the design?
   What could have gone wrong?
                Well, the answer to this lies in a phenomenon called shallow water effects. Actually, a ship is basically built to operate in deep waters but its speed trials are mainly confined to the shallow water areas which affect its speed.
Image Courtesy- www.google.com

                The reason for the drop in speed in shallow waters is that the water which is flowing just below the ship reaches very high speeds fast due to less water depth. We all know that when the velocity is high at any point in a liquid, there is a corresponding drop in pressure at that point which is explained by Bernoulli theorem. This is the same pressure which provides the upthrust and keeps the ship in floating condition, hence the ships will experience a bodily sinkage and its draft will increase. As a greater surface area is under water the frictional resistance will increase which will cause a drop in the speed of the ship.

                 Also, this drop in pressure is not the same everywhere. It is more in the fore part and less in the aft causing the ship to trim by fore which is an undesirable condition. This is known as ship squat.
Image Courtesy- www.google.com

EFFECT ON WAVE PATTERN


This drop in speed is not the only effect in shallow water, another prominent effect is the change in the wave pattern of the ship and hence the wave making resistance of the ship. Actually, the total resistance of a ship can be divided into frictional and residuary resistance. Wave making resistance forms a major part of the later. 
Image Courtesy- www.google.com

This is the general wave pattern developed when a ship moves through the water. As we can see two sets of waves are generated, transverse waves and diverging waves. The diverging waves are contained in an angle of 19O 28’ called angle of envelope. Let’s see what happens in shallow waters.
The general relation between speed of a wave (v) and wavelength (LW) in deep water is
                     V= (g*LW/2π)1/2
Now for a wave in shallow water,
        V= (g*h)1/2
We can see that the wave has a limiting value in shallow waters, varying with depth, known as critical velocity. When the velocity of ship increases, the angle of envelope increases, till the velocity of ship equals critical velocity, where the angle of envelope becomes 90O. At 90O, the transverse waves disappear.
                Now if the speed is further increased, the wave pattern changes completely. The angle of attack increases and the divergent waves become convergent in nature but no transverse waves are formed. So, there is a major change in the wave pattern. We know that the wave making resistance is dependent on the wave pattern. So, there is a difference between wave making resistance in deep and shallow water.

INTERMEDIATE SPEED


It’s pretty much clear that wave making resistance differs for deep and shallow waters. Let’s say a ship is going in deep waters at a speed V.
It will have some wave-making resistance.  When in shallow water the same wave making resistance will occur at a low speed which we call intermediate speed (VI).
This drop in speed is denoted by ΔC= V-VI, which is due to the wave making resistance component.
Now the total resistance of the ship in deep waters will be equal to the total resistance of the ship in shallow waters at a further lower speed. We denote this speed as Vh. This drop in speed in shallow water is denoted by ΔVP. This drop in speed occurs due to the augment in the frictional resistance component in shallow water.
Image Courtesy- Principles of Naval Architecture Volume-2

EFFECT ON TURNING CIRCLE


The diameter of a ship's turn varies with several factors in addition to rudder angle, and water depth is one of them.  For maneuvering, deep water can be assumed when a water depth of more than five times the ship's draught is available. At three times the draught, the shallow-water effects come into action. When depth decreases from twice the draft the effects become more prominent. So, water depth plays a key role in the maneuvering of a ship and if it is neglected it can give wrong interpretations.  The rate of turn depends on the ship's directional stability, and though the rate increases at first on leaving the deep water, it decreases as shallower water is reached. These changes in the rate of turn are comparatively small to be perceived by the ship handler, but in combination with a smaller speed-loss in the turn, as under keel clearance decreases, it results in an increase in the ship's turning-circle diameter.
Image Courtesy- www.google.com

INTERESTING FACTS


• A fishing boat normally can run at 10 knots in deep water. If the water depth is 14 feet, then we can expect a speed loss of about 4%. That means that its speed is reduced by about one-half knot.
• A supply boat is expected to run 15 knots in deep water. However, during sea trials in 20 feet deep water, it did not make this speed. The 14 % typical speed loss means that we would expect a bit less than 13 knots on trial.
• A river workboat is trying to run at 10 knots in 9 ft. of water. It will lose almost a knot and half.

SUMMARY OF WHAT HAPPENS IN SHALLOW WATERS


1.       Ship maneuvering becomes sluggish.
2.       A decrease in ship speed.
3.       Change in the wave pattern.
4.       A decrease in propeller rpm.
5.       Increase in turning circle diameter.
6.       Increase in stopping distance and stopping time.


Article by: Anil Kumar Singh

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