Stability – I : Chapter 10

18. A ship has a very small GM it is decided to fill up SW ballast , one tank at a time , in six tanks whose particulars are :-

Tank	‘i’ about tank’s  centre line
NO 1 DB	280m4
NO 2 O or S	600
NO 3 P or S	350
NO 3 C	650

State the order in which the tanks must be filled so as to keep FSC at a minimum at all times.

19. An unstable vessel is it her angle of loll. The following tanks are available for SW ballast :-

NO  1  DB	400m4
NO  2   P	700
NO  2  S	700
NO  4  P	250
NO  4  S	250
NO  4  C	800

If it is decided to ballast three tanks with SW, one at a time , state what order should be followed so as to keep FSE to a minimum .

20. A vessel of W 8000t, KM 7.9m, KG 7.0m,has a tank 15m long and 12m wide , partly full of HFO of RD 0.95 .

    • Find her moment of her statical stability at 6degree heel
    • If her BM is 4.9m, find her moment of statical stability at 20 degree heel assuming that she is wall- side.

Solution:

Given:
W = 8000t, KM = 7.9m & KG = 7.0m
Area of tank = (L X B)
= (15 x 12) m².

RD = 0.95, θ = 6degree

We know that:
FSC= (LB³ x di )/( 12 x W)
FSC = (15 x 12³ x 0.95)/(12 x 8000)
= 0.256m

We know that GM can be calculated as
GM = (KM – KG)
= (7.9 – 7.0) m
= 0.9m

Again, RM = (W x GM sinθ )
So, RM = (8000 x 9 x sin 6 ⁰ )
= 538.5tm

B).  If the BM is 4.9m find the moment of statical stability at 20 degree heel assuming that the ship is wall sided .

Given:

BM = 4.9 m, θ = 20 degree,
RM = (W x Sinθ (GM + 1/2 BM tan² θ)
RM = (8000 x Sin 20 (9 + ½ x 4.9tan ²20 ⁰)
Hence, RM = 2650 tm

GM fluid = (0 .9 – 0.256)
= 0.644 m.