Deck cargo barges of 180 to 330 feet are the backbone of sea transport in Indonesia. They carry coal out of Kalimantan, nickel ore and heavy equipment to Sulawesi and Maluku, and project modules to sites without a proper quay. Because they have no engine and are towed by a tug, barges bring stowage considerations of their own that differ from a conventional cargo ship.
Basic stability concepts are explained in our guide to ship stability basics for planners. Here we focus on flat-top barges and coal barges.
Common barge sizes
Barge sizes in Indonesia are usually quoted in feet. The figures below are rough, illustrative ranges; real capacity depends on the design, freeboard and load line certificate of each barge.
| Size | Rough dimensions (L × B × D) | Rough cargo capacity |
|---|---|---|
| 180 ft | ± 55 × 15 × 3.7 m | ± 1,500–2,000 t |
| 230 ft | ± 70 × 21 × 4.3 m | ± 3,000–4,000 t |
| 270 ft | ± 82 × 24 × 5.0 m | ± 5,000–6,000 t |
| 300 ft | ± 91 × 24–27 × 5.5 m | ± 7,000–8,000 t |
| 330 ft | ± 100 × 30 × 6.0 m | ± 10,000–12,000 t |
You can estimate a specific barge with the barge capacity calculator and sketch the deck layout with the barge visualizer.
Deck load and zones
A barge deck is designed for a uniform load in tonnes per square metre (t/m²), but its strength is not the same everywhere. The areas over longitudinal bulkheads, transverse bulkheads and web frames are far stronger than the plating between stiffeners. It helps to divide the deck into zones:
- Centre zone over the centre longitudinal bulkhead, suited to the heaviest items.
- Side zones near the shell, strong over the side bulkheads but more affected by roll.
- End zones near the bow and stern, often weaker and wetter.
Point loads from cargo feet or supports must be spread with timber dunnage or a steel grillage and should land over bulkheads or web frames. A quick check is available in the deck load calculator, and the principles are covered in our guide to deck and tank top strength.
CoG, trim and heel of a flat-top barge
Typical targets for the combined cargo centre of gravity:
- Longitudinal: close to midship or the LCF, ending with a slight trim by the stern so the barge tracks well under tow.
- Transverse: on the centreline, so the barge has no permanent list.
- Vertical: as low as possible, especially for tall modules that also add wind area.
A barge is wide and shallow, so BM (≈ B²/12T for a box shape) is very large and the initial GM can be tens of metres. Heel from a slightly off-centre load is small, but that does not make every condition safe. Freeboard, the angle of deck edge immersion, wind load on tall cargo and the full GZ curve must still be checked against the stability criteria that apply to that barge.
Worked example: combined CoG and deck pressure
An illustrative fictional 270 ft barge, treated as a box: L 82 m, B 24 m, lightship 1,200 t with LCG at midship and KG 3.0 m. Longitudinal distances are from midship (+ forward), transverse distances from the centreline (+ starboard).
| Item | Weight (t) | LCG (m) | TCG (m) | VCG (m) |
|---|---|---|---|---|
| A: transformer + grillage | 180 | −10.0 | 0.0 | 7.3 |
| B: module 1 | 150 | +12.0 | +5.0 | 7.0 |
| C: module 2 | 150 | +12.0 | −5.0 | 7.0 |
| D: pipes and fittings | 120 | −22.0 | +1.5 | 6.0 |
| Total cargo | 600 | −1.40 | +0.30 | 6.89 |
Longitudinal moment −1,800 + 1,800 + 1,800 − 2,640 = −840 t·m, so the cargo LCG is 1.40 m aft of midship. Transverse moment +750 − 750 + 180 = +180 t·m, so the TCG is 0.30 m to starboard. Use the combined CoG calculator for longer cargo lists.
Draft and trim. TPC ≈ 82 × 24 × 1.025 / 100 = 20.2 t/cm. The lightship draft is 0.60 m and 600 t of cargo adds about 29.7 cm, giving a mean draft of about 0.89 m at 1,800 t displacement. For a box, BML ≈ L²/12T = 628 m and MTC ≈ 1,800 × 628 / (100 × 82) ≈ 138 t·m/cm. Change of trim = 840 / 138 ≈ 6 cm by the stern, split equally because the LCF is at midship: about 0.92 m aft and 0.86 m forward.
Heel. KB ≈ 0.45 m and BM ≈ 24² / (12 × 0.89) ≈ 53.8 m, so KM ≈ 54.3 m. Combined KG = (1,200 × 3.0 + 600 × 6.89) / 1,800 ≈ 4.30 m, so GM ≈ 50 m. Heel: tan θ = 180 / (1,800 × 50) = 0.002, or about 0.1°. Small, but still worth correcting where possible.
Deck pressure. The 174 t transformer on a 6.0 × 2.5 m base (15 m²) gives 11.6 t/m², above the example zone limit of 10 t/m². A 6 t grillage spreading the load over 8.0 × 5.0 m (40 m²) brings it down to 180 / 40 = 4.5 t/m². This even-spread assumption only holds if the grillage is stiff enough and bears on bulkheads or web frames; for heavy items, ask a naval architect to check.
Sea-fastening and dunnage
Sea-fastening holds the cargo against the forces from roll, pitch and heave during the tow. Design forces come from motion criteria agreed with the owner, insurer or marine warranty surveyor. Widely used marine warranty guidelines give default motion criteria for barges that vary with barge size; use the figures from the guideline that applies to the project.
Common sea-fastening includes steel stoppers welded to the deck, chains or slings with turnbuckles, and bracing for tall items. Timber dunnage adds friction and spreads load, but should not be relied on as the only restraint against sliding. A first estimate can be made with the sea-fastening calculator. For cargo ships, securing assessments follow the CSS Code and the ship's Cargo Securing Manual.
Coal vs project cargo
Coal is loaded in bulk into barges with side walls, usually by conveyor at a jetty or by loaders on a river. The cargo must be trimmed level so the load is spread and the barge does not trim or list excessively. The quantity is measured by draft survey. Coal is listed in the IMSBC Code with hazards such as self-heating and methane emission, so ventilation, temperature monitoring and compacting the pile also matter.
Project cargo on a flat-top barge follows the example above: point loads, grillage, sea-fastening, stability and often marine warranty surveyor approval before departure.
Tug and tow considerations
- Bollard pull of the tug must suit the barge size, cargo and weather on the route.
- Towing gear: bridle, main towline, emergency towline and suitable securing points.
- Wind area: tall cargo adds wind load and can reduce visibility.
- Documents: port clearance from the harbour master (syahbandar) and the barge's load line certificate.
STOWAGE.ID Vessel Stowage helps build and check barge stowage plans from shore, including the combined CoG and deck pressure per zone. STOWAGE.ID is a shore-side planning and decision-support tool; the barge's stability booklet, structural drawings and surveyor approval remain the official reference.