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We now know how a boat floats, but
just having a boat float doesn't do us much good. If
you want a boat to be useful, it needs a way to move itself
through the water. Today, most boats use a motor with
a propeller to do this. There can be all different
kinds of boat motors but they all work on the same idea of
action-reaction forces.
If you ever get a chance to examine the propeller of a boat motor, you will see that the fins on the propeller are twisted almost like a giant screw. When the propeller turns, the screw shape causes it to "grab" the water and push it backwards ("Reaction", Wikipedia). The force of the propeller pushing the water backwards is countered by a force from the water pushing the boat forwards. This force from the water is what causes a boat to move forward.
If you ever get a chance to examine the propeller of a boat motor, you will see that the fins on the propeller are twisted almost like a giant screw. When the propeller turns, the screw shape causes it to "grab" the water and push it backwards ("Reaction", Wikipedia). The force of the propeller pushing the water backwards is countered by a force from the water pushing the boat forwards. This force from the water is what causes a boat to move forward.