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As a child I wondered; "How can something made of steel float?" The answer, once I understood it, made me wonder why it took so long to make steel boats.

Everyone knows that steel is "heavier" than water and that is why it sinks. So as a kid the first thing I wondered about navy ships was; why were they able to float if they were made of steel? The explanation of why steel ships float has to do with "specific gravity" and "density".

The density of something is the mass of the object divided by the volume or the space it takes up. Specific gravity is the comparison of the density of an object to a standard density. In most cases this standard density is the density of water which is defined as 1.0. For example if you read tables of specific gravity

Material Specific Gravity
Barite 4.25
Bentonite 2.4
Calcium Carbonate 2.7
Cement 3.0
Clays 2.6
Diesel Oil 0.84
Dolomite 2.9
Fresh Water 1.0
Galena 6.5
Gypsum 2.3
Iron/Steel 7.8
Iron Oxide 5.1
Lead 11.4
Limestone 2.8
Salt 2.2
Sand 2.6
Air (dry) 0.00129
If something has a higher specific gravity than water it will sink, lower it will float. Since specific gravity is relative density, and water is 1.0, a specific gravity of 2.0 means that the substance is 2 times as dense as water.

You will see that steel is 7.8 times more dense than water, so how can it float? The answer is in the definition of density.

If you remember density is mass divided by volume, the way you decrease the density of something is by increasing its volume, or the space it takes up. By taking steel and shaping it in a way that increases its volume, the density of the object constructed can be made less than that of water. This happens because the increased space is done without allowing the new space to be taken up by something heavy enough to increase the density to higher than 1.

Think of a steel ship as a can that is empty and watertight. The majority of its space is taken up by nothing more than air which has a specific gravity of about 0.00129. So if a make a ship that is like a can, empty inside and watertight it floats. Its volume sufficient that it causes its specific gravity to remain below 1.0 even after adding the weight of its contents.

The steel ship will remain afloat as long as its density remains below that of water. The problem is that if water is allowed to enter the ship it would sink. Ships made of steel have to be sealed so water can not get in and increase its density.

Steel can float only because of the way it is shaped, the space it takes up, and the seal that keeps its watertight. The ability of steel to float is the ability of people to shape metal and solve problems by examining outcomes.