Question

when iron is thrown in water it sink...  but  
WHY THE SHIP AND STEAMER MADE OF IRON FLOAT ON WATER?    

Answer 1

Any object will float if it is less dense than water. Iron ships don't only have iron in their inside; there are spaces with air in between.

Answer 2

The ship is designed in such a way that it displaces the water which is weighing more than the weight of the ship and thus the ship floats .