Question

how archimedes principle is used to design ships and submarines

Answer 1

Archimedes principle states that "when an object is immersed in liquid it feels the loss of weight "the amount of weight loss is equal to the amount of water displaced and its weight so ship displaces its own weight on water

Answer 2

Suppose you want to design your own ship. What do you want is that it would float on Ocean. How do you do that? Use Archimedes Principle! So you want it to be such that Average density of Ship would be less than density of water. But there's a problem because ship is made up of metal which have higher density than water. So that not work. So made it hollow, so air can occupy space in it. Now density of air is very much less than density of water. So average density of ship becomes less than water, so the ship floats.