Copyright © 2007 Dorling Kindersley
Although boats float, their weight makes them settle a little way into the water. This means they create some resistance or drag when they move through the water. The bow (front) of a boat is V-shaped and curved. This raises the boat up as it goes faster, and helps reduce drag. Boats are powered by sails, oars, or engines that turn propellers at the rear. The propellers push water backwards, and this backward thrust moves the boat forwards.
A hydrofoil is a high-speed boat that seems to fly along almost out of the water. The boat has small underwater wings called foils. These work like the aerofoil wings of an aeroplane. As the boat speeds along, the foils generate a force upwards that lifts its hull (body) clear of the water. This reduces drag and helps the hydrofoil go faster.
Submarines can float on the sea, sink just beneath the surface, or dive to the seabed. They dive or surface using tanks that work like a fish’s swim bladder. When the tanks are filled with water, the submarine dives. When they are filled with air, it surfaces. A submarine can select its level in the sea by changing the mixture of air and water in its tanks.