Fresh Water Allowance Calculator
Calculate the fresh water allowance (FWA) of a vessel from its displacement and water density. Get the allowance in millimeters, centimeters, meters, feet, and inches.
Fresh Water Allowance Result
What is fresh water allowance?
Fresh water allowance, or FWA, is the amount by which a vessel's draft can increase when it moves from seawater to fresh water while carrying the same weight.
Fresh water is less dense than seawater, so the vessel must displace a greater volume of water to produce the same buoyant force.
Fresh Water Allowance Formula
The standard freshwater allowance relationship can be written as:
$$FWA = \frac{\Delta} {4 \times TPC \times \Delta\rho}$$
For the standard load-line case, a commonly used simplified result when displacement is in metric tonnes is:
$$FWA(mm) = \frac{\Delta} {4 \times TPC}$$
The calculator below instead uses the equivalent hydrostatic-density relationship:
$$FWA = \frac{\Delta} {A_{wp}} \left( \frac{1}{\rho_f} - \frac{1}{\rho_s} \right)$$
Here Δ is displacement, Awp is waterplane area, ρf is fresh-water density, and ρs is seawater density.
Why displacement alone is not enough for exact FWA
A vessel's draft change depends on how much waterplane area is added or removed as the vessel moves vertically. Two vessels with the same displacement can have different waterplane areas and therefore different changes in draft.
For a practical ship calculation, TPC or waterplane area should be available from the vessel's hydrostatic data.
How is FWA calculated using TPC?
TPC means tonnes per centimeter immersion. It tells you how many tonnes of additional displacement are required to change the vessel's draft by 1 cm.
$$FWA(cm) = \frac{\Delta \left( \frac{1}{\rho_f} - \frac{1}{\rho_s} \right)} {TPC}$$
A more convenient form is normally used with the vessel's hydrostatic data and the relevant density convention.
Typical seawater and fresh water density
A common reference value for seawater is about 1025 kg/m³, while standard fresh water is approximately 1000 kg/m³.
$$\Delta\rho = 1025 - 1000 = 25\ kg/m^3$$
How much is the fresh water allowance?
For a vessel using the standard 1025 kg/m³ seawater and 1000 kg/m³ fresh water reference, the actual FWA depends on the vessel's TPC or waterplane area. It should therefore be taken from the ship's hydrostatic/load-line data rather than estimated from displacement alone.
Fresh water allowance vs change of draft due to density
The two concepts are closely related, but they are not exactly the same calculation. Fresh water allowance is a specific maritime load-line/hydrostatic concept, while a general change-of-draft calculation can use a simplified density ratio:
$$T_2 = T_1 \frac{\rho_1}{\rho_2}$$
The density-ratio method assumes a simplified proportional relationship between draft and displaced volume. FWA calculations using hydrostatic data are more appropriate when accuracy matters.
Fresh Water Allowance FAQ
FWA is normally calculated from the vessel's hydrostatic data, particularly TPC, displacement, and the applicable seawater and fresh-water densities.
Fresh water is less dense than seawater, so the ship must displace more water to produce the same buoyant force.
A commonly used reference value is 1025 kg/m³, although the appropriate value depends on the applicable load-line or hydrostatic convention.
FWA is a vertical draft allowance and is commonly expressed in millimeters or centimeters. The exact value depends on the ship's hydrostatic characteristics.