Practical Considerations
Reducing Free Surface Effect
Free Surface Effect Calculations
Tanks in General Cargo Ships
Construction of Deep Tanks


When a tank (on board a ship), is completely filled with a liquid, the liquid cannot move within the tank when the ship heels. For this reason, as far as stability is concerned, the liquid may be considered a static weight having its center of gravity at the center of gravity of the liquid within the tank.

When the tank (on board the ship), is partially filled by the liquid, when the ship heels, the liquid moves across (flows) to the low side of the tank (i.e.: in the same direction as the heel) such as its center of gravity shifts to the inclined side.

Since the initial stability of a ship is usually measured in terms of metacentric heights, it is useful to assume that the effect of a free surface of a liquid is to raise the center of gravity from G to G2, thus reducing the metacentric height of the vessel and hence we have a proportional (virtual) loss of GM.

If free surface is created in a ship with a small initial metacentric height, the virtual loss of GM due to the free surface may result in a negative metacentric height. This would cause the ship to take up an angle of loll which may be dangerous and in any case is undesirable.

This should be borne in mind when considering whether or not to run water ballast into tanks to correct an angle of loll, or to increase the GM: Until the tank is full there will be a virtual loss of GM due to the free surface effect of the liquid.

It is important to note that the free surface effect depends upon the displacement of the ship and the shape and dimensions of the free surface. It is independent of the total mass of liquid in the tank and of the position of the tank in the ship.


The effect of a free surface of liquid may be most dangerous in a vessel with a small metacentric height and even cause the vessel to become unstable. In such a ship, tanks which are required to carry liquid should be pressed up tight. If the ship is initially unstable and heeling to port, then any attempt to introduce water ballast will reduce the stability. Before ballasting, therefore, an attempt should be made to lower the center of gravity of the ship by pressing up existing tanks and lowering masses in the ship. If water is introduced into a double bottom tank on the starboard side of the vessel will flop to starboard and may possibly capsize.A small tank on the port side should therefore be filled completely before filling on the starboard side. The angle of heel will increase due to free surface and the effect of the added mass but there will be no sudden movement of the ship.

A particularly dangerous condition may occur when a fire breaks out in the upper 'tween decks of a ship or in the accommodation of a passenger ship. Any accumulation of water should be avoided because if water is pumped into these spaces , the stability of the ship will be reduced both by the added mass of water and by the free surface effect.


It is most important, therefore, that the free surface effect in tanks carrying liquids should be reduced to a minimum. One way of doing this is to subdivide wide tanks into two or more narrower ones by fluid (liquid)-tight divisions.


When the ship heels, the liquid flows to the low side of the tank such as its center of gravity shifts from g to g1.This will cause the ship's center of gravity to shift from G to G1, parallel to gg1.

Moment of statical stability = W X G1Z1

= W X GvZv

= W X GvM X Sinq

This indicates that the effect of the free surface is to reduce the effective metacentric height from GM to GvM. GGv is therefore the virtual loss of GM due to the free surface.


1- The reason double bottom tanks are important when it comes to a ship's stability is that the low position in a ship, under double bottoms has an effect on the center of gravity: filling, obviously lowers the center of gravity and increases the metacentric height (GM) and as the additional weight of water is loaded into the ship, the ship settles deeper into the sea which is sometimes very useful as well as necessary.

2- A general cargo ship has generally five (5) holds each divided into upper and lower holds known as the 'tween decks. Stowing cargo on the upper decks is its self dangerous as it raises the center of gravity of the ship. By loading water ballast in double bottom tanks we can lower the center of gravity and thus neutralize the negative effect of stowing cargo on upper decks.

3- These ships carry their own cargo handling equipment for loading and unloading, usually derricks, winches or cranes. During loading/unloading procedures cargo is lifted high up in the air by the on- board equipment which causes the center of gravity of the vessel to be raised. Therefore, double bottom tanks can be used in this case in order to keep the center of gravity of the vessel as lower as possible during cargo handling operations.

4- General cargo ships often travel partially loaded since they discharge part of their cargo in different ports. In addition, in some cases, cargo is discharged from certain holds and not evenly from all the holds since each hold may contain different cargo or cargo with different destinations. Hence double bottom tanks are indispensable in order to cope with trimming problems that occur due to the uneven distribution of weight within the ship and in order to settle the ship deeper into the sea when is not fully loaded.

5- Some other pressing reasons for fitting double bottoms for dry-cargo ships are that:

i) the double bottom acts as a safety precaution should the bottom becomes damaged,
ii) it forms a back bone of a longitudinal frame,
iii) it provides compartments for the storage of fuel oil, lubricating oil, water ballast and fresh water.

6- Deep tanks (such as the fore and aft peak) are often fitted in order to be used to carry liquid cargoes and, therefore, increase profits operational efficiency. These tanks can also be used for water ballast and suitably trim the ship when it is unevenly loaded and provide total propeller immersion.

7- Side ('tween deck) tanks can also be used in cases where cargo shifting within the holds has occurred (since general and dry-bulk cargoes never comprise the total available space) and an extra righting moment is necessary.

8- Load Line Rules impose the incorporation of tanks in the design of general cargo ships.

9- Classification Societies' Rules and Regulations for the construction of ships also impose the incorporation of tanks in the design of general cargo ships.

In short, it is difficult to operate a general cargo ship that is to stow and secure the cargo if the ship did not have double bottoms or other (stability) tanks.


Bulkheads which form the boundaries of a deep tank differ from hold bulkheads in that they are regularly subjected to a head of liquid. The conventional hold bulkhead may be allowed to deflect and tolerate high stresses on the rare occasions when it has to withstand temporary flooding of a hold, but deep tank bulkheads which are regularly loaded in this manner are required to have greater rigidity, and be subjected to lower stresses (the thickness of bulkhead plating is greater than required for hold bulkheads, with a minimum thickness of 7.5mm).As a result the plate and stiffener scantlings will be larger in way of deep tanks, and additional stiffening may be introduced.

The greater plating thickness of the tank boundary bulkheads increases with tank depth, and with increasing stiffener spacing. To provide the greater rigidity the vertical stiffeners are of heavier scantlings and more closely spaced (the stiffener spacing on the transverse bulkheads is usually about 600mm and the stiffeners are much heavier than those on hold bulkheads).They must be bracketed or welded to some other form of stiffening member at their ends. Vertical stiffener sizes may be reduced, however, by fitting horizontal girders which form a continuous line of support on the bulkheads and ship's side. These horizontal girders are brackets at the toes of the end brackets, and at every third stiffener or frame. Intermediate frames and stiffeners are effectively connected to the horizontal girders. Side frames where no horizontal girders are fitted have their scantling strength increased by 15% in way of deep tanks.

Where deep tanks are intended to carry oil fuel for the ship's use, or oil cargoes, there will be a free surface, and it is necessary to fit a center line bulkhead where the tanks extend from side to side of the ship. This bulkhead may be intact or perforated, and where intact the scantlings will be the same as for boundary bulkheads. If perforated, the area of perforations is sufficient to reduce liquid pressures, and the bulkhead stiffeners have considerably reduced scantlings, surging being avoided by limiting the perforation area.

Both swedged and corrugated plating can be used to advantage in the construction of deep tanks since, without the conventional stiffening, tanks are more easily cleaned. With conventional welded stiffening it may be convenient to arrange the stiffeners outside the tank so that the boundary bulkhead has a plain inside for ease of cleaning. When deep tanks were riveted this was not possible as it was necessary to have the stiffening inside for tank testing purposes, the plain outside being caulked if necessary.

In cargo ships were various liquid cargoes are carried, arrangements may be made to fit cofferdams between deep tanks. A pipe tunnel is generally fitted through them with access from the engine room.

Copyright ©, 1997 Marinet