What Is a Bulwark on a Ship?

A bulwark is the solid wall of steel plating that runs along the edge of a ship's exposed deck. It continues the line of the ship's side above the deck, usually finished with a rail along the top and held upright by stays, and its job is to stop people and deck cargo going over the side and to keep some of the sea off the deck.

Many cargo ships have bulwarks at the forecastle and along parts of the main deck. Tankers and other ships with reduced freeboard mostly use open guard rails on the main deck instead, which do the same safety job without forming a solid wall.

What a bulwark does

• It protects crew working on deck from falling overboard, particularly in heavy weather.
• It keeps deck cargo and loose gear on board.
• It reduces the spray and green water that come over the deck edge.
• It carries fittings at the mooring stations, such as fairleads and closed chocks set into the plating.

How high a bulwark must be

The minimum is set by the International Convention on Load Lines, 1966. Regulation 25 requires bulwarks or guard rails at least 1 metre high on the exposed parts of the freeboard and superstructure decks. A lower height can be approved where 1 metre would interfere with the normal operation of the ship, provided the flag Administration is satisfied that adequate protection is given.

How a bulwark is built

On a steel ship the bulwark is steel plate, stiffened along the top by the bulwark rail and supported at intervals by bulwark stays welded to the deck. It is not part of the watertight hull and is not designed to carry the hull's main bending loads.

The deck edge is one of the most highly stressed parts of the hull, so classification rules control how the bulwark is attached there: over the midship part of the ship it is generally not welded to the top of the sheer strake, so that hull bending is not passed into the thinner bulwark plate.

Why bulwarks need freeing ports

Bulwarks also trap water. When a wave breaks over the deck, they turn it into a well that holds the water. Water trapped on deck adds weight high up and surges from side to side, which reduces the ship's stability. Scuppers and deck drains are far too small to clear that volume quickly, so the Load Line Convention requires freeing ports: openings in the bulwark at deck level that let shipped water run straight back overboard.

How much freeing port area is required

Regulation 24 of the Load Line Convention sets a minimum freeing port area on each side of the ship for each well on the freeboard deck. It depends on the length of bulwark in the well, l, in metres:

• where l is 20 metres or less, the area is 0.7 + 0.035 × l square metres;
• where l is more than 20 metres, the area is 0.07 × l square metres.

The length l need not be taken as more than 70 per cent of the ship's length. The formula assumes a bulwark of ordinary height on a deck with standard sheer:

• If the bulwark's average height is more than 1.2 metres, the area is increased by 0.004 square metres per metre of length of the well for each 0.1 metre of extra height. If it is less than 0.9 metres, the area may be reduced by the same amount for each 0.1 metre below.
• In a ship with no sheer, the area is increased by 50 per cent, with an interpolated increase where the sheer is less than standard.
• Wells on superstructure decks need half the area that the formula gives.

As an example, take a well with 30 metres of bulwark on a deck with standard sheer. The basic area is 0.07 × 30 = 2.1 square metres on each side. If the bulwark averages 1.5 metres high, it is 0.3 metres above 1.2 metres, so the area goes up by 0.004 × 30 × 3 = 0.36 square metres, to 2.46 square metres.

Two-thirds of the required area must be placed in the half of the well nearest the lowest point of the sheer curve, where water collects. The convention has separate provisions for ships with trunks and for superstructures open at the ends, and ships assigned reduced freeboards have stricter freeing requirements than those described here. Fishing vessels are outside the Load Line Convention and have their own rules.

How freeing ports are arranged

• The lower edge of each freeing port is kept as near the deck as practicable, so water drains before it builds up.
• Openings are usually rectangular with rounded corners, because sharp corners concentrate stress in the plating. Large or numerous freeing ports may need extra stiffening around the openings.
• Openings in the bulwark must be protected by rails or bars spaced about 230 millimetres apart, so a person cannot be washed through.
• If shutters are fitted, they need ample clearance so they cannot jam, their hinges must have pins or bearings of non-corrodible material, and any securing appliances must be of approved construction.

Freeing ports blocked by stowed gear or lashings, and shutters seized by paint or corrosion, stop the openings working, so they are worth checking on routine deck rounds.

Related reading

Bulwarks and freeing ports are part of the load line system that sets how deep a ship may load: see load line marks and draft markings. For how a bulwark differs from a bulkhead, see bulkhead vs bulwark, and for the decks a bulwark runs along, ship decks and their purpose.