Shallow water

For manoeuvring purposes, the depth less than twice the draft of the vessel is considered to be shallow water.

Now, you must be wondering why so?
It is because at this depth, the seabed causes a drag on the vessel, which changes it’s manoeuvring characteristics.
Now, that we have understood what is shallow water, let us see and understand what are it’se effectson the ship:

As the vessel enters shallow waters, the vibration throughout the ship increases. This is because the hull vibration almost matches with the engine vibrations.
The bow waves increases, which increases the load on the engine. Hence the ship’s speed should be reduced.
Appearance of muddy water at the stern part of the vessel.
Vessel’s turning circle increases. It increases by as much as 3 times than in the deep waters.
Directional stability of the vessel increases. Directional stability means the ability to maintain a
single heading, i.e. it requires more helm to change the vessel’s heading.
The steering of the vessel becomes sluggish as compared to that in deep waters.
Pivot point of the vessel shifts a little bit aft, about 1/3rd of ship’s length from bow. This is the
reason for vessel’s steering being sluggish.
Squat increases. Squat is defined as reduction in UKC as the vessel moves through the water.
Stopping distance increases when astern movement is given. Although propeller is moving
astern but there is a void space in astern and there is not enough water for griping, hence
resulting in increase of the stopping distance.