When any object is immersed in a liquid, it would be subjected to an upward force which is termed as the buoyant force. Due to the upthrust, the weight of the dipped bodies in liquids appears to be lesser than its original weight in the air. The buoyancy force does facilitate swimming and also prevent the ships from sinking. The spring balance experiment does prove the existence of buoyant force. An object suspended from a spring balance does have different weights in the air and in any liquid.
An ancient philosopher, Archimedes did discover that the buoyancy force acting on any object is equivalent to the weight of the liquid displaced by the object.
His discovery is applicable to the objects placed in all the fluids. The rising balloons, the flying aeroplanes, etc do prove his discoveries.
In the case of flying aeroplanes, the buoyant force on them is equivalent to the weight of the air displaced by them.
Any object made of iron does sink in water as the density of iron is more than water. But the ships are similar to hollow objects. They contain iron or steel outer surfaces. Due to their hollowness their average density is lesser than that of water. Hence, the weight of the water displaced by ships would be equal to the whole weight of the ships.
Archimedes did discover that the buoyant force is directly proportional to the density of the liquid. The more the density, the higher the buoyant force and vice versa. For instance, iron is less denser than that of mercury. Hence, an object of iron floats in mercury. But, iron is denser than water. Hence, an iron object does sink in water. Thus it is obvious that all solids float in fluids if their densities are lesser than the fluids.
Archimedes did understand that there ae three special cases related to his study on buoyancy force. The weight of an object can be more than the buoyancy force. The weight of the objects is equivalent to the buoyancy force. The weight of the objects is lesser than the buoyancy force.
In the first case, the objects do sink. In the second instance, the objects float. In the last case, the objects do rise until the buoyancy force equals the weight of the objects, then they start floating!.
The buoyant force is not influenced by the nature of the solid bodies. But it does depend on the volume of the solid body.