Displacement

Why does water overflow when you get into a full tub? The water overflows because your body takes up some of the room in the tub and two things cannot be in the same place at the same time. So your body pushes, or displaces, come of the water someplace else–probably onto the floor! This is called displacement.

Density

Some things are denser than others. A  brick is denser than a sponge. Steel is denser than water. Water is denser than air. When something is very dense, that means it is packed tightly together. If a boat is less dense than the water it displace, it will float. If it is denser than the water, it will sink. When steel is in form of a steel boat, it is not denser than water. A boat is not solid steel. It has space inside where there is no metal at all.

Buoyancy

When something floats, we say it has positive buoyancy. When it sinks, we say that it has negative buoyancy. Submarines use special ballast tanks to switch between positive, neutral, and negative buoyancy in order to submerge and surface.

Stability

Good stability is what keeps a boat from tipping over. Things have good stability when the center of gravity is low. If you put all the cargo low in a ship, it will be stable. But if you put it all high up on the deck,or off to one side, the ship could tip over. Because sailboats and tall ships have tall masts, extra weight must be added to the bottom of the boat to keep the center of gravity low. Extra weight that is added to a boat is called ballast.