Any structure built with hands or machines requires foundations. Foundations that touch upon, and weigh themselves over the surface of the earth, as a preliminary permission to exist as a structure itself. The hull has the same function, except that it floats above the earth, where water substitutes for air, and is the single most important part of any ship.
It is the body of the cruise ship kept afloat by a design structure that maintains buoyancy, protecting everything inside the ship from the sea water, and determines how efficiently (and comfortably) the ship moves through the water. It is cast and forged as a massive steel structure you see disappearing below the waterline, at the bottom. Its sides cut through waves, and support thousands of passengers, crew, pools, restaurants, theaters, and engines.
So, whenever someone asks, “what is a hull?”, you can comfortably answer. But, if you really want to know more, so that you can discourse on the modern cruise ship, it is pertinent to understand this most sophisticated piece of engineering on the planet a carefully shaped, multi-layered steel envelope that balances speed, stability, safety, fuel efficiency, and passenger comfort.
Let’s explore everything about the cruise ship hull: its definition, design, shape, materials, thickness, depth, underwater appearance, cleaning process, safety features (including double hulls), and related terms. All information is current as of 2025–2026, based on naval architecture sources, cruise industry reports, and technical data.
What Exactly Is the Hull of a Cruise Ship?
The hull of ship is the main body, without which the ship simply cannot be a ship. It is as simple as that. When this watertight outer steel structure extends from the bow (the pointed front of the ship or “nose of a ship”) to the stern (rear), and from the uppermost continuous deck (weather deck) down to the keel (the ship’s structural backbone at the very bottom), you can say this is it. In simple words:
- The hull is everything below the main deck that is in contact with the water.
- It is the bottom of a cruise ship and the curved sides that give the vessel its characteristic elliptical shape.
- It provides buoyancy (by displacing water), structural strength, and hydrodynamic efficiency.
The cruise ship hull shape is created with the help of thousands of steel plates welded together into a seamless, watertight envelope. Unlike the upper decks (which are mostly for passengers), the hull’s primary job is to keep the ship afloat, resist wave impacts, and support the enormous weight above (up to 230,000 gross tons on the largest ships like Icon of the Seas).
Types of Hulls Used on Cruise Ships
Most modern cruise ships are designed keeping water displacement in mind.
And because of this technical and engineering concern, modern cruise ships are constructed with the use of displacement hulls. A displacement hull pushes water aside as it moves forward, creating buoyancy by displacing a volume of water equal to the ship’s weight. This is the only practical hull type for large, heavy vessels like cruise liners.
Other hull types exist but are not used on large cruise ships:
Planing hulls: These are mostly designed keeping speedboats in mind as they have rapid speeds. Planing hulls lift out of the water at high speed, skimming over the surface of water. But they are not suitable for 200,000-ton vessels.
Semi-displacement hulls: Semi-displacement hulls combine speed and weight. They are hybrid builds for medium-speed craft (e.g., ferries).
Multi-hull designs (catamaran or trimaran): Twin hull cruise ship or tri hull cruise ship are very rare on large passenger ships due to size, cost, and stability trade-offs. They are, however, meant for luxury yachts or sports sailing boats. Some small expedition vessels use catamarans for shallow-water access, but mainstream cruise ships do not.
Most cruise ships have monohull displacement hulls with a very fine entry (sharp bow) and a relatively wide, flat-bottomed midsection for stability and passenger comfort.
Cruise Ship Hull Design and Shape
The cruise ship hull shape is carefully engineered for efficiency, stability, and seakeeping. This is a critical process that will determine whether a ship masters and survives sea turbulence or sinks due to structural issues. Key types of bow shapes are:
Fine entry (bow): The bow (front) is sharply pointed with a bulbous bow (a rounded protrusion below the waterline) that reduces wave-making resistance by 5–15% and improves fuel efficiency.
Deep V or U-shaped midsection: Most modern cruise ships use a U-shaped hull (shallow draft with rounded bilges) as this is extremely important to handle the weight tonnage of the whole ship. Deep V (sharper angle) may certainly be good for smooth rides, cutting through water, but it has stability issues. Therefore, the U-shape is preferred as it provides more interior volume and stability.
Wide beam: Cruise ships are very wide (up to 215 feet on Icon-class) with a nearly rectangular underwater cross-section below the waterline. This gives excellent stability. The ship can roll only a few degrees even in heavy weather which reduces sea-sickness on a cruise.
Flared bow and tumblehome: The flared bow shape tapers at the bottom and flares outward above the waterline. This is useful to deflect waves. In contrast, the tumblehome has the upper hull tumbling inward (shaped like wine-glass) to reduce wind resistance and improve ship aesthetics.
Keel: The keel of a ship is the bottom centerline consisting of a heavy steel spine that runs the full length, dividing the bow into two halves, providing longitudinal strength and stability. This is the first part of the laying of the bow.
The hull is painted red or black below the waterline with antifouling paint to prevent marine growth (barnacles, algae) that increases drag.
How Thick is the Hull of a Cruise Ship?
The thickness of a cruise ship hull varies by ship size and function. Let’s look the types of standard thickness used in ships:
Bottom plating (keel area): With 20–32 mm (0.8–1.25 inches) of thickness, it is meant for strength and ice resistance in some ships.
Side shell plating: With 12–25 mm (0.5–1 inch) thickness below, thinning at higher up, this size is mostly meant to manage the overall structural loads and sea wave impacts.
Bow and ice-strengthened areas: Up to 35–40 mm is best for protection against ice impacts and increases resistance to ice segments, floating dangerously.
Double bottom: Most modern cruise ships have a double bottom (two layers with 1–2 meters space between) for fuel, ballast, and extra safety. The inner bottom is 10–15 mm thick.
Modern steel is used for increasing thickness. It is high-strength (AH36/DH36 grade), allowing thinner plating than older ships (e.g., Titanic had 25–38 mm plates, but they were made of low carbon, mild steel with impurities in alloy). Double bottoms are very useful in adding redundancy; if the outer hull is breached, the inner layer ph2revents flooding.
How Deep Is the Hull of a Cruise Ship?
Draft (how deep the hull sits in the water) is the vertical distance from the waterline to the lowest point of the keel. The depth also depends on how much tonnage the ship has, which puts pressure against the buoyant action of the hull.
Typical cruise ship draft: 25–35 feet (7.6–10.7 meters) submerged below the sea surface.
Oasis-class (e.g., Oasis of the Seas): Approximately 30–33 feet (9.1–10 m).
Icon-class: Up to 35 feet.
Smaller ships (e.g., Azamara): 20–25 feet.
The hull itself extends the full draft depth (the depth of submersion when the vessel is fully loaded), with most of the submerged volume in the lower 10–15 meters.
What Does the Hull of a Cruise Ship Look Like Underwater?
If you are curious, you may dive into the water, where you’ll find that in the underwater (diver or ROV view), the cruise ship hull looks smooth, streamlined, and massive.
Key features of underwater hull includes:
Red or black antifouling paint: Often bright red below the waterline, fading to black as you go deeper. It helps prevent the formation of algae and barnacles that can drag the ship slow.
Bulbous bow: A large rounded bulb protruding forward, reducing drag from water resistance.
Propellers and rudders: Massive bronze propellers (up to 20 feet diameter) at the stern. This is a marvel of engineering, but also dangerous, especially when they are spinning.
Stabilizer fins: Retractable fins midship for roll reduction.
Sonar domes: Rounded protrusions for navigation. Usually housed in bulbous bow. It provides information on ground depth beneath sea surface and provides clues to change direction, especially if an obstacle appears. Cruise ships don’t use this. But military vessels do.
Intakes and discharges: Water inlets/outlets are designed to manage the flow of water from the bow ballasts. As the ship travels, fuel expenditure lightens the tonnage, reducing the waterline depth and consequently stability of the ship. Hence water is pumped into ballasts to increase bow submerged depth.
The hull is surprisingly clean on well-maintained ships, thanks to regular cleaning. Let’s look at how this is done.
How Do They Clean the Hull of a Cruise Ship?
Cruise ship hull cleaning is critical. Barnacle formation not only creates problems in the way the ship bow looks, but it also increases water drag by 20–30%, raising fuel consumption by millions annually.
Methods:
In-water hull cleaning: Scuba divers or ROVs (remotely operated vehicles) use robotic brush karts or high-pressure water (eco-friendly, no toxic chemicals) to remove algae and barnacle formations. This cleaning task is performed in port every 3–12 months.
Hull grooming: This is frequent, proactive light cleaning (every 1–3 months) as opposed to reactive deep cleaning to prevent heavy fouling. Most cleaning operations are done in submerged conditions as opposed to deep cleaning in dry dock conditions.
Dry docking: This task is done every 2–5 years. In this process, the ship is lifted out of the water for full scraping, repainting, and inspection. Antifouling coatings last 3–5 years, so they need to be repainted if necessary.
New tech: Robotic crawlers, ultrasonic systems, and non-toxic silicone paints.
Regulations: IMO (International Maritime Organization) bans toxic paints.
Nowadays, major focus is on eco-friendly methods to protect marine life.
Conclusion: The Hull is Everything for a Cruise Ship
The cruise ship hull is far more than the bottom of a ship it’s the foundation of safety, efficiency, and comfort. From its carefully shaped cruise ship hull design to its robust steel construction, double bottoms, and regular cruise ship hull cleaning, the hull enables the magic of cruising.
Next time you stand on the promenade deck, look down at the waterline and appreciate the engineering marvel below. The hull is what makes the cruise possible.
Enjoy! ??
