Question

a steel needle sinks but a steel ship floats why

Answer 1

An object will float in water if its density is less than that of water. A steel needle sinks since its density is more than water. The interior of a ship is hollow. Therefore, its average density is less than that of water, therefore, it floats.

Answer 2

It is defined by the weight of displaced water. If a nail is dipped, the volume of water it displaces weighs less than that of steel due to lower density thus the nail sinks. However ship works differently. it is shaped such that it will contain air within its hull and therefore the average density of the ship as a whole including cavities and all the air space is less than the water and therefore it floats.  the amount of water it displaces, weighs more than that of hull, which makes it float.