1b Centre of Gravity

7/24/2019 1b Centre of Gravity

1/10

Centre of gravity/buoyancy (

1

CENTRE OF GRAVITY (G)

Is the point through which the tota weight force (!f) of the ship

can be consi"ere" to act verticay "ownwar"s. It#s position wi

"epen" upon the "istribution of the weight of the vesse an"

everything on boar".

$he position of % wi not &ove as the ship hees (provi"e" that

weights are not cause" to &ove within the ship). It always acts

vertically downwards!

G G

Wf Wf

!hen weights are shiftedon boar"' loadedor discharged% wi

&ove. !henever % is cause" to &ove the shift of %# &ust be

cacuate".


2/10


GG1= w d

W

w# is the weight shifte".

"# is the "istance through

which the weight is shifte".

!# () is the ship#s

"ispace&ent

Effect of shifting a weight a!ead" on #oa!d

!henever a weight area"y on boar" is shifte" % wi &ove

parallelto an" in the same directionas the shift of the centre of

gravity of the weight (g).

The vertical position of G is expressed in terms of metres above

the keel (KG).

G

G1d

w

$

"

ship displaces #$$$ t and has an initial KG of %.# m.&alc'late the final KG if a weight of $ t is moved vertically

'pwards from the lower hold (Kg .$ m) to the 'pper deck (Kg

.# m)


3/10


3

nswer

GG*+ w d + $ (.# , .$) + $.$*- m

#$$$

Initial KG + %.#$$ m

GG* + $.$*- m

/I01 KG + %.#*- m

KG has increased which wo'ld make the ship less stable.


4/10


*

GG1= w d

W w

w# is the weight oa"e".

"# is the "istance between % of

the ship an" g of the oa"e"

weight.

!# () is the ship#s initia

"ispace&ent.

Effect of oading a weight

!henever a weight is oa"e" % wi &ove "irecty towards the

centre of gravity of the oa"e" weight (g).

G

G1

$

"

ship displaces *#$$ t and has an initial KG of .# m.&alc'late the final KG if *$$$ t of cargo is loaded into the lower

hold at Kg 2.$ m.


5/10


+

nswer

GG*+ w d + *$$$ (.# , 2.$) + $.#3 m

w *#$$ *$$$

Initial KG + .#$$ m

GG* + $.#3 m

/I01 KG + .%* m

KG has decreased which wo'ld make the ship more stable.


6/10


,

GG1= w d

W%w

w# is the weight "ischarge".

"# is the "istance between % of

the ship an" g of the

"ischarge" weight.

!# () is the ship#s initia

"ispace&ent.

Effect of discha!ging a weight

!henever a weight is "ischarge" % wi &ove "irecty away

fro& the centre of gravity of the "ischarge" weight (g).

G

G1

$

"

ship displaces *-$$$ t and has an initial KG of #.2$ m.&alc'late the final KG if *$$$$ t of cargo is discharged from

the lower hold (Kg 2.$ m).


7/10


-

nswer

GG*+ w d + *$$$$ (#.2 , 2.$) + .-4# m

, w *-$$$ , *$$$$

Initial KG + #.2$$ m

GG* + .-4# m

/I01 KG + -.*4# m

KG has increased which makes the ship less stable.

It wou" be very te"ious to "o a cacuation for every singe

weight that was either shifte"' oa"e" or "ischarge" fro& the ship.

In practice &o&ents about the ee# are use" to "eter&ine the

fina % of the ship.

&O&ENT' (t) = WEIG*T (t) +I'TANCE ()


8/10


&oents a#o,t the -ee

2a&pe

A ship "ispaces 10000 t an" has a % of *.+ &.

$he foowing cargo is wore"

4oa" 10 t at g ,.0 &5

-30 t at g 3. &.

6ischarge , t fro& g .0 &5

100 t fro& g ,. &.

7hift , t fro& g . & to g ,.0 &.

Cacuate the fina %.

FINA. $G = '/& OF T*E &O&ENT'

FINA. +I'0.ACE&ENTWEIG*T (t) $G () &O&ENT' (t)

'hi (2) 13333433 5463 56333433

.oad (2) 173433 8433 973433

.oad (2) 9:3433 :473 7::8433

+ischa!ge () 8;433 7433 1:8433

+ischa!ge () 133433 8473 873433

< +ischa!ge () ;8433 7473 1;=473


9/10


8

"

ship displaces *%$$ t and has a KG of . m.

The following cargo is worked5

1oad5 *- t at Kg 4.$ m6

7ischarge5 2%- t from Kg %. m6

t from Kg #.2 m.

hift5 *-- t from Kg -.$$ m to Kg 2.$ m.

&alc'late the final KG.

WEIG*T (t) $G () &O&ENT' (t)

'hi (2) 15733433 8477 ;;:75433

.oad (2) 18;433 9473 173=483

+ischa!ge () :5;433 5487 1839498

+ischa!ge () 788433 64:8 1576498

< +ischa!ge () 1;;433 ;433 1635433


10/10

Centre of gravit /b (

10

CENTRE OF >/OYANCY(>)

Is the geo&etric centre of the un"erwater vou&e of the ship. It is

the point through which the tota force of buoyancy (9f) is

consi"ere" to act verticay upwar"s.

It#s position wi constanty &ove as the ship &oves in a seaway.

> >1

>f >f

$he position of 9 has nothing to "o with the "isposition ofweights within the ship.

$he centre of gravity (%) is assu&e" to re&ain in the sa&e pace

(provi"e" weights "o not shift within the ship) as the ship hees

but the centre of b'oyancy constantly moves as the ship pitches8

rolls and heaves.

:or a bo2;shape" vesse on even ee 9 is haf the "raught.

Documents

1b Centre of Gravity