Fresh water allowance and dock water allowance explained

The fresh water allowance (FWA) is how far a ship may submerge her load line in fresh water, stated in millimetres on her International Load Line Certificate. The dock water allowance (DWA) is the part of it that applies in brackish dock water: DWA = FWA × (1025 − ρ) / 25, with ρ in kg/m³. Both rest on the Load Lines Convention of 1966.

Why a ship floats deeper in fresh water

A floating ship displaces her own mass of water. Fresh water holds less mass per cubic metre than sea water, so the same ship needs more volume under water and sinks until she has it. The convention works with salt water at 1.025 and fresh water of unit density, 1.000.

The size of that sinkage follows from the ship's TPC, the tonnes needed to immerse her one centimetre in salt water. With Δ the displacement in metric tonnes (MT):

1. Waterplane area A = 100 × TPC / 1.025 m², because one centimetre of salt water over A weighs TPC tonnes.
2. Extra volume in fresh water = Δ / 1.000 − Δ / 1.025 = Δ × 0.025 / 1.025 m³.
3. Sinkage = extra volume / A = Δ × 0.025 / (100 × TPC) m = Δ / (4 × TPC) mm.

The result assumes the waterplane area stays the same over those few centimetres.

The fresh water allowance in the Load Lines Convention

IMO adopted the International Convention on Load Lines on 5 April 1966; it entered into force on 21 July 1968. Its 1988 Protocol, in force since 3 February 2000, revised certain regulations in the technical annexes.

In the 1966 text, as published in the UN Treaty Series, Regulation 40(7) of Annex I obtains the minimum freeboard in fresh water (density 1.000) by deducting Δ/40T centimetres from the minimum freeboard in salt water. Δ is the ship's salt water displacement at her summer load waterline, T her TPC in salt water at that same waterline. Δ/40T centimetres is Δ / (4 × TPC) millimetres, the result derived above. If that displacement cannot be certified, Regulation 40(8) takes 1/48 of the summer draught as the deduction.

The formula survives in national rules. The UK notice MSN 1752 (2000), issued under regulations that incorporate the 1988 Protocol amendments, gives it as Δ/4T with T in metric tons per centimetre. The US rules in 46 CFR 42.20-75 (September 2026) keep Δ/40T, in inches.

On the ship, the allowance appears in two places:

• The certificate states the allowance for fresh water in millimetres for all freeboards other than timber, with a separate figure for timber freeboards.
• The load line mark, amidships on each side, carries a line marked F abaft the vertical line. The gap between the summer line and F is the fresh water allowance, and the same gap applies when loading in fresh water to the other load lines; TF marks the tropical fresh water line.

Dock water allowance: the formula and its units

Article 12(2) of the convention lets a ship in fresh water of density 1.000 submerge the applicable load line by the FWA on her certificate. For any other density the allowance is scaled to the gap between 1.025 and the density of the water she is in. Written out:

• DWA = FWA × (1.025 − ρ) / 0.025, with ρ in t/m³
• DWA = FWA × (1025 − ρ) / 25, with ρ in kg/m³

Both give the same result. The second form needs ρ in kg/m³: dock water of 1.0210 t/m³ goes in as 1,021.0. Entered as 1.021, it returns a DWA of more than 13 metres. ρ is the actual density of the water the ship floats in (Article 12(2)).

Worked example: fresh water allowance and DWA

A fictitious bulk carrier: displacement at the summer load waterline 47,165 MT and TPC 36.9 MT/cm, both in salt water; assigned summer freeboard 3,125 mm. She loads in dock water of 1.0210 t/m³, and her zone and season call for the summer load line.

Step	Calculation	Result
FWA	47,165 / (4 × 36.9)	319.546 mm, 320 mm on the certificate
Dock water density	1.0210 t/m³ × 1,000	1,021.0 kg/m³
DWA	320 × (1,025 − 1,021.0) / 25	51.2 mm, used as 51 mm
Minimum freeboard in this dock	3,125 − 51	3,074 mm
Freeboard read at the mark, ship upright	reading	3,090 mm
Margin	3,090 − 3,074	16 mm
TPC in dock water	36.9 × 1.0210 / 1.025	36.756 MT/cm
Cargo still to load	1.6 cm × 36.756	58.8 MT

We round the FWA to the nearest millimetre, the unit of the certificate, and the DWA down to whole millimetres, so the allowance errs on the safe side. The TPC in dock water is the salt water TPC scaled by ρ / 1.025; tonnes are rounded to one decimal. For this ship the DWA runs with the density as follows:

Density (t/m³)	1.0000	1.0050	1.0100	1.0150	1.0210	1.0250
DWA (mm)	320	256	192	128	51.2	0

Is the ship on her marks?

A ship loaded down to the appropriate load line is said to be on her marks. Article 12(1) keeps that line above water when she puts to sea, during the voyage and on arrival; paragraphs (2) and (3) are the only exceptions. The DWA therefore holds only for the water she is in. Loaded to a freeboard of 3,074 mm in the dock, she rises by about the DWA on reaching sea water and floats at about her summer mark. Article 12(3) adds an allowance for ports on rivers and inland waters: the ship may load deeper by the weight of the fuel and other consumables she will use before reaching the sea.

Density correction in a draft survey versus the dock water allowance

A draft survey and the load line both allow for water lighter than 1.025 t/m³, but they answer different questions. A draft survey reads the draft, takes the displacement from the ship's tables and corrects it by displacement × (ρ − 1.025) / 1.025, with 1.025 the reference density of the tables (UNECE draught survey code, 1992); in our draft survey example, dock water of 1.0210 t/m³ gives a correction of −182.710 MT at the final survey. The DWA keeps the mass fixed and asks how far the ship may sink. How the density reading and its correction are recorded: the draft survey report.

FOSS

FOSS is software for inspection companies and the port, with a sector specialist at the table. Readings are entered once, offline on board if need be, and nine report types come from that input.