Transverse Stability

How do we determine whether a ship is seaworthy or not?

Although there are many factors that will affect seaworthiness, we can generally agree that a ship is seaworthy when it is fit to carry the cargo in protected condition and deliver it to the destination port.

Assuming that the propelling machinery are all in good condition, the ship needs to be strong, have sufficient freeboard and is stable.

This page tells a bit about transverse stability.

transverse stability of ship

transverse stability of ship

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Whenever a ship floats on the water, there are 2 forces acting on it – the gravity force and the buoyancy force.When the cargo in the ship are evenly distributed, the ship will be upright. The sum of the gravity forces of cargo and the ship will be acting at one point – the Center of Gravity, G, acting downwards.

Similarly, the Center of Buoyancy of the ship will be acting at one point B, acting upwards.

However, during rough weather, the ship may be tossed about by the waves or the wind to heel towards one side. At this moment, the Center of Buoyancy will shift to another position, B1. This buoyancy force acting upwards, tends to turn the ship back towards upright position.

A ship is said to be in Stable Equilibrium if on being slightly inclined, tends to return back to the original position. However, a ship will be in Unstable Equilibrium when she tends to move further from that original position on being tilted slightly. A ship in Neutral Equilibrium will tend to neither return nor move further from that position.

Looking at the drawing on the left, a ship at upright is in Stable Equilibrium when the center of gravity, G is below M.

A ship is in unstable equilibrium when G is above M.

At neutral equilibrium, G and M coincides.