Ship displacement is the weight of water displaced by a vessel when afloat, which equals the ship’s own weight. It is calculated using Archimedes’ principle, typically with the formula:
Where L = length, B = breadth, T = draft, Cb= block coefficient, and ρ = water density
🌊 Concept of Ship Displacement
- Definition: Ship displacement refers to the mass or weight of water displaced by a ship’s hull when floating. By Archimedes’ principle, this equals the ship’s own weight.
- Units: Traditionally measured in long tons (2,240 lbs), but today metric tonnes (1,000 kg) are more common.
- Types of Displacement:
- Lightweight displacement: The ship’s weight without cargo, fuel, passengers, or stores.
- Loaded displacement: The ship’s weight when fully loaded to its maximum draft.
- Standard displacement: Used in naval contexts, excludes fuel and reserve feed water.
- Full load displacement: Includes everything – cargo, fuel, crew, provisions.
⚖️ Principles Behind Displacement
- Archimedes’ Principle: A floating body displaces a volume of water equal to its own weight.
- Draft Marks: Ships have draft marks along the hull that indicate how deep they sit in the water. By reading these marks, naval architects can calculate displacement.
- Density Factor: Displacement varies depending on whether the ship is in seawater (1025 kg/m3) or freshwater (1000 kg/m3).
📐 Formula for Ship Displacement
The general formula is:
- L : Length of the ship at waterline (m)
- B : Breadth (beam) of the ship (m)
- T
: Draft (vertical distance from waterline to keel) (m)
- Cb: Block coefficient (ratio of actual underwater volume to the volume of a rectangular block with same dimensions)
- ρ : Density of water (kg/m³)
This formula gives the displacement volume, which when multiplied by water density gives the displacement weight.
Below is the Ship’s Displacement Calculator; You fill in the corresponding data into the fields in the spreadsheet below, then click the Calculate button and you will see the display Displacement Weight.
Ship’s Displacement Calculator
✅ Key Takeaways
- Ship displacement = ship’s weight = weight of water displaced.
- Measured using Archimedes’ principle and draft marks.
- Formula involves length, breadth, draft, block coefficient, and water density.
- Critical for ship design, stability, and load management.
