For an object to float on the water’s surface, it must sink enough to displace a volume of water equal to its own weight. For example, if a boat is to carry three people, their fishing gear, an outboard motor, and a supply of fuel—a total weight of about 500 kg (1,100 lb)—then the boat must be made long and large enough to displace 500 kg (1,100 lb) of water without sinking below the water level. Boat designers also have to take into account the weight of the boat itself. The heavier the material used to build the boat, the larger the boat has to be.
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