Bridge visibility, often called line of sight, is one of the constraints that most often caps deck stack heights on container ships, especially in the forward bays. A ship can still have free slots, enough GM and compliant lashing, yet a single extra tier in the foremost bay can stop the master from seeing the sea surface close enough ahead of the bow. This guide covers the idea behind the SOLAS rule, the sight-line geometry and how to check it while building a stowage plan.
What SOLAS V/22 asks for
SOLAS Chapter V, Regulation 22 deals with navigation bridge visibility for ships above a certain length (broadly, ships of 55 m and over built after the rule came into force). In our own words, the core points are:
- Blind distance ahead. From the conning position, the sea surface must not be hidden for more than two ship lengths or 500 m ahead of the bow, whichever is less, across a sector of about 10° either side of right ahead. This holds for every condition of draught, trim and deck cargo.
- Blind sectors from cargo. Obstructions outside the wheelhouse, such as container stacks, cranes or lashing bridges, may only create narrow blind sectors. There are limits on the width of each blind sector, on the total of all of them, and a minimum width for the clear sectors in between.
- Horizontal field of view. The conning position needs a wide view forward and to each side, reaching slightly abaft the beam.
The detailed sector angles and the definition of ship length should be read in the current SOLAS text from IMO or in the ship's own documents. For a planner the key phrase is "all conditions of deck cargo": meeting the rule is not settled once at the design stage, it is decided again by every voyage's loading plan.
Sight-line geometry: similar triangles
Picture a straight line from the observer's eye on the bridge that just grazes the top edge of the most critical stack and carries on down to the sea. Where it meets the water is the edge of the visible area.
Notation:
- H = eye height above the waterline (m)
- h = stack-top height above the waterline (m)
- d = horizontal distance from the eye to the forward edge of the stack (m)
- D = horizontal distance from the eye to where the sight line meets the water (m)
Similar triangles give D / H = d / (H − h), so D = d × H / (H − h). The blind distance ahead of the bow is D minus the distance from the eye to the bow.
The formula makes two things clear. As h approaches H the denominator shrinks and D grows fast, so one extra tier matters a lot. And a stack far forward (large d) hurts more than an equally tall stack near the bridge. So check every deck bay, including the forecastle and any bow equipment, and keep the largest D.
Worked example (illustrative vessel)
A fictional container feeder, rule length 145 m, bridge aft. The limit is the lesser of 2 × 145 = 290 m and 500 m, so 290 m. The eye at the conning position is 120 m aft of the bow. The forward edge of the stack in the foremost bay is 20 m aft of the bow, so d = 100 m.
| Case | H (m) | h (m) | H − h (m) | D (m) | Blind distance ahead of bow (m) | Result |
|---|---|---|---|---|---|---|
| A. Base, even keel | 30.0 | 22.0 | 8.0 | 375 | 255 | Complies (35 m margin) |
| B. One extra tier (2.59 m) | 30.0 | 24.59 | 5.41 | 555 | 435 | Does not comply |
| C. 1.4 m trim by the stern | 29.5 | 22.5 | 7.0 | 421 | 301 | Does not comply (11 m over) |
| D. 1.0 m parallel sinkage | 29.0 | 21.0 | 8.0 | 363 | 243 | Complies |
How case C works: the LCF is taken 70 m aft of the bow, so the eye is 50 m aft of the LCF and the stack 50 m forward of it. A 1.4 m change of trim over a 140 m LBP gives a slope of 0.01. The eye drops 50 × 0.01 = 0.5 m and the stack top rises 0.5 m. H − h loses 1.0 m, and a compliant ship becomes non-compliant. Trim is covered in more depth in ship trim and draft, and you can work out trim changes with the draft & trim calculator.
Case D surprises many people: the deeper ship with the same stacks is slightly better off, because the slope of the sight line is unchanged but the eye is closer to the water. The critical case is usually a light draught with a lot of empties stacked high.
Stack height and container type
Stack height depends on the number of tiers and the container type. A standard box is 8'6" (2.59 m) high and a high cube is 9'6" (2.90 m). Four high cubes stand about 1.2 m taller than four standard boxes, close to half a tier. See also container sizes.
Practical habits that protect line of sight:
- Put high cubes and the tallest stacks in bays close to the bridge, not in the foremost bay.
- Use the ship's visibility table, where the loading manual has one, which gives the maximum stack height per bay for given draught and trim combinations.
- Plan ballast so the final trim is not excessively by the stern. Some stern trim is usually wanted for the propeller, so agree the compromise with the chief officer.
- Check the outer rows too, because the 10° sector either side can be blocked by wing stacks.
Slot and tier positions are explained in container ship bay planning and the glossary entry for bay-row-tier.
The link with air draft
Air draft is the height of the ship's highest point above the waterline, including container stacks, masts and antennas. In simple terms: air draft = height of the highest point above the keel − draught at that position. It matters when passing under bridges and overhead cables, or working under certain terminal cranes.
Visibility and air draft both depend on stack height, but draught pushes them in different directions. A light draught raises the air draft (harder to clear a bridge) and slightly worsens the blind distance. A deep draught lowers the air draft. Check both in every departure condition and, where relevant, during the voyage, for example after fuel consumption has changed the trim. See draft for the definition.
Checking it in your planning workflow
In STOWAGE.ID Vessel Stowage, the line-of-sight check is calculated from the stack height of each bay plus the draught and trim of the planned condition, and flagged as it approaches the limit. It is a shore-side planning and decision-support tool. The official reference remains the vessel's class-approved loading computer and stability booklet, together with the judgement of the chief officer and master. For the wider container ship workflow, see container ship stowage.