Lecture 9: Hull Forms, Hydrostatics and Stability Flashcards
What do you understand by a metacenter?
When a vessel heels( at an angle doesn’t move) , the submerged volume will shift:
Shifts of B from centreline(CL) to the side B1 (at a lower position)
If vertical line is extended from the new centre of buoyancy(B1):
The point at which this line meets CL of the ship is termed the “transverse metacentre” (denoted as M in the diagram)
Vertical distance between keel and metacentre= KM
What do you understand by the Centre of Gravity?
Longitudinal position of CG respect to any reference point on ship:
Longitudinal Centre of Gravity(LCG)
Reference pt of locating it is either after or forward perpendiculars
Vertical Distance Between Keel and CG= KG
What do you understand by centre of buoyancy (CB)?
Longitudinal position of CB respect to any reference point on ship:
Longitudinal Centre of Buoyancy(LCB)
Reference pt of locating is either forward or after perpendiculars
Vertical Distance Between Keel and CG=KB
What do you understand by metacentric height?
- Most concerned parameter
- A vital parameter in IMO CODE OF STABILITY OF SHIP
- Measures the ship’s initial stability
GM= KM - KG
What is Tonnes per centimetre Immersion (TPC)?
How much of weight to change draft of ship by 1cm IN SALT WATER
E.g TPC is 100 tonnes, 100 tonnes of weight to change draft by 1cm.
What do you mean by center of floatation
It is where trimming of ship occurs. (where ship will tilt)
Centre of Floatation: point centroid of the sea of the water plane
LCF referencing points w.r.t either perpendiculars or midship
What is the Moment to Change Trim by 1 centimetre?
- For a particular draft, it is the longitudinal moment(about the LCF) required to bring about a trim of 1 centimeter.
- Predict the resultant trim caused by any change of weights onboard
- An important hydrostatic parameter required by stability/trim stability
What factors make up the stability triangle?
- When the ship is inclined, the centre of buoyancy shifts off the centreline while the centre of gravity stays in the same location.
- The forces of buoyancy are equal and act along parallel lines but in opposite directions. This results in a rotation called a couple.
- The rotation returns the ship to where forces of buoyancy and gravity balance out.
What are the 4 ways to correct the incline of ships?
- Lowering existing weights
- Loading/Adding weights below the centre of gravity of ships
- Discharge weights at locations above CG of ships
- Remove all Free Surface Effects(FSE) e.g top up slack tanks
What is FSE?
- Occurs in partially filled tanks or compartments containing liquid.
-Liquid sloshes freely within the tank when the ship rolls. - This movement impacts the vessel’s stability, especially during heavy rolling or pitching motions.
What are the 4 impacts of FSE on Vessel’s Stability?
- Reduction of Metacentric Height (GM):
Lowers GM, reducing stability.
Ships become more prone to rolling and have less ability to right themselves. - Increased Rolling Amplitude:
Liquid shifts to the lower side during rolls, amplifying rolling motion.
Can result in excessive and frequent rolling. - Delayed Righting Moment:
Slows the ship’s ability to return upright after a roll.
Especially dangerous in rough seas where quick recovery is crucial. - Increased Risk of Capsizing:
Extreme rolling from FSE can lead to a high risk of capsizing.
Larger ships with fine hull forms (e.g., container ships) are more vulnerable to stability changes.
What are the 5 Conditions for Parametric Rolling to occur?
- Wave Encounter Period = Ship’s Roll Period:
Rolling amplifies when wave frequency matches the ship’s natural roll period. - Head or Following Seas:
Occurs when waves hit head-on or from behind, causing the hull to alternate between wave crests and troughs. - Fine Hull Forms:
Long, slender hulls (e.g., container ships) are more prone; VLCCs/capesize vessels less so. - Specific Wave Heights/Lengths:
Moderate to large waves with wavelengths close to the ship’s length cause buoyancy changes along the hull. - High-Speed Operations in Rough Seas:
Increases frequency of bow and stern dipping into wave crests/troughs, intensifying stability/buoyancy changes.
What are the 5 risks with parametric rolling?
- Excessive Rolling:
Rolling motion can reach dangerous angles (over 30°).
Potential consequences:
Loss of cargo (containers may fall overboard on container ships), which can alter the ship’s CG and further destabilise the vessel
Damage/ Strain to ship structures and cargo.
Injury to crew members. - Capsizing:
In extreme cases, unchecked parametric rolling can lead to capsizing.
Risk increases if rolling synchronizes with external forces like wind or waves. - Loss of Ship Control:
excessive rolling »_space;> difficult to maintain control over the vessel.
less responsive to steering inputs, making collision avoidance or course corrections difficult. - Strain on Ship Structure:
● Significant strain on the ship’s hull and superstructure, leading to structural failures or cracks. - Increased Fuel Consumption:
● Creates additional resistance in the water resulting in higher fuel consumption.
What is the angle of list and the angle of loll?
Angle of List: The angle at which a vessel heels to one side or the other due to the distribution of weight.
Angle of Loll:
- Occurs when a vessel reaches negative stability.
- After heeling, the vessel comes to rest. ( at an angle)
- Happens when the vessel is at neutral stability (ZERO GM).
- This condition is dangerous, as it can lead to capsizing if the lever changes from righting to capsizing.
- High risk of the vessel overturning.
What are the princiiples of a ship’s stability?
- Intact Stability
→ Ability of ship to return to an upright position after being inclined by external forces(e.g waves, wind that causes rolling) - Damage Stability
→ Ship’s ability to remain stable and afloat after experiencing damage that compromises hull’s watertight integrity (flooding: water get into ship)
