Why does a ship made up of a metal heavier than water floats whereas a metal block sink in water

The ship is built in a shape such that its volume is greater than the volume of solid metal used to built. 

When ship is immersed in water, ship is subjected to upward thrust. This upward thrust is propotional to the immersed volume of ship

Due to the huge volume of ship, ship is getting upward thrust equal to downward gravitational force due to its own weight. 

Hence ship floats in water 

 

In case of solid metal block, its volume is less. Hence when metal block is immersed in water,

developed upward thrust is less than the weight of block. Hence metal block sinks due to its own weight