How Does a Cruise Ship Float?
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Buoyancy and displacement
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Buoyancy is the key to why 70,000-ton cruise ships can float. Ships are designed such that when they push down on the ocean, they displace a mass of water equivalent to their own mass. Thus, the pressure of the water pushing up on the bottom (or hull) of the boat actually counters the downward force of the boat's gravity, keeping the ship afloat.
Materials and design of ship
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The materials that the ship is made of and the design of the ship are the key to creating this balance. Ships are designed so that they are not as dense as the water on which they float. Though they are typically made of materials that are denser than water, such as extra-strength steel, there is actually a lot of empty space in them. Engineers achieve this by making the hull rounded, wide and deep, thus dispersing the weight of the ship across the body of the ship. These hulls, in essence, push water out of the way. The rounded edges (the hull looks like a U) allow water to more easily flow over the hull, thus facilitating a smooth ride.
These boats tend to move very slowly because of all the resistance they encounter in the water. However, they are also very stable (not much rocking), which is prefect for cruises.
Other factors keeping ships afloat
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In addition to stabilizing the boat, the extra-strength steel hull hull protects it from major damages. Typically, to ensure a boat stays afloat after a collision, engineers design a double-hull---a hull within a hull. And, they add bulkheads to the hull's interior. These watertight dividers contain water in compartments so it does not flood the entire ship.
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