What Makes a Cruise Ship Float?

Displacement and Buoyancy

Cruise ships can way over 71,500 tons, typically with two hulls made of extra-strength steel. The actual construction is a hull within a hull for added protection should the boat hit an obstacle.
When floating on the ocean, the ships displace a mass of water that equals their own mass. In essence, the water pushes up on the hull of the boat, countering the downward force of the boat's gravity.

Design Determines Displacement that Allows a Ship to Float

The design of the ship determines its buoyancy. Engineers make sure the vessels are not as dense as the water they float on by creating rounded U-shaped hulls that are deep and wide. This type of hull disperses the weight of the ship across the body of the ship, which pushes the water out of the way. It also leaves plenty of empty space in the boat.

For example, a ship designed like a super dense bowling ball would just sink. On the other hand, a balloon-like ship designed with no density would just float on top of the water and roll over. However, because it's designed more like a soccer ball, with a good balance of empty space and weight, it can actually displace water.

Other Important Factors Involved in Keeping a Ship Afloat

Boats designed in a U-shape tend to move very slowly because the water resists its progress. However, they don't rock as much, which makes them perfect for cruises. These ships also have their mass concentrated in the hull, which ensures they stay upright. One final factor ensuring the stability of the boat is the bulkheads added to the hull's interior. These are watertight dividers that help protect the boat if both hulls are breached and water enters the boat. They contain water in compartments thereby preventing flooding of the entire boat.