Hotel Service Unit 16 Q2

A vessel in water experiences various types of motion, which can be described using specific terminology and categorized into two main categories:

1. Rotational Motions:

• Roll: The angular motion of the vessel about its longitudinal axis (the imaginary line running from bow to stern). It’s the side-to-side tilting or rocking motion caused by waves or wind.
• Pitch: The angular motion of the vessel about its transverse axis (the imaginary line running across the ship from side to side). It’s the up-and-down movement of the bow and stern, often caused by waves.
• Yaw: The angular motion of the vessel about its vertical axis (the imaginary line running vertically through the ship’s center of gravity). It’s the side-to-side movement of the bow and stern, causing the vessel to deviate from its intended course.

2. Translational Motions:

• Surge: The linear motion of the vessel along its longitudinal axis. It’s the forward and backward movement, mainly caused by the ship’s propulsion or waves.
• Sway: The linear motion of the vessel along its transverse axis. It’s the side-to-side movement, often caused by wind, currents, or waves.
• Heave: The linear motion of the vessel along its vertical axis. It’s the up-and-down movement, primarily caused by waves.

Terminology:

• Amplitude: The maximum extent of a motion from its equilibrium position (e.g., the maximum angle of roll or the maximum vertical displacement during heave).
• Period: The time it takes for a complete cycle of motion (e.g., from one peak of a roll to the next).
• Frequency: The number of cycles of motion per unit time, usually expressed in Hertz (Hz) or cycles per second.
• Phase: The position of a motion within its cycle at a particular instant, often expressed as an angle.

Factors Affecting Vessel Motion:

• Sea State: The size and direction of waves play a major role in influencing a vessel’s motion.
• Wind: Wind speed and direction can cause significant sway, yaw, and roll, especially for vessels with large exposed areas.
• Currents: Ocean currents can affect the vessel’s speed and course, leading to translational motions.
• Vessel Design: The hull shape, size, and stability characteristics of the vessel also influence its response to external forces and its overall motion in the water.

Importance of Understanding Vessel Motion:

• Seakeeping: Analyzing and predicting vessel motion is crucial for designing ships with good seakeeping qualities, ensuring comfort and safety for those onboard.
• Navigation and Maneuvering: Understanding how the vessel responds to different motions is essential for safe navigation and maneuvering, especially in challenging conditions.
• Cargo Operations: Vessel motion can impact cargo handling and stability. Proper understanding and control are necessary to prevent cargo shifts and ensure safe operations.
• Crew and Passenger Comfort: Excessive motion can cause discomfort and seasickness. Minimizing unwanted motions is important for passenger comfort and crew well-being.

In summary, understanding and describing the various motions of a vessel in the water using appropriate terminology is crucial for naval architects, marine engineers, and ship operators. This knowledge helps in designing vessels with good seakeeping characteristics, ensuring safe and efficient operations, and providing a comfortable experience for those on board.

A vessel floating in the water has six degrees of freedom, representing its potential movements in three-dimensional space. These six degrees of freedom are:

1. Surge: This is the linear motion of the vessel along its longitudinal axis (fore and aft). It refers to the vessel’s movement forward or backward.
2. Sway: This is the linear motion of the vessel along its transverse axis (port and starboard). It represents the side-to-side movement of the vessel.
3. Heave: This is the linear motion of the vessel along its vertical axis (up and down). It describes the vertical movement of the vessel as it rides the waves.
4. Roll: This is the rotational motion of the vessel about its longitudinal axis. It’s the side-to-side tilting or rocking motion, often caused by waves or wind.
5. Pitch: This is the rotational motion of the vessel about its transverse axis. It’s the up-and-down movement of the bow and stern, typically due to encountering waves.
6. Yaw: This is the rotational motion of the vessel about its vertical axis. It represents the swinging or turning motion of the bow and stern, causing the vessel to deviate from its course.

In summary:

• The first three degrees of freedom (surge, sway, and heave) represent the translational movements of the vessel in three-dimensional space.
• The remaining three degrees of freedom (roll, pitch, and yaw) describe the rotational movements of the vessel around its three axes.

Understanding these six degrees of freedom is crucial for analyzing and predicting a vessel’s behavior in the water, designing effective stabilization systems, and ensuring safe navigation and operation in various sea conditions.

A bilge keel is positioned longitudinally (lengthwise) along the hull of a vessel, typically on both sides, at the turn of the bilge. The turn of the bilge is the curved section where the bottom of the hull transitions into the side. The bilge keel extends outwards from the hull, creating a projection into the water.  

1. Bilge keel | shipbuilding – Britannica

2. Bilge keel – Wikipedia

How Bilge Keels Reduce Vessel Motion

Bilge keels primarily reduce a vessel’s roll motion, which is the side-to-side rocking or tilting movement caused by waves or wind. They achieve this through two main mechanisms:  

1. An Investigation into the Effect of Bilge Keels to the Roll Motion Response of Fishing Vessel

1. Increased Hydrodynamic Resistance:

• When the ship rolls, the bilge keel on the lower side dips deeper into the water.
• This creates additional surface area in contact with the water, leading to increased drag or resistance against the rolling motion.   1. Bilge keel – Wikipedia en.wikipedia.org
• The increased resistance opposes the roll, dampening its amplitude and slowing down the rolling motion, making it less severe.   1. Bilge keel – Wikipedia en.wikipedia.org

2. Vortex Generation and Energy Dissipation:

• As the bilge keel moves through the water during roll, it generates vortices (swirling water currents) along its edges.   1. (PDF) Influence of bilge-keel configuration on ship roll damping and roll response in waves www.researchgate.net
• These vortices dissipate energy from the rolling motion, further reducing the roll amplitude.   1. Bilge-Keel Influence on Free Decay of Roll Motion of a Realistic Hull 1 asmedigitalcollection.asme.org
• The shape and size of the bilge keel, as well as the ship’s speed and roll angle, influence the formation and effectiveness of these vortices.   1. (PDF) Influence of bilge-keel configuration on ship roll damping and roll response in waves www.researchgate.net

Additional Benefits:

• Reduced Stress on the Hull: By dampening the roll motion, bilge keels reduce the stresses and strains on the ship’s structure caused by excessive rolling.   1. Roll Motion of a Ship and the Roll Stabilising Effect of Bilge Keels – ResearchGate www.researchgate.net
• Improved Comfort: Reduced roll motion enhances the comfort and stability of passengers and crew, particularly in rough seas.
• Enhanced Safety: In certain situations, bilge keels can also provide some protection against grounding or collision by acting as a buffer between the hull and underwater obstacles.

Limitations:

• Effectiveness: Bilge keels are most effective in reducing roll motion at moderate speeds and in moderate sea conditions. They may be less effective in extreme rolling conditions or for vessels with high centers of gravity.
• Increased Drag: Bilge keels increase the hydrodynamic resistance of the hull, leading to a slight reduction in the vessel’s speed and fuel efficiency.   1. Bilge keel – Wikipedia en.wikipedia.org

Overall, bilge keels are a simple yet effective passive stabilization device that enhances the stability and comfort of vessels by reducing roll motion. While they may not eliminate roll completely, they significantly improve the vessel’s seakeeping characteristics and contribute to a smoother ride.  

1. Roll Motion of a Ship and the Roll Stabilising Effect of Bilge Keels | The Journal of Navigation – Cambridge University Press