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Ranger Hope © 2008, contains edited material courtesy of A.N.T.A publication’s.

 

1. Explain the meaning of the term ‘centre of gravity’.

2.  Explain the meaning of the term ‘centre of buoyancy’.

3.  With the aid of a sketch, explain the effect on a vessel’s centre of gravity when a weight is added to the vessel.

4.   With the aid of a sketch, explain the effect on a vessel’s centre of gravity when a weight is removed from the vessel.

5.  With the aid of a sketch, explain the effect on a vessel’s centre of gravity when a weight is shifted on the vessel.

6.  With the aid of a sketch, explain the meaning of the term ‘metacentric height’ (GM) of a vessel.

7.  With the aid of a sketch, explain the meaning of the term ‘righting lever’ (GZ) of a vessel.

8. With the aid of sketches, explain how the relative positions of LCG and LCB determine the way a vessel trims.

9.  Explain the meaning of the term ‘Equilibrium’.

10.  Explain the meaning of the term ‘Stable Equilibrium’.

11.  Explain the meaning of the term ‘Unstable Equilibrium’.

12.  Explain the meaning of the term ‘Neutral Equilibrium’.

13.  While in port you notice that your vessel is very tender.  You want to increase the vessel’s stability before you sail out.  List all the actions that you can take.

14.  Explain the meaning of the term LCF.

15.  Describe the information contained in a typical simplified stability data booklet supplied to small vessels.

16.  For what purpose is the simplified stability data supplied to vessels?

17.    When a weight is added on a vessel, its

          a) draft decreases and freeboard increases

          b) draft increases and freeboard decreases

          c) draft and freeboard remain unchanged

18.    When a vessel’s centre of gravity is raised, its stability is

          a) increased

          b) decreased

          c) unaffected

19.    If a vessel’s LCB is aft of LCG, the vessel will

          a) trim by the bow

          b) trim by the stern

          c) not trim at all

20.    If a vessel’s LCB is forward of LCG, the vessel will

          a) trim by the bow

          b) trim by the stern

          c) not trim at all

21.    The longitudinal centre of flotation (LCF) is the centre of a vessel’s

          a) underwater volume

          b) main deck area

          c) water plane area

22.    When a vessel trims, it rotates about the

          a) LCG

          b) LCB

          c) LCF

23.    When a weight is lifted by a crane, the centre of gravity of the weight is transferred to

          a) the point of suspension

          b) the base of the crane

          c) the actual position of the weight

24.    A stiff vessel has a

          a) negative GM

          b) large GM

          c) small GM

25.    A tender vessel has a

          a) negative GM

          b) large GM

          c) small GM

26.    A vessel’s roll period is governed by its

          a) GM

          b) length over all

          c) LCF

27.    If a vessel’s GM is reduced, its roll period

          a) becomes shorter

          b) becomes longer

          c) is unaffected

 

Check your answers and study the Workbook for topics you are unsure of then try the Stowage Assignment.

 

ANSWERS

1.  The centre of gravity can be imagined to be a point, through which all of the vessel's weight acts vertically downward.

2.  The centre of buoyancy is the centre of the underwater volume of the vessel.

3.   The centre of gravity moves towards an added weight.

Weight Added

4.      The centre of gravity moves away from a discharged weight.

Weight Removed

5.      The centre of gravity moves parallel to the movement of a weight which is already on board.

Shifting Weights

6.      The Metacentric height (GM) is the vertical distance between the metacentre and the centre of gravity.

7.      GZ is the righting lever. When a vessel is heeled by an external force, the centre of buoyancy moves towards the lowside, the force of buoyancy acts vertically upwards through B, (the centre of buoyancy), the force of gravity (weight) acts vertically downwards through G. The horizontal distance between the vertical lines drawn through these points is the size of the righting lever (GZ).

8.  If the position of LCG and LCB are as shown in Fig. 1, then the actions of buoyancy and weight will cause the vessel to rotate as shown by the arrow.  The stern will sink deeper, the bow will rise higher.  As the vessel rotates, the shape of the underwater volume will change and LCB will move to the new centre.

 

Figure 1  LCB Forward of LCG - Vessel trims by the stern

When LCG and LCB are in the same vertical line, the rotation will stop the vessel will be trimmed by the stern as shown in Fig 2

 

Fig 2  LCG and LCB in same vertical line  -  no trimming moment.

If the vessel had started with LCB aft of LCG then the rotation would cause a trim by the bow.

9.      Equilibrium is the term used to describe a vessel that is afloat.  It is a word made up of two words namely equal, and balance.  A vessel will float when the forces of weight and buoyancy are equal, and they balance - that is both B and G are in the same vertical line, and the vessel is not being acted on by an external force (a force other than buoyancy or weight).

10.  A vessel which will tend to return to the upright after being heeled by an external force, is said to be in stable equilibrium.  When G is below M the vessel is in stable equilibrium.

11.  If G is above M the ship is said to be in unstable equilibrium.  It will not remain upright.  It will heel to an angle called an angle of loll.  At that angle of loll it will have 'picked up' stability and will return to the angle of loll if disturbed by an external force.

12.  If G and M coincide, then, theoretically the vessel will have no reason to remain upright.  Also, if it is heeled, it will have no tendency either to heel further or to return to the upright.

13.  Stability can be improved by:

·        Lowering weights already on board.

·        Adding weight lower down.

·        Removing high weights.

·        Lowering suspended weights.

·        Keeping tanks completely full or completely empty.

14.  LCF is the centre of the shape of the waterline at which the vessel is floating.

15.  The booklet is set out in an approved format and contains the following information.

·        The vessels name, official number, port of registry, gross and net tonnages, dimensions, operating displacement, deadweight and draught.

·        A profile view of the vessel showing and naming all compartments, including tanks.

·        The capacity and the centre of gravity, vertical and longitudinal, of all spaces used to carry fish, water, fuel, stores etc.

·        Tank calibrations for every tank holding 2 tonnes or over, plus the free surface effect of every tank.

·        Information about the following hydrostatic particulars.

      (a)                           Displacement in salt and fresh water.

      (b)                           K M

      (c)                           T P C

      (d)                           L C B

      (e)                           L C G

      (f)                            Trim information

          These values are recorded for various draughts.

·        Sample loading conditions such as Lightship, Loaded departure from port, worst operating condition, etc.

·        Guidance notes and warnings dealing with such things as recommended distributions for fuel, water, cargo etc., recommended operating procedures and warnings about dangerous practices

16.       The purpose of this information is to let the master of the vessel know under what conditions the vessel will have sufficient stability.  If you operate a vessel so that its condition is better than the worst condition that is still safe, then you will know that your vessel has sufficient stability for normal conditions.

17.    b)

18.    b)

19.    a)

20.    b)

21.    c)

22.    c)

23.    a)

24.    b)

25.    c)

26     a)

27.    b)