The centre of buoyancy (B) is the geometric centre of the underwater volume of a ship’s hull — the point through which the total upward buoyant force effectively acts.
Why B Moves
When a ship is upright, B sits directly below G on the ship’s centerline. As the ship heels to one side, the shape of the underwater hull volume changes (more volume submerged on the low side, less on the high side), causing B to shift toward the low side.
How This Creates Stability
This shift in B, while G stays in the same position (assuming no cargo has moved), creates a righting lever — the buoyant force acting through the new B position, combined with gravity acting through G, generates a moment that tends to rotate the ship back upright. This is the fundamental mechanism behind ship stability.
B, G, and M Together
- B — centre of buoyancy, shifts as the ship heels
- G — centre of gravity, fixed unless weight is added, removed, or shifted
- M — metacentre, the point where the line of buoyant force (through the shifted B) crosses the ship’s centerline; for small angles of heel, M’s position is approximately constant
The relationship between these three points — captured in GZ (righting arm) and GM (metacentric height) calculations — is the core of practical ship stability assessment.
Exam Relevance
Centre of buoyancy behavior, its relationship to GZ curves, and stability calculations built on B’s movement are extensively tested Ship Stability topics in deck officer competency exams.