Ship Stability: Key Concepts

• Ship stability is the vessel's ability to stay upright and return to equilibrium after disturbances from external forces.
• Understanding stability is crucial for safe navigation and preventing capsizing.

Fundamental Concepts

• Center of Gravity (G): The point where the ship's entire weight is concentrated.
• Center of Buoyancy (B): The point where the upward buoyancy force (equal to the weight of displaced water) is concentrated.
• Metacenter (M): The intersection point of the buoyancy force's vertical line of action with the ship's centerline when slightly inclined.
• Righting Arm (GZ): The horizontal distance between the lines of action of buoyancy and weight forces, indicating the restoring force.
• Metacentric Height (GM): The vertical distance between G and M, a key indicator of initial stability; a larger GM means greater stability.
• Righting Moment: The product of the ship's weight and the righting arm (GZ).
• Damage Stability: The ship's stability after sustaining hull breach and flooding.
• Statical Stability: Ship stability when water level is calm and ship is inclined.
• Dynamical Stability: Ship stability with dynamic water level (waves) and inclination.
• Free Surface Effect: Liquids in partially filled tanks create a virtual rise in G, reducing stability. Wind and Waves: External forces exerted on the ship influence stability.
• Trim: Difference in draft between the bow and stern.
• Equilibrium: State where the ship is at rest, G and B are vertically aligned.
• Heel: Transverse inclination caused by external forces.
• List: Transverse inclination due to uneven weight distribution.

Types of Stability

• Intact Stability: Stability when the ship is undamaged and operating normally.

Factors Affecting Stability

• Loading: Cargo, passenger, and equipment weight and distribution affect G and stability.

Ship Stress

• Ship stress refers to the internal forces within the ship's structure due to external loads.
• External Loads: Waves, cargo, and engine thrust can affect the internal forces.
• Understanding stress is crucial for structural integrity and preventing failure.
• Tensile Stress: pulls or stretches the material.
• Compressive Stress: Pushes or compresses the material
• Shear Stress: Acts parallel to the surface, causes sliding or deformation
• Bending Stress: Occurs when a material is subjected to a bending force, causing it to deform.
• Torsion Stress: Occurs when a material is subjected to a twisting force causing it to deform

Ship Dimensions

• Air draft is the distance from the waterline to the highest point of the vessel.
• Deadweight: The maximum weight a vessel can carry, including cargo, fuel, provisions, and crew, without compromising its structural integrity.

Ship Tonnage

• Gross Tonnage: Measures total internal volume.
• Net Tonnage: Measures cargo space volume.
• Lightweight Tonnage: Weight of the ship itself.
• Deadweight Tonnage: Weight of cargo, fuel, supplies, crew, etc.

Deadweight and Cargo Capacity

• Deadweight is not just the maximum weight a ship can carry.
• Deadweight also depends on cargo type and stability requirements.
• Heavy cargo (iron ore) results in lower deadweight capacity than lighter cargo.
• Deadweight calculation: Displacement loaded minus displacement light provides lifting capacity.

Ship Hydrostatics

• Ship hydrostatics deals with forces on a floating ship.
• Deals with buoyancy, stability, and trim for safe/efficient operation.
• Forward Perpendicular (FP): Vertical line at the intersection of the designed load waterline and the forward face of the stem.
• After Perpendicular (AP): Perpendicular to the waterline at the after side of the rudder post/center line of the rudder stock.(if no rudder post).
• Length overall (LOA): The maximum length from the forward most point to the after most point.
• KEEL (K): The lowermost point of the ship at any point along its length.
• Length between perpendiculars (LPP or LBP): Longitudinal distance between the forward and aft perpendiculars.
• Length of waterline (LWL): Hull length intersecting the water surface.

Bilge system

• Bilge system removes water/fluids from the lowermost areas.
• Fluids such as leaks, condensation, washing, or firefighting efforts.

Stability

• Ship stability deals with how a ship behaves at sea.
• Applies to calm water and waves, intact or damaged stability.
• Stability calculations focus on gravity, buoyancy, and metacenters.

Ballast System

• The ballast system controls stability by adjusting draft and trim.
• It involves filling or emptying ballast tanks with seawater to achieve the desired buoyancy and prevent the ship from becoming unstable.

Stability Instruments

• Loading Control System software calculates loading, trim, and stability.
• Can be used for intact or damaged vessels.
• Aims to adjust loading for required trim, stability, and strength.
• Aims to estimate damaged stability by indicating types of flooding.