How Cruise Ships Float, Steer and Stay Upright at Sea

A cruise ship floats because of buoyancy. Its wide, hollow hull pushes aside an enormous amount of water, and that displaced water pushes back up with a force equal to the ship’s weight. As long as the hull can displace that much water without running out of freeboard, the ship rides on the surface, however heavy it looks.

The rest of this guide covers what keeps that ship upright, afloat after damage, pointed where the captain wants, powered, anchored and on course in the dark.

How do cruise ships float?

This is Archimedes’ principle: any object placed in water is pushed up by a force equal to the weight of the water it displaces. A cruise ship sinks into the water only until the weight of the water it has pushed aside equals the weight of the whole ship. At that point the two forces balance and the ship stops sinking.

Steel is far denser than water, so the trick is shape. A solid block of steel sinks. A cruise ship is mostly empty space, with air in the hull, the decks and the cabins, so its average density is well below that of seawater. A broad, deep hull displaces a lot of water without having to sit very far into it.

Weight still matters. Fuel, water, cargo, supplies and passengers all change how deep the ship sits, and crews track the loading throughout a voyage.

How much of a cruise ship is underwater?

Much less than most people expect. The part of the hull below the waterline is called the draft, and on a large modern cruise ship it is only a fraction of the ship’s total height. The same ship can tower many decks above the water, so most of what you see is above the surface.

A few reasons it sits so shallow:

Wide, hollow hulls. Cruise ships are broad and mostly empty inside, so they displace a lot of water without sitting deep.
Light upper decks. Cabins, public rooms and the superstructure are built relatively light, so they add height without pushing the hull much lower.
Spread-out weight. The mass is distributed over a large footprint, which keeps the draft modest.

Draft also tells you the minimum water depth a ship needs, which is why a cruise ship can enter many ports and channels that a deep-draft tanker cannot. Exact drafts vary by ship and loading. The size comparisons in how big modern cruise ships are give the wider picture.

How do cruise ships not tip over?

Cruise ships are designed to be self-righting. The heaviest items, including engines, fuel, water tanks and machinery, sit low in the hull. That keeps the center of gravity low, so when a wave or gust heels the ship over, the shape of the submerged hull creates a “righting moment” that pushes it back toward level. The further it leans, within design limits, the harder it pushes back.

Three things help:

Low weight. Heavy equipment is packed into the lowest decks, below the waterline.
Ballast. Tanks that can be filled with seawater let the crew fine-tune balance and compensate as fuel and water are used up.
Stabilizers. Most large ships have retractable fins below the waterline that extend like small wings. They do not prevent tipping, but they sharply reduce rolling in rough seas.

Stability is regulated, not left to chance. The International Maritime Organization’s SOLAS convention and related intact-stability rules set how a passenger ship must be designed and loaded, and ships carry approved stability information so officers can check loading conditions. In normal wind and waves the physics keeps a modern ship upright. Capsizing is associated with something going badly wrong, such as serious flooding, rather than weather alone.

Why don’t cruise ships sink?

The most important feature is watertight subdivision. The hull is split into sealed compartments, and heavy watertight doors can close to contain flooding to one section. SOLAS damage-stability rules require passenger ships to be able to stay afloat and stable with a defined amount of flooding, which is why a localized breach does not have to become a disaster. Newer passenger ships are also required to be built for “safe return to port,” meaning key systems must keep working after a casualty so the ship can get to a safe harbor under its own power.

Backup systems add to that:

Multiple bilge and ballast pumps to remove incoming water and rebalance the ship.
Redundant engines and power, so the ship stays maneuverable if one system fails.
Constant monitoring, so the crew spots flooding or fire early.
Lifeboats and rafts for everyone aboard, plus the muster drill, for the rare case an evacuation is needed.

No ship is truly unsinkable. Damage beyond what the design allows for can overwhelm these systems, and a handful of cruise ship losses over the decades show that. They stand out because they are rare. For the realistic scenarios, such as a grounding or collision, the layered design is built to hold. The U.S. Coast Guard also inspects foreign-flag passenger ships that embark passengers in U.S. ports, so uscg.mil is the place to look for how that oversight works. For the wider risk picture, see are cruise ships safe.

How do cruise ships steer?

Older ships steered with a fixed propeller and a rudder that deflected water to turn left or right. That still works, but the ship has to be moving for the rudder to bite.

Most large modern cruise ships use azimuth pods, often called azipods. An azipod is a propeller in a pod under the hull that can rotate through a full circle, so the ship steers by aiming its thrust instead of deflecting water past a rudder. That gives much finer control, especially at low speed.

At the front, tunnels through the hull house bow thrusters, small propellers that push the bow left or right. With pods at the stern and thrusters at the bow, the captain can:

Rotate the ship in place by pushing the bow one way and the stern the other.
Move sideways to ease the hull against a pier.
Hold position against wind and current while waiting to dock.

This is why many ships dock with little or no tug assistance. The same systems support dynamic positioning, where the ship holds a fixed spot using thrusters and propulsion when there is no suitable berth or anchorage. Joystick-style controls on the bridge blend the pods and thrusters into one motion.

How do cruise ship anchors work?

An anchor holds a ship mostly through its chain, not just its own weight. The crew pays out far more chain than the water is deep, often several times the depth. The extra chain lies along the seabed, and its weight and friction do most of the holding. It also keeps the pull on the anchor nearly horizontal, which lets the anchor’s flukes dig into the bottom instead of being pulled straight out.

A large cruise ship’s anchor weighs many tons, and the links are individually very heavy. Ships usually carry two anchors at the bow for redundancy, raised and lowered by a powerful winch called a windlass.

The system only works where the chain can reach the bottom and lie along it, so ships do not anchor in the open ocean. At deep ports without a pier, a ship may anchor in shallower water offshore or hold position with its thrusters, while tenders carry guests ashore.

What do cruise ships use for fuel?

Most cruise ships burn marine fuel oil, with a growing number of newer ships running on liquefied natural gas (LNG). Fuel use is large. A big ship at cruising speed can burn many tons of fuel per day, and the figure rises quickly with speed, which is one reason itineraries are planned around moderate cruising speeds.

The main fuels:

Heavy fuel oil. A thick, cheap residual fuel traditionally used on big ships. It has to be burned with exhaust scrubbers or swapped for lower-sulfur fuel to meet current rules.
Marine gas oil and low-sulfur marine diesel. Lighter, cleaner distillate fuels used where regulations demand them.
LNG. Found on a growing number of recently built ships. It sharply cuts sulfur and particulate emissions compared with oil, though it is not emission-free.

Regulations drive much of the choice. The IMO’s global sulfur cap on marine fuel took effect in 2020, and designated emission control areas have stricter limits. Details are on the International Maritime Organization site. Lines are also testing biofuels, methanol, batteries and shore power (plugging into the local grid in port), so the mix is slowly shifting.

Fuel does not only move the ship. It also feeds the generators that run everything else on board, so a ship burns fuel even while docked.

How do cruise ships generate electricity?

A cruise ship makes its own electricity with large onboard generators, usually driven by diesel or dual-fuel engines. Most modern ships are diesel-electric: the engines spin generators, and that electricity runs the electric motors that turn the propellers as well as the cabins, galleys, air conditioning, pools and lighting.

Compared with engines directly driving propellers, that has advantages:

Flexibility. One shared supply can be shifted between propulsion and hotel loads.
Efficiency. The ship runs only as many generators as current demand needs.
Redundancy. With several separate generator sets, the ship keeps going if one unit is down for maintenance or fails.

A large ship has the electrical demand of a small town. Air conditioning, kitchens, laundries, water-making plants and thousands of cabins all draw power, and the propulsion motors add a bigger load at speed. Power is distributed through an onboard grid with switchboards and backups. More ships can now take shore power in compatible ports, shutting down engines while docked. The freshwater plants that eat much of this power are covered in where cruise ships get fresh water.

How do cruise ships navigate at night?

Almost exactly the way they do by day, using instruments rather than eyesight. The bridge team relies on:

GPS for the ship’s exact position.
Radar, which detects vessels, land and obstacles in darkness, fog and rain.
Electronic charts (ECDIS), digital nautical charts showing depth, hazards and the planned route with the ship’s live position overlaid. The underlying chart data comes from national hydrographic offices such as NOAA in U.S. waters.
AIS, which broadcasts and receives identity, position, course and speed so vessels can see and avoid each other.

The bridge is staffed around the clock by officers in rotating watches, with lookouts keeping a visual and radar watch. The bridge is kept dark, with dimmed lighting on the instruments, to protect night vision and avoid glare on the windows. Because the systems are redundant and the watch is always manned, sailing at night is routine.

Part of our How Cruise Ships Work hub.