The massive U-shaped hull of the Cunard liner Queen Elizabeth as she undergoes her annual overhaul

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How Cruise Ships Float

In order for ships to journey across the open sea, they must withstand the tremendous burden, or weight of the ship, along with the crew, luggage, supplies and passengers. They do that with a little help from the principles of density and buoyancy. Cruise ships can weigh upwards of 71,500 tons (65,000 tonnes). They displace the equivalent amount of water when they press down on the ocean, which meanwhile pushes up and keeps the ship afloat, or buoyant.

That's why when engineers talk about how heavy a ship is, you'll hear them talk about displacement instead of weight. To keep from sinking, the cruise ship has to displace its weight in water before it's submerged. That's a lot easier to do if the cruise ship is constructed in a way so that it's less dense than the water below it. Think of it as the difference between dropping a bowling ball in the water and trying to submerge a beach ball. The bowling ball can't displace enough water before it's submerged, so it sinks. The beach ball does the opposite and floats.

Engineers help ships to achieve buoyancy by choosing lightweight, sturdy materials and dispersing the weight of the ship across the hull. The hull, or body of the ship below the main deck, is typically very wide and has a deep base line, or bottom. Large ships such as freighters, naval vessels and transport and cruise ships commonly utilize displacement hulls, or hulls that push water out of the way, to stay afloat.

A round-bottom displacement hull looks like a large rectangle with rounded edges to dissipate drag, or the force exerted against a moving object. The rounded edges minimize the force of the water against the hull, allowing large, heavy ships to move smoothly along. If you somehow hoisted a cruise ship out of the water and looked at it standing a few hundred feet away, the hull would look like a huge capital letter "U," depending on the size of the keel. The keel runs from the bow to the stern and acts as the backbone of the ship.

Like just about everything in our lives, round-bottom displacement hulls have advantages and disadvantages. Unlike a boat with a v-hull, which rises out of the water and s­kirts the waves, round-bottom hulls move through the water, making them extremely stable and seaworthy. Passengers on these ships rarely feel any rocking or side-to-side movement.

Ships with round-bottom hulls move fluidly, but the resistance of the water makes them extremely slow. They can only go so fast before the addition of more engine power reaches a point of diminishing returns. Nevertheless, the need for stability and a smooth ride outweigh overall speed, thus making the round-bottom displacement hull a good fit for cruise ships.

The hull serves not only as stability but also as protection. Reefs, sandbars and icebergs can tear apart fiberglass, composite materials and even steel. To prevent catastrophic damage, shipbuilders typically construct cruise ships using extra-strength steel and insert double hulls as an extra precaution. A double-hull design is a hull within a hull, like a tire with an inner tube.

Unfortunately, accidents do happen. In order to prevent cruise ships from sinking should something penetrate the first two lines of defense, vertical watertight dividers known as bulkheads are installed throughout the interior of the hull. These dividers keep damaged ships afloat by containing incoming water into a compartment or compartments, thereby preventing the whole ship from flooding.

Now that we've learned how these massive ships float, let's look at the various propulsion systems that propel them from port to port.