Bulkheads are the vertical walls inside a ship’s hull. Every internal wall on a vessel is called a bulkhead, with the single exception of the outer hull itself. They serve three core purposes: strengthening the ship’s structure, dividing the interior into functional rooms, and creating sealed compartments that can contain flooding if the hull is breached.
How Bulkheads Keep a Ship Afloat
The most critical job of a bulkhead is flood control. Watertight bulkheads divide the hull into separate compartments, so if one section floods, the water stays contained rather than spreading through the entire vessel. This is what allows a damaged ship to maintain enough buoyancy to stay afloat, or at least stay afloat long enough for evacuation. Each watertight bulkhead must be strong enough to hold back a full head of water reaching the top of the wall, and they must extend continuously from the bottom of the hull up to the bulkhead deck (the uppermost deck to which watertight walls reach) without gaps, steps, or recesses.
International safety rules govern how far apart these bulkheads can be. On passenger vessels longer than about 65 feet or carrying more than 49 passengers, the maximum spacing between main watertight bulkheads cannot exceed one-third the length of the bulkhead deck. The math gets more specific from there, factoring in freeboard (how high the deck sits above the waterline) and the ship’s depth, but the principle is simple: shorter spacing means smaller compartments, which means less water can enter any single section.
The Collision Bulkhead
One bulkhead gets special treatment: the collision bulkhead, located near the bow. This wall is specifically designed to protect the rest of the ship if the front end is crushed in a collision. Under international maritime safety rules (SOLAS), it must sit at a minimum distance of 5% of the ship’s length from the bow, or 10 meters, whichever is smaller. It can’t be placed too far back either, with a maximum distance of 8% of the ship’s length or 5% plus 3 meters.
The collision bulkhead is held to stricter standards than other watertight walls. On vessels operating in ocean or coastwise routes, it must extend all the way up to the weather deck or one deck above the bulkhead deck, whichever is lower. Almost no holes are allowed through it. Where penetrations are absolutely necessary (for essential piping, for example), they must be placed as high and as far inboard as possible and fitted with watertight seals.
Transverse vs. Longitudinal Bulkheads
Bulkheads run in two directions. Transverse bulkheads cross the ship from side to side, and longitudinal bulkheads run along the ship’s length from bow to stern. They handle different structural loads.
Transverse bulkheads are the primary flood barriers. They chop the hull into a series of compartments from front to back, and they resist forces that try to rack or twist the hull sideways. Longitudinal bulkheads, by contrast, contribute to the ship’s overall bending strength. Because they run parallel to the keel, they help the hull resist the enormous forces of waves trying to bend it upward at the ends (called hogging) or push it down in the middle (called sagging). Longitudinal bulkheads that form part of the hull’s global structure are held to stricter engineering standards, including additional checks for buckling under stress.
On tankers and bulk carriers, you’ll often find one or two longitudinal bulkheads running down the centerline of the cargo area, dividing the hold into port and starboard tanks. This prevents a free surface effect: if a partially filled tank stretches the full width of the ship, liquid sloshing from side to side can cause dangerous rolling. A centerline bulkhead cuts that risk dramatically.
Fire-Rated Bulkheads
Not all bulkheads are about water. Many are rated for fire resistance, acting as barriers that stop flames and heat from spreading between compartments. Marine fire bulkheads fall into three classes.
- Class A bulkheads are built from steel or equivalent metal. They block smoke and flame for a full hour. Depending on their insulation rating (A-60, A-30, A-15, or A-0), they also limit how much heat passes through. An A-60 bulkhead keeps the unexposed side from rising more than 139°C (250°F) above its starting temperature for 60 minutes. These are used around high-risk areas like engine rooms, galleys, and control stations.
- Class B bulkheads are made from noncombustible materials and block flame for 30 minutes. They run from deck to deck and are commonly used between staterooms, along corridors, and around stairway enclosures.
- Class C bulkheads are also noncombustible but have no specific fire or heat resistance requirements. They simply divide spaces where fire risk is low.
Which class goes where depends on what’s on each side of the wall. A bulkhead separating a machinery space from a stateroom gets a much higher rating than one between two open deck areas. Detailed tables in maritime regulations specify the exact class required for every combination of adjacent spaces.
Corrugated and Flat Bulkheads
Walk into the cargo hold of a bulk carrier or chemical tanker and you’ll likely see bulkheads with a distinctive zigzag profile. These corrugated bulkheads use a folded pattern (like a giant accordion) instead of a flat plate with welded-on stiffeners. The corrugation itself provides the structural strength, which brings several practical advantages: they weigh less than flat stiffened bulkheads, they’re easier to clean and inspect because there are no frames or brackets trapping cargo residue, and they suffer fewer corrosion problems because moisture doesn’t collect in hidden corners.
Flat bulkheads reinforced with horizontal or vertical stiffeners are still common throughout most ship types, particularly in areas where equipment, doors, or complex piping layouts make a corrugated design impractical.
Keeping Bulkheads Sealed
A bulkhead is only as good as its weakest point, and the most vulnerable points are where pipes, cables, and ducts pass through. Every penetration through a watertight or fire-rated bulkhead requires a type-approved sealing system, tested and certified to maintain the wall’s rating. A typical seal consists of a steel frame welded into the bulkhead opening, modular rubber or composite blocks that pack tightly around each cable or pipe, and wedge bolts that compress everything into a watertight, fireproof, and sometimes gastight barrier.
These seals are considered critical safety barriers. A single poorly installed cable transit can compromise an entire watertight compartment, which is why classification societies and port-state inspectors pay close attention to their condition during surveys. Over time, vibration, thermal cycling, and corrosion can degrade seal integrity, making regular inspection essential to the ship’s safety.