Ship Draft Displacement & Deadweight Calculator

Compute total displacement tonnage, deadweight cargo capacity (DWT), Tons Per Centimeter immersion (TPC), and Fresh Water Allowance (FWA) from draft readings.

Mean Hydrostatic Draft (Meters) Average of forward, mid, & aft draft

Lightship Weight (Metric Tons) Empty vessel weight without cargo/fuel

Length Between Perpendiculars LBP (m)

Molded Beam B (Meters)

Block Coefficient (Cb)

Dock Water Density (t / m³) Sea: 1.025 | Fresh: 1.000

Current Vessel Displacement (Δ)

59,207 MT

57,763 m³ Underwater Volume

Deadweight on Board (DWT)

49,707 MT

84.0% of displacement

Immersion Rate (TPC)

54.0 MT / cm

137.2 Long Tons / inch

Fresh Water Allowance (FWA): 274 mm (10.8 Inches)

Dock Water Allowance (DWA): 0 mm (At standard sea water 1.025)

Waterplane Area (Awp approx): 5,268 m² (56,704 sq ft)

Equivalent Draft in Imperial: 38' 8-1/2" (38.71 Ft)

Draft Increase per 1,000 MT: 18.5 cm (+7.3 Inches)

Draft Survey Standard: Conforms to UN ECE / IMO international code of uniform standards for ship draft survey measurement.

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Ship Hydrostatics: Displacement, Deadweight & Plimsoll Marks

Draft surveys determine the exact mass of cargo loaded or discharged onto bulk carriers and tankers by measuring the ship's displacement before and after port operations. Because seawater density changes with salinity and water temperature, accurate hydrostatic calculations require correcting for waterplane immersion and freshwater allowance.

Key Naval Architecture Governing Formulas

Underwater Molded Volume (∇):
∇ (m³) = LBP × Beam × Mean Draft × Cb
Displacement (Δ):
Displacement (Metric Tons) = ∇ × Water Density (ρ)
Standard ocean saltwater density is 1.025 t/m³ (1025 kg/m³).
Deadweight Tonnage (DWT):
DWT = Displacement - Lightship Weight
Deadweight includes payload cargo, fuel bunker, lube oil, ballast, fresh water, and provisions.
Tons Per Centimeter Immersion (TPC):
The weight required to sink the vessel by exactly one centimeter:
TPC = (Awp × ρ) / 100
Where waterplane area Awp ≈ Cwp × L × B (with waterplane coefficient Cwp ≈ Cb + 0.06).
Fresh Water Allowance (FWA):
When a ship transits from saltwater to freshwater, water density drops from 1.025 to 1.000, causing the ship to sink deeper to maintain buoyant equilibrium:
FWA (mm) = Δ / (40 × TPC)
FWA (inches) = Δ / (4 × TPI)
Dock Water Allowance (DWA):
DWA (mm) = FWA × [ (1.025 - ρ_dock) / 0.025 ]

Frequently Asked Questions

What is the difference between Displacement and Deadweight (DWT)?

Displacement is the total actual weight of the ship and everything on board (equal to the weight of water displaced by the hull). Deadweight (DWT) is the carrying capacity alone—the difference between the loaded displacement and the bare empty ship (lightship).

Why can a ship submerge its Summer load line in fresh water?

Because fresh water is less dense (1.000 t/m³) than ocean sea water (1.025 t/m³), the ship sinks deeper by the Fresh Water Allowance (FWA). Once the ship reaches the open ocean, the higher density of saltwater will automatically lift the vessel back up to its legal Summer load line.

What is TPC used for in cargo operations?

Chief mates use TPC to control cargo loading rates and prevent exceeding maximum permissible draft limits. For example, if TPC is 50 MT/cm, loading 500 metric tons of iron ore will increase draft by exactly 10 centimeters (0.10 m).

How is mean draft calculated during a draft survey?

A standard draft survey calculates Mean Draft using the "Quarter Mean" (Keel Mean-of-Means) formula: Draft_mean = (Draft_fwd + 6 × Draft_mid + Draft_aft) / 8, which accounts for midship hull hogging or sagging.