The danger of mobile fluid in a tank

You must have Adobe Flash to view this animation. If you don’t have flash you can download it for free here.
Fluids can move very quickly. Have you seen how quickly water moves when it is shaken or tipped in a glass? If fluids are allowed to move from one side of a ship to the other, the ship will change its balance. This is a great danger to the ship because it can make the ship topple over and sink. 
Oil tankers have many storage tanks below the main deck. There are at least two tanks side by side so oil can’t move from one side of the ship to the other. Most tankers have three or four tanks side by side.