How Does a Cruise Ship Stay Afloat?

Built to float

Cruise ships are built with lightweight sturdy materials that spread out the weight of the ship across the hull. The hull helps keep the ship afloat by pushing water out of the way as it moves. The body of the ship that sits below the main deck is built wide with a deep rounded bottom. The rounded bottom dissipates the force pushed against the ship as it moves. Cruise ships weigh an average of 71,500 tons before the people and cargo are added. To hold this weight the principles of density and buoyancy are used.

Density and buoyancy

Density is a physical property of matter, the heaviness of an object with a constant volume. Each element and compound have an individual density associated with it. In regards to water, if an object floats on water, the object is less dense than the water. If the object sinks, it is more dense than the water. Buoyancy is the upward force that keeps an object afloat. In order for an object to float it's buoyancy must be greater than it's weight. If an items weight is more than it's buoyancy, it will sink. Cruise ships use their weight to displace the equivalent amount of water when they press into the ocean. The ocean then returns this push, causing the ship to become buoyant.

Water displacement

In order not to sink, a ship must displace it's weight in the water before it is actually submerged. For this to happen, the ship must be built to be less dense than the water that it will float in. For example, If you were to place a block of wood in water, some of the water is displaced and the level of the water goes up. If you were to weigh the displaced water, you would find that it weighs the same as the wood. For an object to float, the density of the object must be enough that it displaces an amount of water that is equal to the weight of the object.

Resources

• Buoyancy
• Hyperphysics
• Density