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You are here: Home / Ship Handling / Shallow Water Effect on Ship

Shallow Water Effect on Ship

Shallow water effects on a ship w.r.t “Draft & Maneuverability.”

• As the hull moves through shallow water, the area which it displaces is not so easily replaced by surrounding water, therefore, leading to a state of partial vacuum as the propeller and rudder are still working. The vessel take-ups longer to answer her helm, and response to engine movements becomes sluggish.

• In these circumstances, vibration will be set up, and it will be extremely difficult to correct a yaw or sheer with any degree of rapidity.

• At normal speed it is found that steering becomes erratic when the depth of water is equal to, or less than, one and a half times the deepest draught, i.e. a vessel drawing 8 m maximum draught will develop unsteady steering in water of depth 12m or under.

• When a ship Nearing an extremely shallow depth of water, such as a shoal, she is likely to take a sudden sheer, first towards it and then violently away. This is called smelling the ground, and the movements of a sluggish ship may suddenly become astonishingly lively.

• Due to the fact that the water displaced by a hull moving through shallow water is not easily replaced, the bow wave and stern wave of the vessel increases in height. Further, the trough which normally exists under the quarter becomes deeper and after part of the ship is drawn downwards towards the bottom. By reducing speed, the wave heights and trough depth will be diminished, and the vessel will not, therefore, close the bottom or squat.

• The speed of a vessel moving in shallow water should always be moderate; if the speed is increased the keel will close with the ground and the ship will sheer about unpredictably. If the bow wave and stern wave are observed to be higher than is prudent speed should be reduced – but not suddenly. If the speed is taken off rapidly the stern wave will overtake the vessel and cause her to take a sheer, which in a narrow channel could be disastrous.

Related Article:   Ship Interactions when Navigating in a Channel

Summary of shallow water effect on manoeuvring include:

  1. Bow wave increases.
  2. Manoeuvering becomes sluggish.
  3. The engine load increases.
  4. The ship speed over water reduces.
  5. Stopping distance and time increases.
  6. The turning circle increases to a great extent.
  7. Rolling and Pitching reduces.
  8. The ship may start to vibrate.

Effect of Shallow Water on Turning:

Effect of Shallow Water on Turning

Shallow Water Effect

 

Bow cushion and bank suction effect:

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

Related:

  • Basic Propeller Types
  • UNCLOS - Salient Features, Objectives, Maritime…
  • Shipboard Plate and Construction Terms
  • Ballast Water Management - Merchant Ships
  • Magnetic Compass & Ship Magnetism - Simplified Explanation
  • Slow Speed Control

By Cult of Sea Filed Under: Ship Handling Tagged With: bank suction, bow cushion, bow wave, pivot point, shallow water effect, Ship Handling, sinkage, sluggish, smelling the ground

Comments

  1. M.A.Siddiqui says

    May 2, 2018 at 12:42 pm

    Thank u

    cultof se
    You are excellent and very helpfull for us
    I need your help for Class -1(Master,s Oral Exam
    )
    Regards
    M.A.Sddiqui

    Reply
    • cultofsea says

      May 4, 2018 at 10:56 pm

      Really glad that this could be of help to you. Please contact us for any queries.

      Reply
  2. Jyotirmay Lahiri says

    February 27, 2019 at 12:15 am

    This site is very helpful. I recommend to all the first mates candidates to check your queries through this site.

    Reply
  3. Gavin says

    October 24, 2019 at 3:26 pm

    Lots of good info on this website, thanks!
    Could you explain in more detail why stopping distance and time increase in shallow water? I definitely grasp what you mean when you wrote “As the hull moves through shallow water, the area which it displaces is not so easily replaced by surrounding water, therefore, leading to a state of partial vacuum…” So from that I would think that the reduced pressure near the stern would make it easier to slow down a ship. I’d like to understand better why that isn’t the case.

    Reply
  4. Captain Jin says

    December 28, 2021 at 12:21 pm

    Thank you VM!

    I’d like to know the effects on rate of turn whilist entering the shallow water?

    Reply
  5. Rinof Francis Soares says

    February 26, 2024 at 7:39 am

    Please clarify speed through water in Shallow water, does it increase or decrease.

    Please clarify with example.

    Thanks

    Reply

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