How do our Naval ships, made of steel, weighing thousands of tons, stay afloat?

The concept is quite simple.

An object which sinks completely displaces an amount equal to its volume; Archimedes' Principle states that when this happens, the weight of the object is reduced by its volume times the density of the fluid. If the weight of the object is less than this quantity, it will float; the amount of fluid displaced is directly related (via Archimedes' Principle) to its weight.

Displacement is often used as a measure of the size of ships. The displacement of a vessel is defined as the weight of the amount of water it displaces when afloat. The size of a naval ship typically is reported as the number of long tons of water she displaces, but the measure is less often used for passenger ships or cargo ships. See tonnage.

Also, a displacement hull pushes its way through water, being supported entirely by displacement, as opposed to a hydroplaning hull, which glides over the surface of the water while being partially supported by hydrodynamic forces caused by the motion of the hull across the water, or a hydrofoil, which lifts the hull from the water entirely, using underwater wings.

Its average density is still much less than the density of water because of all the empty space inside the ship. Now how does the ocean "know" that its density is greater than the ship's and lets it float is an other question. You should look up the Archimedes Principle for that.

because they are mostly full of air. This makes up for all the steel. The weight of the water displaced by the boat is equal to the weight of the boat and all the contents. Replace all the air in the boat with water and it sinks.

The weight is distributed across a wide area, so the water pressure below the boat equals the pressure exerted by the weight of the boat.

The ship weighs less than the amount of water it displaces.

By arquimedes principle.

You must to read in books about physics, but not texts books...

It is the Archimedes Principle that can answer this question.

I can't believe it. Thanks for the belly laugh. I mean, you are playing with our minds, right?

Basic bouyancy and Archimedes's principle is third grade stuff. Either you missed a few days or you are spoofing us.

Look up displacement sometime.

Magic.