What is free surface effect and what is its impact on the ships stability when the ship is listed?

• If a tank is completely filled with the liquid, the liquid cannot move and acts as a static weight.

• So, the centre of gravity of the ship remains unchanged.
• The graphic shows the listed ship when the tank is filled with liquid.

• Assume that the ship floats at the same draft with the same KG and with the tank partially filled.

• Now, if the ship lists, the liquid flows to the lower side causing G of the liquid to shift to G1.

• This result in the centre of gravity of the ship G moving G1.

• This effect is called a free surface effect.

• As a result, the vessels suffers an apparent loss of GM which is equal to GGv and hence the metacentric height is GvM.
• GZ, the righting lever is also reduced by an amount which is equal to GG1.

• This virtual loss of GM can be calculated. It is called a free surface correction or FSC.

• In order to indicate whether FSC has been applied or not the GM before subtracting FSC is called solid GM.
• The GM attained after applying FSC is called a fluid GM.
• In all stability calculation, you will use fluid GM.

Points to remember

• If the area of the free surface is constant, the weight of liquid in the tank does not affect the FSC.
• Within the ship, the position of the tank does not affect the FSC.
• When a tank is either empty or full, FSC is zero.
• To reduce the total FSC for the ship, the number of slack tanks must be kept to a minimum since the FSC of each slack tank contributes to the total FSC for the ship.
• By fitting longitudinal division in the tank equally spaced, the FSC can be reduced to 1/n2 times the undivided value, n=number of spaces. E.g: 3 spaces are produced by 2 longitudinal divisions and the FSC reduces to 1/9th value of undivided tank.
• FSC will make the situation worse before the bottom weight increases to a sufficient level to bring G down, if time is taken to decide and start filling a DB tank to improve stability. The smallest tank on the lowest side should be filled first, if the ship is at an angle of loll.