Usually, the hull of a ship contains a large volume or air. Why is this?
Because the hull of a floating ship must displace a mass of more WATER than the mass of the ship..........
You might get a better answer in the Physics section, however, I will give this a go.
#"Archimedes principle"# states that a body wholly or partially submerged in a liquid is subjected to an upward buoyant force equal to the weight of the fluid that the body displaces. Steel is more massive than water, and thus a steel boat must displace a weight of water GREATER than the weight of the hull. The bigger the hull the more water it displaces..........and the more buoyant the hull.
The principle (so the intrawebs tells me) was formulated by Archimedes of Syracuse in the year 216 pre-Christian era:
#"Any object, wholly or partially immersed in a fluid,"#
#"is buoyed up by a force equal to the weight of the fluid"# #"displaced by the object."#
Mercury has a density of approx. #13.5*g*mL^-1#. Would objects tend to be more buoyant in liquid mercury than in water?
And so the greater the volume of the hull, and steel hulls can be made quite voluminous due to the structural properties of steel (certainly more so than wood), the more buoyant the vessel, and the more containers it can carry. How does Archimedes principle lavoro in relation to helium, or dihydrogen, or hot-air balloons?