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Describe the various effects on a ship behavior, which can be expected as a result of entering shallow water.

When there is limited UKC the restriction in the velocity of the water flow which causes a drop in pressure. This reduces the buoyancy force of the v/l. since the weight of the ship unchanged the v/l will tend to sink further thereby increasing draught in order to resolve equilibrium. There is also likely to be a change in trim because the LCB is likely to change thereby creating a trimming moment.

EFFECTS:  

1.      Vessels take longer to answer helm.

2.      Response to engine movements becomes sluggish.

3.      Vibrations will be set up.

4.      Extremely difficult to correct a sheer.

5.      When a ship is nearing an extreme shallow depth of water such as shoal. She is likely to take a sudden sheer, first towards it and then away.

6.      The bow waves and astern waves of ship increase in height.

7.      The trough which normally exist the quarter become deeper and the after of the ship drawn downwards towards the bottom.

8.      Increase of time due to squat.

9.      The increase in the propeller speed, increase efficiency of the rudder but will not increase the ship’s speed.

10. Transverse thrust of the propeller will change.

11. Minimum RPM to maintain steerage is more than normal.

12. Color of water changes.

(a)  Explain the term synchronous rolling and describe the dangers if any associated with it.

This occurs when the natural period of roll is equal to the apparent period of wave. When this occurs the wave gives the ship a push each time she rolls (like a swing) causing her to roll more and more heavily. This effect is known as synchronous rolling.

DANGERS:

1.      Possible danger of capsizes.

2.      Cargo shifting due to heavy rolling.

3.      Possible cargo damage and structural damage, personnel injury.

4.      Dangers of free surface effect.

5.      Possible machinery / Nav. Aids damage.

6.      Ship is more vulnerable if engine break down occurs.

(C) Describe the action which may be taken by the ship’s officer when it becomes apparent that the vessel is experiencing synchronous rolling.

1.      Alter course this will alter the apparent period of the waves, an alteration of course towards the is likely to be particularly effective, as it reduces the apparent period of the wave.

2.      Alter speed (effective if the area not abeam).

3.      Change GM or distribution of weights aboard the vessel by ballasting/deballasting / shifting weights.

a)     What is meant by squat and explain how does it occur.

SQUAT:

This is a term used to define changes in draught and trim which occurs when the depth of water beneath the vessel is less than one and a half time the draught of the vessel when travelling at a significant speed.

CAUSES:

                  When there is a limited clearance under the keel the restriction increases the velocity of water flow which causes a drop in pressure thereby reducing the buoyancy force on the vessel. This effect is increased still further when vessel is in the confined channel since the velocity of water flow must increase due to further restriction.

                Since the weight of the vessel remains unchanged the ship will have to sink further thereby increasing her draught in order to restore equilibrium. There is likely to is a change in trim since the LCB likely to change therefore creating a trimming moment. Where LCF is greater than LCB there will be a trimming moment at astern, where LCF is less than LCB there will be a trimming moment by the head and where LCF = LCB there will be no trimming effect and maximum squat will be of equal value at fwd and aft.

b)     List the factors, which effect the magnitude of squat.

1.      Speed of the ship.

2.      Draught / water ratio.

3.      Propeller revolution.

4.      Form of bow waves.

5.      Length / breadth ratio.

6.      Block co-efficient.

7.      Change width / beam ratio.

8.      Initial trim.

c)     Describe the overall effect of shallow water on the maneuvering characteristics of a vessel.

1.      Speed of the vessel decreases as squat is directly proportional to square of speed.

2.      R.P.M. decreases and high R.P.M. increases astern trim.

3.      Higher the draught to depth of water ratio greater the squat which results in lesser U.K.C.

4.      Vibration may occur.

5.      In shallow water squat causes abnormal bow and stem wave to build up there by the type of bow effects wave making and pressure distribution.

6.      Steering is effected because the water displaced by the hull is not so easily replaced by other water and the propeller and rudder might be working in partially vacuum conditions. The vessel takes long to answer her helm and response to engine movement become sluggish.

7.      It will be extremely difficult to correct a yaw or sheer with any degree of rapidity.

8.      The moving vessels bow wave, stem wave and trough increase in amplitude.

SIGNS OF SQUAT  

1.      Speed decreases.

2.      RPM decreases.

3.      Vibration may occur.

4.      Steering is affected vessel become sluggish to maneuver.

5.      Ship made waves increase in amplitude.

6.      Ship wake changes color and becomes muddy.

a)     Itemizes the contents of an approved ship’s stability book.

1.      General particulars (e.g., ship’s name, port of registry, GT, NRT, LOA. Breadth, DWT, Draft to summer load lines.

2.      General arrangement plan.

3.      Capacities and C.O.G. (cargo spaces, fuel, F.W, Ballast tanks, stores etc.)

4.      Estimated weight and disposition of passengers and crew. 

5.      Estimated weight and disposition of dk cargo (including 15% allowance for timber dk.cargo)

6.      Dead weight scale (displacement, DWT, TCP, MCTC)

7.      Hydrostatic particulars (Displacement, TPC,  MCTC, LCB, LCF, KM)

8.      Free surface information (including an example)

9.      KN tables cross curves (including an example)

10. Pre-worked ship conditions (light ship. Ballast. Arr / Dep, service loaded Arr. / Dep. Homogenous loaded Arr./Dep. Dry Docking etc.). To include for each condition profile diagram indicating disposition of weights, statements of light weights plus disposition pf weight onboard, Metacentric height (GM curve) statical stability (GZ curves). Warning of usage conditions.

11. Special procedures (cautionary notes)

12. Inclining experiment report.

13. Information for longitudinal stresses (For v/ls over 150 m in length).

14. Loading / Discharging / Ballasting sequence for long vessels.

15. Worked KG example of “icing”.

16. Maximum Draught Forward and Aft.

17. Wind heeling moment for high deck cargoes.

18. Maximum height of deck cargoes.

19. Damage stability conditions.

A.    Flooding and damage stability requirements for type A and type B ships.

B.     Flooding and damage stability requirements in the flooded conditions.

C.     Flooding and damage stability information to be presented from flooding conditions.

D.    Flooding and damage stability typical sketches required.

b)     Give example of special cautionary notes for the Master, which may be included in this book.

1.      Required minimum bow height always maintained the Forward draught should not exceed.

2.      Sequence of Ballasting to enable adequate stability throughout the voyage.

3.      Warning against large angle of heel, produced by strong beam wind.

4.      Dangers of icing if the vessel is trading in severe winter conditions.

5.      Incase of Timber deck cargo absorption of water should be considered up to 15% of its own weight.

6.      Special precautions when loading bulk grain.

7.      Recommended minimum draught for heavy weather conditions.

8.      In case of vehicle ferry, the KG of the compartment for carriage of vehicles shall be based on the estimated center of gravity of vehicle and not the volumetric KG of the compartment.

9.      Information’s to enable free surface effect.

10. Any special features regarding the stowage or behavior of cargoes.

If the calculated Metacentric height during Dry Docking is found to be in adequate. Explain clearly the practical measures that can be taken to remedy this, prior to Dry Docking.

1.      Reduces the trim to the minimum so that the critical period reduces significantly.

2.      When the vessel takes the blocks, the “G” will rise due to the “P” force, which acts vertically upwards, from keel blocks.

3.      Therefore, calculate the maximum trim taking into account the virtual loss of GM not more than 0.2 m, so that the vessel can have the adequate GM when she is sitting on the blocks.

4.      Any free surface in the tanks should be removed or reduced to as little as possible either by emptying the tanks or pressing it up to the full conditions.

5.      Sound all the tanks before entering the Dock, to be aware of quantities aboard and note all the soundings in the sounding book.

6.      Empty the wing tanks if possible. Stow derricks, cranes and riggings in stowed position re-arrange the deck cargo, or cargo in between deck if any, to L.H, Ballast the D.B. tks. (press up).

Q. NO. 6  DEC’90

A.     State the surveys required in order that an international load line certificate remains valid.

1. Annual survey.        2. Renewal survey every 5 years.

B.     List the items and state the nature of the exam. Required for each item at these surveys.

Preparation should be commenced three months before the expected date of the surveys.

1.      Check all access openings at ends of enclosed structure are in good condition, all daubs, clamps, and hinges should be free and well greased.  

2.      Check all cargo hatches and access to holds for water tightness, especially battening device such as cleats and wedges.

3.      Securing of portable beams.

4.      Tarpaulins must be in good condition and two for each hold.

5.      Check all machinery space openings on exposed decks.

6.      Check all ventilator openings are provided with water tight closing.

7.      All air pipes must be provided with permanently attached satisfactory means for closing and openings.

8.      Check all manholes and flush scuttles are water tight.

9.      Inspect cargo ports below free board deck for water tightness.

10. Non-return valves on over board discharge are operating satisfactorily.

11. Side scuttles must have internal water tightness.

12. All freeing ports to be in good working condition.

13. All guard rails and bulwarks in satisfactory condition.

14. Rigged lifelines required to be filled in certain areas.

15. De-rust and paint the deck line, load line marks and draft marks.