Why Doesn’t an Iron Ship Sink?
This is a very natural question for you to ask right after you were told why iron sinks in water. The explanation is that by making an iron ship in the way we do, we fix it so that it holds a lot of air in between the bottom and sides, making the combination of the two—the iron ship and the air in it—lighter than the water on which it sails. Men thought at one time that a ship would sink if made of iron, and therefore built all of their ships of wood. Finally one inventor made a ship of iron and it was one of the wonders of the world. When we found that iron ships would float if they were built to retain sufficient air to keep them from sinking, we made the hulls of most ships of iron for a time. Now, however, the best ships are made of steel, which is even better.
If you bore a hole in the bottom of a ship, the water will run in if the ship is in the water, and the ship will sink, because the water coming in drives out the air; and when the ship is full of water, the water in it, with the ship itself, are heavier than the water on which it sails, and the ship will go down. Filling a ship with water makes the iron part of the ship just like a bar of iron, so far as its sinking qualities are concerned.
Of course, an iron ship must be made long enough and broad enough so that when it is completed there will be sufficient air contained within the hull to make the combination lighter than water. Always, therefore, when a ship is to be built, competent engineers must go over the plans of the vessel and calculate the air capacity, so as to make sure she will float.
Nowadays it would be difficult to sink a modern vessel by boring one small hole in the bottom, because the bottom and sides are lined with enclosed steel air-chambers, and a ship will keep afloat even if one or a number of holes are made. The reason is, of course, that when you bore a hole into one of these air-chambers the water rushing in will fill that air-chamber with water, but as there is no connection from the inside with the rest of the ship, the water can get no further.