Cruise ships are huge floating cities, and they float using buoyancy. But how does a cruise ship stay afloat? And why is it so hard for some of them to sink?
When a ship moves through the water, its weight causes underwater pressure to bring it down. But because this substance cannot be compressed like air, it can quickly become denser and thus counter the force of gravity on us at sea level – leaving us with some extra “buoyancy” (or lift).
When you’re on vacation at sea, two things can make your trip miserable: rough seas and sinking ships. You’ve probably heard stories about massive luxury liners tossed around like toys in high winds or completely capsizing when large waves hit their hulls.
As terrifying as these events sound, maritime disasters have become rare, thanks mainly to passenger vessels and ocean conditions improvements. Still, if you decide to take a cruise, pack an inflatable life raft, just in case! Most cruising today takes place in relatively calm waters with small swells.
But what makes a vessel “float?” It turns out that the answer has less to do with physics than we might think. Instead, it all comes down to one simple principle: Buoyancy. A boat floats because its weight is greater below the waterline than above it.
That means that even though the entire craft sits lower underwater, the overall effect is that it stays primarily level. This phenomenon works similarly to how a person stands upright while seated on a chair without tipping backward from gravity, pressing downward onto their head. The body counteracts this force by shifting up through muscles and bones to stay balanced.
Since boats also contain wooden planks (called beams) instead of muscle and bone, buoys accomplish the same task underwater as our bodies would otherwise perform on dry land.
So now you know that the secret to stability isn’t height but rather weight distribution. What happens when passengers get added to a cruise ship? Find out next.
The Basics
Before we talk about adding people on board, let’s review the fundamentals behind how cruise ships float. Most cruises travel across open oceans where they encounter little wind and wave action. Even under ideal weather conditions, a full-capacity cruise liner will experience only 1 g (1 foot per second squared), which equates to a gentle rocking motion known as heave [Source: NOAA].
Imagine riding in a car, going 50 miles per hour (80 km/hr) for comparison purposes. The vehicle experiences 0.5 g of acceleration at this speed — not much more than walking pace. With no additional weight aboard, such vehicles float in deep enough water to avoid collision damage during storms.
Now imagine boarding dozens upon dozens of humans into those same cars. Suddenly, each individual adds extra mass to the entire structure and increases drag against the current of the moving water. The larger group must sit higher within the cabin or move closer together outside.
If they choose the former option, they’ll need roomier cabins and bathrooms or add bulkheads between decks to create separate areas for dining, sleeping, and other activities. Either way, they exert more pressure on the ship’s outer skin to keep it stable.
First, consider the area beneath a typical cruise ship’s hull to understand how this process works. Imagine taking a vertical slice along the ship’s length from top to bottom. Below the waterline, you’d see several compartments separated by walls composed of wood, steel plates, or fiberglass insulation.
These spaces form different levels or decks and doors leading back and forth between them. On top of this arrangement rests the actual cargo hold or storage space. Within this layer lies everything from refrigerators to swimming pools.
However, if you look closely at any particular deck, you’ll notice another piece of interior architecture: stabilizer trusses. These support structures run perpendicular to the main beam walls inside the ship and help distribute the load between the inner and outer surfaces.
Without them, heavier objects placed near the ship’s center could cause it to tip forward. Stabilizers serve similar functions in airplanes, too.
Stabilizer frames don’t account for every single aspect of keeping a ship steady. Next, we’ll learn about ballast systems and their contribution to stability.
On July 9, 2009, Carnival Destiny set a new record when it reached speeds exceeding 30 knots (35 mph, 56 kph)! However, this was accomplished without relying on engines since the ship had four nuclear generators onboard, producing electricity.
Adding Stability
Ballasting refers to placing excess material beneath the keel to increase the amount of overall buoyancy. By doing so, ballasted vessels achieve increased stability due to reduced rolling and pitching motions. Ballasts come in many forms, including heavy lead weights and plastic bags filled with sand, gravel, and concrete blocks.
Other types include liquid or gas propellors, which pump air or hydraulic fluid through extraordinary chambers underneath the boat’s exterior surface. One example consists of interconnected pneumatic cylinders that push upward whenever a valve opens, allowing compressed air to escape and creating positive internal pressurization.
The valves close again when the cylinder bottoms out, forcing the opposite result. Another common type involves pumping seawater through tubes directly beneath the floorboards via a motorized system powered by solar panels.
In addition to increasing stability, these methods allow operators to trim off excess weight from the bow or stern. They extend from the bottom of the boat once underway, pushing the whole thing toward the middle until the desired position is achieved.
This reduces fuel costs since fewer batteries and motors are required to maintain balance. Some designs even incorporate retractable ballast tanks for easy stowing away when traveling slowly.
While ballasts certainly improve stability, they aren’t perfect. First, they reduce maneuverability, making turning sharply in shallow water harder. Second, they require regular maintenance and may eventually fail.
Finally, although ballast technology dates back centuries, modern versions still rely heavily on manual labor to operate appropriately. This often results in workers climbing atop the ship and manually adjusting hundreds of pounds weighing mere ounces.
Since ballast relies mainly on physical forces generated internally, it leaves the question of how cruise ships manage to stay upright unanswered. How can these enormous vessels remain stable when carrying thousands of passengers plus tons of precious cargo? We’ll investigate next.
Passenger capacity significantly affects a ship’s ability to handle choppy waters. While sailing past Ireland’s Blennore Head in April 2008, Royal Caribbean’s Oasis of the Seas experienced a sudden loss of power after losing propulsion control due to a generator failure.
Despite the ship’s automatic controls automatically turning off auxiliary power sources during emergencies, the situation quickly progressed into a major fire caused by overheated wiring. Fortunately, none of the 2,600 passengers died or sustained serious injuries, and the blaze burned itself out before reaching the engine rooms.
What’s Inside The Hull?
Although ballast provides excellent strength and stability, it doesn’t protect against structural failures. Therefore, a sturdy hull is essential. Modern cruise ships feature advanced composite materials explicitly designed for safety. Commonly used composites include carbon fiber, aramid fibers, and glass epoxy resins. Although generally stronger than traditional metals, these substances are lighter and easier to work with.
Carbon fiber is prevalent among manufacturers because it combines exceptional tensile strength with lightness. Its fibrous strands are woven tightly together to produce highly durable products. However, carbon fiber is usually formed by hand because it requires intense heat.
Aramid fibers consist of long chains of nylon molecules linked together. Their solid yet flexible nature allows them to withstand tremendous amounts of stress. Like carbon fiber, aramid fibers are created by heating raw materials to extreme temperatures to melt their bonds and join them together.
Glass epoxy resins combine bisphenol A and epichlorohydrin resin compounds with polystyrene beads soaked in amine curing agents. Resin components react chemically with the epoxy groups on neighboring particles, binding them together permanently.
A cruise ship’s structure includes external sheathing layers, internal bracing and cross-members, and secondary supports. Sheathing consists of thin sheets of plies of interlocked metal bent at right angles. Internal braces act as reinforcements for weaker sections, especially in corners.
Crossmembers connect beams to prevent buckling and twisting due to lateral loads. Secondary supports ensure that critical structural elements like elevators and staircases receive adequate reinforcement.
Despite all the precautions taken, accidents sometimes occur. During November 2005 transatlantic voyage from Port Canaveral, Florida, to Fort Lauderdale, a cruise ship named Pride Of America collided with a container crane, damaging the latter’s boom arm.
Luckily, nobody suffered severe injury or fatality. Two years later, another cruise ship named Celebrity Summit struck rocks while passing Bermuda. Again, fortunately, no lives were lost.
Just like driving a car, steering a cruise ship can be dangerous. Large yachts typically navigate by tacking, pulling straight ahead to the port and starboard sides simultaneously. Tacks involve rotating the wheel 90 degrees clockwise to alter course slightly, depending on whether the ship is headed northward or southward.
This method prevents sharp swings but puts considerable strain on the rudder mechanism. Smaller vessels employ electronic positioning systems that use GPS satellites to determine location and direction.
