Free Surface Effect

Question 1

When a vessel is heeled what happens to the slack tanks and the ships COG?
How does this affect the vessels GM?

Answer 1

• When a vessel is heeled the water moves to the low side.
• G moves parallel and in the same direction as the weight shifted. So, there is a large transverse shift of G and a small vertical shift up.
• GZ has reduced therefore there has been a virtual rise in G.
• Difference between GMs (solid) and GMe (effective) is called the virtual loss of GM or FSE.

Question 2

Summarise Chapter 3.1 in the IS Code

Answer 2

Chapter 3.1 Effect of Free Surfaces of Liquids in Tanks
- Initial Metacentric height and righting lever curve should be corrected for FSE in tanks.
- FSE should be considered whenever the filling level is less than 98%.
- Initial Metacentric Height Correction based on I.Mom at 5° heeling angle ÷ displacement. Correction of Righting Lever based on real shifting moment of cargo liquids.
- Two types of tanks; tanks with fixed filling levels and tanks with variable filling levels. If fixed, FSE correction should be for that fixed level. If variable the FSE correction should be the maximum value attainable.
- Vessels engaged in liquid transfer operations can have loading conditions representing initial, intermediate and fan stages of filling or discharge.
- Correction to initial metacentric height, transverse moments of inertia should be calculated at 0° angle of heel.
- Righting lever Curve can be corrected in two ways; actual moment of fluid transfer for each angle of heel or moment of inertia, calculated at 0° angle of heel, modified at each angle of heel calculated.
- Really small tanks that when calculated have little to no effect do not need to be included.
- Empty tanks do not need to be included provided their residue does not create FSE.

Question 3

What information should a stability booklet have about tanks?

Answer 3

Tank sounding table, COG, Free surface data

Question 4

What density are FSM given to you in a stability data booklet?

Answer 4

• Given to you as 1.000 (FW)
• For different density’s you need to apply the relative density when calculating the FSM

Question 5

How do I calculate the FSC?

Answer 5

FSC = FSM / △

Question 6

How do I calculate the Moment of inertia (I) of a rectangular free surface (tank)?

Answer 6

I = lb³/12

Question 7

Is FSE independent of weight of liquid in the tank?

Answer 7

• Yes, the FSE is based on area of the tank

Question 8

Is FSE independent of the position of the tank in the vessel?

Answer 8

• Yes, FSE has the same moment of interia provided the area of free surface in the tank is unchanged

Question 9

How does fitting longitudinal subdivisions reduce the FSE of a tank?
What is the formula used when you reduce a tank via longitudinal subdivisions?

Answer 9

• FSE reduction is proportional to the breadth³. The movement of water has reduced transversely.
  1/n²
  (1 = the original undivided value)
  (n= number of spaces)

Question 10

When should you consider complete FSE?

Answer 10

• As soon as you start ballasting a tank

Question 11

What is pocketing?

Answer 11

• When we calculate for FSM we assume one value but this is not strictly true in reality. When the tank is nearly full or nearly empty the FSE will reduce. This is pocketing.

Question 12

How would adding transverse subdivisions affect the FSM of a tank?

Answer 12

• There would be no change in the FSM
• The same mass is moving the same distance transversely

Question 13

What happens to the FSM when you increase the length of a tank?

Answer 13

• FSM will increase, directly proportional, FSM ∝ L

Question 14

What happens to the FSM when you increase the breadth of a tank?

Answer 14

• FSM will increase, directly proportional, FSM ∝ breadth³

Question 15

What happens to the FSM when you increase the density of fluid?

Answer 15

• FSM will increase, directly proportional, FSM ∝ 𝜌

Question 16

What happens to the FSM when you increase the depth of fluid in tank?

Answer 16

• Will have no effect on FSM, apart from pocketing

Question 17

What happens to the FSC when you increase the △ of the vessel?

Answer 17

• FSC will reduce, inversely proportional,
  FSC ∝ 1/△

Question 18

Describe FSE?

Answer 18

• As a vessels angle of heel changes the water within a slack tank tends to remain horizontal.
• This results in a wedge of liquid being transferred from side to side as a vessel rolls
• The weight movement produces a capsizing moment known as FSE, which reduces the ships stability and so can be considered as an effective rise in KG value.

Question 19

Sketch a graph to show how the effective KG of a ship changes as a DB tank is ballasted.

Answer 19

Question 20

Sketch a transverse picture of a vessel at a angle of heel under the influence of FSM

Answer 20