• No results found

Ship Stability, Statical Stability, Free Surface Effect ,Correction of and Angle of Loll.


Academic year: 2021

Share "Ship Stability, Statical Stability, Free Surface Effect ,Correction of and Angle of Loll."


Loading.... (view fulltext now)

Full text



Group Award Code: G8F5 15

Unit Code: F0LD 34 Ship Stability

Outcome 3 – Statical Stability

3.5 Free Surface Effect

& Correction of and



To give the student an understanding of:

the creation of Free Surface Effect (FSE);



To give the student an understanding of:

how FSE can be reduced/eliminated;



The Student will be able to:

Describe with the aid of sketches the

effects of FSE in part filled compartments

containing fluids;

calculate the reduction in GM caused by

FSE, as an adjustment to KG/GM (Single

Weights), or by the inclusion of FSM‟s into

the Moment about the Keel table (Multiple




The Student will be able to:

Calculate the new FSE if a compartment is


Describe the correct procedures required

to carry out the correction of and angle of

loll without putting the vessel in further


Free Surface Effect

Showing the

vessel at rest

with a part

filled undivided

double bottom

tank. The GM

shown is the



, all of

which are on

the centreline

of the vessel.



Free Surface Effect

When the vessel in stable equilibrium is inclined by an external force, buoyancy is lost on the raised side and an equal amount

created on the submerged side.

This creates a shift of buoyancy from b to b1 in the vessel, moving the overall buoyancy of the vessel along a parallel line from B to B1.

This creates a righting lever of GZ. K B G W L M W1 L1 b b1 B1 Z Δ Δ


Free Surface Effect

As the ballast moves to the low side this causes a shift of weight of g to g1

This causes a shift of the overall centre of gravity of the vessel G along a parallel

line to a new position of G1.

This reduces the

righting lever to G1Z1. K B G W L M W1 L1 B1 Z g g1 G1 Z1


Free Surface Effect

If a perpendicular line is drawn upwards through G1 to the centreline of the

vessel, the GZ can be redrawn between the

centreline and the BM line. This gives G2Z2 which is equal to G1Z1.

The distance along the centreline measured

between G and G2 is the “virtual loss of GM”.

This is also known as the Free Surface Effect (FSE).

M G2 Z2 G1 G Z Z1 θ° Vir tua l Lo ss o f G M ( FS E)


Calculating FSE

The stability information required by law to be

supplied to a vessel must include information on

the effect of free surface of liquid in the tanks

and also how to correct the GM for this effect.

Information is usually supplied for each tank in

the form of "Free Surface Moments".

FSE = Free surface moment




Calculating FSE

If there are several Free Surface Moments

involved, then they should all be added,

then divided by the displacement.

FSE = Σ Free surface moments


Key Points

 FSE does not depend upon the weight of liquid in the

tank, providing the area of the free surface remains unchanged.

 FSE does not depend upon the position of the tank

within the ship.

 FSE is zero if a tank is full or empty

 Every slack tank contributes it‟s own FSE to the total

FSE for the ship therefore to reduce FSE keep the number of slack tanks to a minimum.

 If it is decided to improve stability by filling a DB tank

then FSE will worsen the situation before the increased bottom weight is sufficient to bring G down. If at an angle of loll then fill the smallest tank, on the lowest side first.


Execise 1

A vessel has a KM of 5.13m, KG = 4.82m and the FSE = 0.11m. Calculate the effective (fluid) GM.

KM 5.13 m

KG - 4.82 m GMSOLID 0.31 m

FSE - 0.11 m (FSE is always negative) GMFLUID 0.20 m GMFLUID is the effective GM



FSM and therefore FSE can be reduced to the

fitting of equally spaced longitudinal divisions in

the tank.

To Calculate the subdivided value the FSM or

FSE is divided by the new number of

compartments (n) squared










Example 1

A tank has a FSM of 3586 tm. Calculate the FSM if the tank is fitted with:

(i) A single longitudinal bulkhead,

(ii) A further two longitudinal bulkheads. (i) FSMSUB = FSM = 3586 = 396.50 tm n2 22 (ii) FSMSUB = FSM = 3586 = 224.13 tm n2 42 1 2 4 3 2 1


Example 2

A tank has a FSE of 0.26 m. Calculate the FSE if the tank is fitted with:

(i) A single longitudinal bulkhead,

(ii) A further two longitudinal bulkheads.

(i) FSESUB = FSE = 0.26 = 0.065m n2 22

(ii) FSESUB = FSE = 0.26 = 0.016 tm n2 42 1 2 4 3 2 1






Can be combined with








To give






(Δ x n



(Δ x n




Example 3

A vessel displacing 8000 tonne has a DB tank half full, it has a free surface moment 2880 tm. Calculate the free surface effect if:-

i) the tank is undivided

ii) there is a centreline division

iii) there is a centreline division and two equally spaced longitudinal bulkheads.


Example 3

(i) The tank is undivided

FSE = FSM = 2880 = 0.36m Δ 8000


Example 3

(i) The tank is undivided

FSE = FSM = 2880 = 0.36m Δ 8000

(ii) There is a centreline division

FSESUB = FSM = 2880 = 0.09 tm (Δ x n2) (8000 x 42)

2 1


Example 3

(iii) There is a centreline division and two equally spaced longitudinal wash bulkheads

FSESUB = FSM = 2880 = 0.023 tm (Δ x n2) (8000 x 42) 4 3 2 1


FSE and the Angle of Loll

FSE causes a virtual rise in G

If the vessel is tender she will have a small



If the FSE is greater than the GM


then the

vessel will have a negative GM


and will be

in unstable equilibrium.

An unstable vessel could capsize, but more


FSE and the Angle of Loll

The best way to avoid this is to keep the number

of slack tanks to a minimum during the voyage.

Wherever possible tanks should be either empty

or pressed up.

Whilst the vessel is on passage she will use

FW, DO & FO, so some slack tanks cannot be


To avoid an angle of loll due to FSE the vessel‟s



must be large enough to withstand any

anticipated rise in G during the voyage.


List vs Angle of Loll

Angle of List


+ve GM


Stable Equilibrium.


G off the Centreline.


Corrected by moving

G back to the

Centreline – by


weights towards the

“high side”.

Angle of Loll


-ve GM


Unstable Equilibrium.


G on the Centreline.


Corrected by

lowering G below M


Correcting an angle of Loll

Lowering G below M to make the vessel stable

will correct an angle of Loll.

This can be achieved by:

 Moving cargo to a lower position;

 Jettisoning top-weight (in an emergency);

 Reducing FSE by pressing up/emptying tanks;  Filling low ballast spaces such as DB tanks.

Filling an empty tank will introduce FSE causing

a further virtual rise of G, so this must be done

with caution and adopting the following


Correcting an angle of Loll


top up tanks that are already slack.

2. calculate the FSE which will arise before pumping into empty tanks. This will ensure that the rise of G during the operation is acceptable.


Correcting an angle of Loll

4. Start with the smallest tank on the LOW side first.

(If a tank on the high side is filled first, the ship will start to right herself but will then tend to roll over suddenly in an uncontrolled fashion as she passes through the

upright. She will then „whip‟ through to a larger angle of loll on the other side. She may even capsize if the

momentum gathered is sufficient.)

When the low side is filled first, the angle of list will increase initially, but in a slow and controlled fashion. After some time, the weight of the ballast water added will be sufficient to lower the ship‟s COG (despite the extra FSE), to cause the angle of list to decrease. By this method the inclining motions of the v/l take place in a gradual and controlled manner.


Correcting an angle of Loll


now fill the opposite tank on the high side.

6. fill tanks alternately, low side first, until the v/l returns to positive GM.


FSM and Moments about the


Since Moments about the Keel and Free Surface

Moments are both Vertical Moments, they can

be combined into the same table to calculate


The KG calculated automatically will be the



The FSM can just be added to the Loaded


Example 1

A vessel of  = 17,922 tonnes is initially upright, KG = 12.66m, KM = 14.24m. The FSM‟s of the various tanks add up to 1225tm. Calculate the GMf after the following cargo operations if KM is constant.

Weight (t) Kg (m) Discharge: 624 14.88 1,296 8.71 Load: 3,042 6.69 312 13.27 397 14.88


Example 1

Weight (t) KG (m) Moment about the Keel (tm)

Loaded Discharged Loaded Discharged

17 922 12.66 226 892.52 624 14.88 9 285.12 1 296 8.71 11 288.16 3 042 6.69 20 350.98 312 13.27 4 140.24 397 14.88 5 907.36 FSM 1225.00 21 673 - 1 920 1 920 258 516.10 - 20 573.28 20 573.28 Ʃ19 753 Ʃ237 942.82


Example 1

KGf = Ʃ Moments about the Keel = 237 942.82

Ʃ Weights 19 753.00 KGf = 12.045m KM 14 240 m KGf - 12 045 m GMf 2.195 m The Final GMf is 2.20m


Related documents

In conclusion, the finding of this cross-sectional study of pre and post menopausal women show no evidence of a significant difference in BMD between Ocs users and never user

Antihypertensive therapy in hypertensive patients imme- diately post stroke may be effective and cost-effective compared with placebo from the acute hospital perspec- tive at

in a Case Previously Reported as "Nutritional Anemia in an Infant Responding to PERNICIOUS ANEMIA IN AN EIGHT YEAR OLD GIRL: Additional

Biofilm production by selected strains of bacteria (a) and yeasts (b) in the presence of Textus bioactiv, Textus multi dressings, and culture broth without dressing.. The figures

Screening of cytotoxic activities using WiDr and Vero cell lines of ethyl acetate extracts of fungi-derived from the marine sponge

In summary, we have presented an infant with jaundice complicating plorie stenosis. The jaundice reflected a marked increase in indirect- reacting bilirubin in the serum. However,

Abstract: This study examines the unique experience of participants who during their reintegration back into the community, following a conviction for sexual offending, re-

In order to analyze the issues of environmental impacts and economic structural change, an integrated econometric–emission model in continuous time has been developed to