Displacement of water by something that is lighter than water.
The screw you mention weighs more than the water around it per unit of volume. The ship has a 'hollow' hull, to simplify the answer. Of course within the hull is fuel, machinery and all the stuff we think of that ships need. Some of the those things are lighter than the water, some heavier - but the net effect is a hull volume that is lighter per cubic foot than the water it is floating in.
So in effect it is the 'open space' produced by the shape and sized of the ship's hull that keeps water out that creates bouyancy, while the screw can only displace the volume occupied by the solid metal in it. The solid metal is heavier per cubic foot (or cubic inch, cubic centimeter) than water and therefore cannot float by itself.
The screw you mention weighs more than the water around it per unit of volume. The ship has a 'hollow' hull, to simplify the answer. Of course within the hull is fuel, machinery and all the stuff we think of that ships need. Some of the those things are lighter than the water, some heavier - but the net effect is a hull volume that is lighter per cubic foot than the water it is floating in.
So in effect it is the 'open space' produced by the shape and sized of the ship's hull that keeps water out that creates bouyancy, while the screw can only displace the volume occupied by the solid metal in it. The solid metal is heavier per cubic foot (or cubic inch, cubic centimeter) than water and therefore cannot float by itself.