The Science Of Cruising: How Cruise Ships Float

Ship designers calculate how much water the hull must push aside to support the ship’s total weight — including the vessel itself, cargo, fuel and passengers. Hulls are designed to displace enough water to generate a buoyant force equal to their enormous weight. The shape of the hull helps spread the ship’s weight over a large area, allowing it to push aside more water without sinking too deeply. Designers carefully fine-tune hull dimensions to balance buoyancy, stability and fuel efficiency, ensuring the vessel can float smoothly even under heavy loads. This precise shaping also keeps the ship stable, minimizing rolling (the side-to-side rocking motion of the ship) and pitching (the up-and-down tilting motion of the bow and stern) in rough seas.

While the principle of buoyancy has been around for thousands of years, modern cruise ship design didn’t take off until the 1960s, the decade that many of today’s most prominent cruise companies were formed. With air travel making ocean liners obsolete for many travelers, a new business model emerged that prioritized the journey as a vacation in itself with hotel-like amenities and luxury features. Now, cruise ships dazzle with features including record-breaking amusement parks, fine dining restaurants, rejuvenating spas and Broadway-style shows.

Misconceptions About How Cruise Ships Float

Passengers may have concerns around cruise ship safety: Will a ship remain stable in rough waters? Can a big wave flip a ship? What happens if the generator fails? In actuality, experts say that the careful construction, operation and regulation of cruise ships make them extremely safe places to be.

Maritime meteorologist James Van Fleet says it’s even safe to take a cruise during hurricane season, one reason being the constant observation on the ship. “There are officers on board always monitoring information provided by the National Oceanic and Atmospheric Administration (NOAA). They’re stationed throughout the ship to make sure everyone is safe should inclement weather arise,” he says. And in rough waters, cruise ships make use of “stabilizers that act like airplane wings,” Van Fleet says. “These stabilizers essentially lift the ship over waves the same way a wing lifts a plane over rough air. This keeps the ‘rise and fall’ from being too noticeable.” 

In the rare event that part of the main system fails, backup power and propulsion systems keep modern cruise ships resilient. They use multiple diesel generators (or hybrid systems) to produce electricity for everything on board, and if one fails, the others automatically pick up the load. Emergency generators, placed high in the ship, provide power to critical systems like navigation, lighting and communications during a blackout.

Many ships use azimuth thrusters, which can rotate 360 degrees, for propulsion and precise steering. If one unit malfunctions, others keep the ship moving and maneuverable, a key technology used on larger ships. This built-in redundancy means that even in the event of an equipment failure, cruise ships can maintain essential operations and continue to safely navigate the seas.