1. Floating of Ships
An iron nail sinks in water. Due to its small volume, the upthrust which is equal to the weight of water displaced is less than the weight of the nail, and therefore it sinks. Moreover, the density of the nail is more than the density of water and therefore it sinks.
A ship is also made of iron but it floats. This is because the bottom of the ship is built in the form of a large shell, or hull as it is called. It is therefore able to displace a volume of water the weight of which is equal to its own weight, causing it to float. Moreover, due to the large air spaces inside the ship which is included in its volume the density for the ship becomes less than the density of water, due to which it floats.
A submarine can be made to float, dive or surface. It floats when its weight is equal to the weight of the water displaced by it. To make it dive, water is pumped into is ballast tanks so that its weight becomes more than the weight of the water it displaces. To make it rise, compressed air is used to force the water out of the ballast tanks into the sea until is weight becomes less than that if the water displaced.
3. Airships and Balloons
A gas less dense than air e.g. hydrogen or helium is called into the balloon. As the gas is being filled, the size of the balloon increases. More and more air is displaced and therefore the upthrust on the balloon keeps increasing. After some time, the weight of the air displaced becomes more than the weight of the balloon and the gas inside. Thus, the upthrust us greater than the downthrust and the balloon rises.