Load Lines 1966 (as amended)
The Plimsoll mark, the world load-line zones, reserve buoyancy and watertight integrity — explained with interactive visuals for cadets, mates and masters.
Welcome to the International Convention on Load Lines, 1966 — the rules that decide how deeply a ship may be loaded.
Main themes of the Convention
Legal cornerstone (since 1968)
Adopted 5 Apr 1966, in force 21 Jul 1968. Sets universal minimum freeboards to limit overloading — historically the #1 cause of foundering. Plimsoll's 19th-century campaign codified into a global standard.
Assigned freeboard
Calculated from tabular freeboard + corrections for block coefficient, depth, superstructures, sheer and bow height. Resulting waterlines are stamped into the hull as the load-line marks.
Zones, areas & seasonal periods
The 1966 Convention divides the world ocean into permanent Tropical / Summer / Winter zones plus seasonal areas (incl. WNA). Each ship must respect the mark applicable to her position and the month.
Watertight & weathertight standards
Chapter II prescribes hatch covers, doors, ventilators, air pipes, freeing ports and side scuttles. These are the conditions of assignment — break them and the freeboard is no longer valid.
Survey, certification & PSC
International Load Line Certificate (ILLC) — initial, annual, renewal surveys. Port-State Control inspectors check the marks, the certificate, and the openings/fittings on every visit.
1988 Protocol & 2003 amendments
Harmonized survey/certification system (HSSC) + major Annex B updates: revised strength & intact-stability rules, tougher hatch-cover, bow-height and reserve-buoyancy requirements for bulk carriers post-Derbyshire.
The Plimsoll mark — interactive
Click any seasonal mark on the right to see the corresponding waterline rise or fall on the hull. The centre of the disc aligns with the Summer (S) line; the two pairs of letters (e.g. AB / CD) identify the assigning classification society.
Drag to load the ship. Click any mark on the mast to change the applicable seasonal line — the waterline must stay at or above the yellow band.
Summer (Salt Water)
The reference line. Centre of the Plimsoll disc aligns with S.
Global load-line zones — live by month
The Convention's Annex II splits the oceans into permanent and seasonal zones. Drag the month slider — bands re-colour, the WNA box switches on (Nov–Mar) and off, and each band shows which mark (T / S / W / WNA) the ship must respect right now. Click a band to load that mark into the Plimsoll diagram above and check compliance with the cargo slider.
A bulker steams Hamburg (53°33′N 9°59′E) → Recife (8°03′S 34°53′W) on the actual commercial track: Dover Strait, Ushant, Finisterre, Madeira, Canaries, Cape Verde, then SW across the equator. The live panel tracks her latitude, the zone she is in, and the load-line mark she must respect right now. Slide the month — northern winter/summer flip, the southern hemisphere flips opposite, the WNA box appears Nov–Mar, and the ship's applicable mark updates live. Click any band to load that mark into the Plimsoll diagram above.
Voyage quick-check — what the crew actually does at sea
8 short questions tied to the Hamburg → Recife passage above: load-lines, COLREGs, SOLAS drills & BWM duties.
Q1. Hamburg → Recife in February. Which load-line mark governs the maximum departure draft?
Q2. On Day 0 in Hamburg dock water (ρ ≈ 1015 kg/m³), the chief officer applies the DWA. What does this allow?
Q3. Crossing the Bay of Biscay in heavy weather, what is the bridge team's PRIMARY routine action under SOLAS Ch. V?
Q4. Passing through the English Channel TSS off Ushant, which COLREG rule applies?
Q5. As bunkers burn off, the predicted draft on Day 7 falls BELOW the W mark. Correct action?
Q6. South of the Canaries the ship enters the Tropical zone. Under the BWM Convention, what must the crew do with ballast taken in Hamburg?
Q7. Approaching the equator, the master orders a fire & abandon-ship drill. Under SOLAS Ch. III these must be held:
Q8. On arrival Recife (Tropical), the ship is floating exactly at the T mark. Is this compliant?
Load planning — done before the ship sails
The Master and Chief Officer don't wait until arrival to check freeboard. The intended voyage is plotted across the zone chart, the deepest seasonal mark the ship will cross is identified (e.g. WNA in January, Summer in the tropics), and loading is calculated so that at every zone the relevant mark stays above the water — accounting for fuel and water burnt off between ports.
Numerical worked example — Hamburg → Recife in February
How much cargo can the Chief Officer book so the relevant mark stays above the water in every zone the ship will cross? All figures rounded for illustration.
- Summer draft (S): 10.50 m
- Summer DWT: 30,000 t
- TPC at S waterline: 35 t/cm
- FWA: 200 mm
- Lightship: 6,000 t
- Bunkers + FW + stores at departure: 2,200 t
- Daily consumption (HFO+MGO+FW): 44 t/day
- S → W: −21.9 cm → W draft = 10.28 m
- S → T: +21.9 cm → T draft = 10.72 m
- W DWT = 30,000 − 21.9×35 = 29,234 t
- T DWT = 30,000 + 21.9×35 = 30,766 t
- Hamburg dock ρ = 1015 kg/m³ → DWA = 80 mm
- DWA uplift = 8.0 cm × 35 = +280 t
Governing mark at Hamburg in February = W. Max cargo bookable ≈ W DWT − lightship − (bunkers+FW+stores) + DWA uplift = 29,234 − 6,000 − 2,200 + 280 = ≈ 21,314 t of cargo.
| Leg | Days | Zone (Feb) | Mark | Mark draft (m) | Predicted draft at arrival (m) | Margin |
|---|---|---|---|---|---|---|
| Hamburg → 50°N (English Channel) | 2 | Winter Seasonal — Northern | W | 10.28 | 10.26 | ✓ 2.5 cm |
| 50°N → 36°N (Bay of Biscay) | 3 | Summer Seasonal N (Feb → W) | W | 10.28 | 10.22 | ✓ 6.3 cm |
| 36°N → 13°N (NE Trades) | 4 | Tropical Seasonal N (Feb → S) | S | 10.50 | 10.17 | ✓ 33.2 cm |
| 13°N → 8°S Recife | 5 | Tropical Permanent | T | 10.72 | 10.11 | ✓ 61.4 cm |
Daily fuel + FW consumption reduces draft by ≈ 1.3 cm/day (44 t ÷ 35 t/cm). The ship therefore rises as she sails south. Departing exactly at the W mark in Hamburg dock water, every later zone is met with positive freeboard margin — the W mark on the Atlantic crossing is the governing constraint, not the T mark at Recife. If the voyage were reversed (Recife → Hamburg in Feb), the chief officer would have to under-load at Recife so that the W mark is not immersed on arrival in northern winter latitudes.
Reserve buoyancy & watertight integrity
The freeboard mark is only meaningful if the hull above it stays watertight. These two ideas are the engineering soul of the Convention.
Reserve Buoyancy
The volume of the watertight hull above the waterline. It is what keeps a ship afloat when she pitches into a wave, takes a list, or is damaged. ILLC freeboard rules guarantee a minimum reserve-buoyancy ratio for every cargo ship.
- Greater freeboard → greater reserve buoyancy → better survivability in heavy weather.
- Why WNA is the deepest freeboard mark: small ships, big North Atlantic seas, winter.
- Loading past the seasonal mark is an offence — and may be unseaworthy.
Watertight & Weathertight Integrity
Reserve buoyancy only counts if the hull above the waterline keeps the sea out. ILLC Chapter II prescribes the openings, fittings and structural standards that must remain intact for the assigned freeboard to be valid.
- Hatch coamings & covers — minimum heights, gaskets, securing devices (Reg. 14–16).
- Ventilators & air pipes — coaming heights, automatic closures (Reg. 19–20).
- Doors in superstructures — weathertight, opens outward (Reg. 12).
- Freeing ports — area & arrangement to clear shipped water fast (Reg. 24).
- Watertight bulkheads — subdivision preserves buoyancy after damage.
One-glance recap
- • Plimsoll disc + horizontal line through its centre = S (Summer).
- • Mast on right of disc carries TF, F, T, S, W, WNA.
- • Always apply FWA / DWA when moving between salt & fresh water.
- • Respect the zone & season of your position.
- • Keep hatches, vents, doors & freeing ports weathertight.
- • ILLC certificate to be on board, valid, and matching the marks on the hull.
Educational schematic — for navigation, always refer to the ship's load-line certificate, the Plimsoll mark on the hull and Admiralty publication NP 131 / Chart 5128.