If you put a nail or a lump of iron in a vessel of water it sinks at once. A piece of wood of the same size floats. So, until fifty years or so ago, people thought all boats and ships must be built of wood, or they would sink.
Take a sheet-iron pan from the kitchen and put that on the water. It floats. It weighs just as much as the lump of iron that sinks, but the weight is spread or distributed over a larger volume of water That is all. It has been made lighter than the total amount of water it rests upon. A ship is just such a hollow vessel, whether made of iron or wood. When there is nothing in it, a ship stands high, almost on the surface of the water. As it is loaded with goods and people, it rides deeper. Load your sheet iron pan with a cargo of toys. Watch it go deeper. Don't fill it to the top. That would make it as heavy as if it were solid. Then it would sink.
If you live in a lake or sea-port town, you will find that all ships have a water-line painted plainly around the hull. This is the safety loading line. No ship owners are allowed to load a vessel so heavily that that water line sinks below the surface of the water. Air spaces must be left, to keep the ship and its cargo lighter than the water that is beneath them. In the old days overloaded wooden vessels often sank. Today, iron ships ride the ocean safely.
Source: http://chestofbooks.com/reference/The-New-Student-s-Reference-Work-Vol5/Why-An-Iron-Ship-Does-Not-Sink.html#.ViIG-fkrLIU#ixzz3ooQryes9
