Study Material

# Why don’t boats and ships sink?

Why don’t boats and ships sink?
The air that is inside a ship is much less dense than water. That’s what keeps it floating! The average density of the total volume of the ship and everything inside of it (including the air) must be less than the same volume of water.

If you build a wooden raft, you can sail on it. If you build a raft out of iron or some other metal, it will sink. The reason that a wooden raft does not sink, but an iron one, lies in the different density of wood and iron. Wood is a less dense material than water, so the buoyant force of water is greater than the force of gravity acting on a wooden raft (or more than its weight). Iron is denser than water, and its buoyant force cannot overcome the weight of an iron raft.

In the old days, ships and boats were built mainly from wood. Now they are mostly made of metals. What is the focus? Why don’t ships sink? Maybe there is a lot of wood inside the ship, and it “wins” iron?

Of course, if you take a large board and sheathe it on top with a thin sheet of metal, then the whole structure will not sink. After all, its average density will be less than the density of water. If, for example, the density of wood is 600 kg/m 3 and the board has a mass of 100 kg, and the iron sheathing has a density of 7800 kg/m 3 and a mass of 10 kg. Then the total mass will be 120 kg, and the total volume will be 100 / 600 + 10 / 7800 ≈ 0.1667 + 0.0013 = 0.168 (m 3 ). From here we find the average density of the structure 120 / 0.168 ≈ 714 (kg / m 3 ). This is less than the density of water (1000 kg / m 3 ), which means that the structure will float.

However, it’s actually even easier. Why sheathe a tree? You can just leave an empty cavity inside and make sure that water does not get there. More precisely, not empty, but filled with air. Air density is only 1.29 kg/m 3 .

That is why ships made of metals float. Inside them there are large cavities filled with air. As a result, the average density of the ship is less than the density of water, and the buoyancy force keeps the ship afloat.

If water gets into the cavity of the ship, then it will certainly sink. In order to minimize the possibility of flooding, partitions are built in the underwater part of the ship. The result is compartments in which water from one cannot enter the other. If the ship gets a hole, then only the compartment in the place of the hole will be flooded. The rest will remain filled with air and will keep the ship afloat.

In any case, the ship has weight. This weight is equal to the weight of the water, the volume of which the ship “occupies” with itself in the sea.

As you know, ships do not sail just like that, but carry various cargoes and people. An empty ship weighs less, which means less will “settle” in the sea. If it is loaded, the ship will sink deeper into the water. With excessive load, the ship can generally go under water and drown.

Therefore, a special line ( waterline ) is marked on the hull of ships. The vessel must not be submerged so that this line is submerged. Otherwise, any strong wave, splashing water on the stern, can easily flood the ship.

On the other hand, an empty vessel should not be too light. Otherwise, its underwater part will be too small in relation to the surface. In this case, waves and wind can capsize the ship.

A ship loaded at the waterline displaces the largest volume of water. The weight of this water is called the displacement of a particular ship. The carrying capacity of a vessel is the difference between the displacement and the empty weight of the vessel; or, more simply, the difference between a loaded ship, when it has a draft at the waterline, and the weight of the ship without cargo. This article now totally covers our topic boats and ships sink, cheers!!!!