Many a time as you stand looking at the faraway horizon at the silhouette of a battleship receding into the distance, you may have wondered, "What keeps these monsters afloat? I mean, metal is supposed to sink, right? So why doesn't the big metal ship just sink to the bottom of the ocean?"

HYDRODYNAMICS

Well, this phenomenon largely depends on a principle founded by a famous Greek physicist and mathematician called Archimedes. According to his principle,

the magnitude of the buoyant force is equal to the weight of water it displaces. Put in another way,

Buoyant force= Weight of Fluid Displaced

Since the ship is airtight and does not have a leak anywhere, and is relatively hollow inside, the ship displaces the amount of water equal to its weight. Thus, the metal ship actually floats, and does not sink.

Now, based on this principle alone, the ship would only be able to float around. A big metal ship, say, a battlecruiser, would not be able to propel itself so smoothly through the water unless some other principles were used.

If a boat were to have a flat bottom, it would not be of very much use. This is because of the tremendous friction caused by the pressure of, say, a 100-ton ship exerted on the large surface area of the base of the ship, the area in contact with the sea.

Thus, to have maximum speed, the base of the ship must be curved and the boat must sit out of the water as far as possible, to reduce the amount of negative friction (friction resisting the movement of the boat) acting on the base of the boat and thus slowing it down. Although this fact is not very clear when one is viewing a battlecruiser in the sea (they seem to have flat, oblong sides) actually the underbelly of the cruiser is curved to offer the least resistance when moving.

In order for the Battleship to have the maximum weight before sinking, the hull is designed in such a way that it displaces the maximum amount of water (to receive the most support from the water) and yet the minimal amount of surface area (to reduce the negative friction).

Another important point to consider is that of centre of gravity. It has to be placed low, preferably well below the waterline of the sea, to prevent the boat from capsizing easily, especially during storms. Why?

The answer is very simple, and can be traced back to the concept of equilibrium and moments. Once the ship is tipped over to one side by a high wind, the centre of gravity would exert a equal and opposite moment, or turning effect, which would right the ship again.

Ship designers usually use dense materials, called chainplates, to weight the base of a ship down. They are incorporated into the hull.

Well, you've done it. These are the physics principles which go on behind a sailing ship, be it a minesweeper or destroyer.

Let's move on to the case study...

CASE STUDY:

EISENHOWER CARRIER