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How does Archimedes’ principle explain why a ship floats?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

Archimedes’ principle explains that any object submerged in a fluid—whether partially or completely—is acted upon by an upward buoyant force. The magnitude of this force is exactly equal to the weight of the fluid displaced by the object.1

For a ship to float, it must displace a volume of water that weighs exactly the same as the ship itself, including its cargo and equipment. When a ship is launched, it sinks into the water until the weight of the displaced water matches the ship's total weight; at this point, the upward buoyant force and the downward force of gravity are in balance, allowing the ship to remain in static equilibrium.123

If more weight is added to the ship, it sinks deeper into the water to displace a greater volume, thereby increasing the buoyant force to match the new, higher total weight. This principle is fundamental to naval architecture, ensuring that ships are designed with sufficient underwater volume to support their intended load while remaining stable.132