Stability and Ship Design Flashcards
what is the difference between heeling and listing?
Heeling refers to the controlled inclination or leaning of a ship intentionally induced by the crew or external forces, whereas listing refers to the unintentional or uncontrolled inclination or leaning of a ship to one side. Unlike heeling, listing is not part of normal navigation or sailing; instead, it is typically an abnormal condition resulting from external forces or internal issues.
how does listing happen?
- total weight of ship increase and the ship sits deeper in the water as displacement increases.
- the position of G will go from G to G1 to the direction of the added weight, but off the centre line.
- With the new displacement (with the new weight) acting down and the buoyancy acting up, they are not acting in the same vertical line.
- NOW - the vessel will start to incline towards the weight, this is called listing.
- As it lists, the underwater geometry of the ship changes: B moves off the centreline to B1- the new centre of underwater volume.
- Inclination stops, and the ship is at rest when buoyance and gravity are now acting in the same vertical direction, but off the centre line.
how does heeling happen?
- when a vessel is affected by an external force, the only thing that changes is the position through which the force of B is acting upwards (what is called heeling).
- Just because the ship is tilting/inclining, the weight doesn’t change, which in this example is inclination by 10C.
- As the shape of underwater volume changes, the force of buoyancy now acts through position B1, but unlike in the listing example, G remains in the same place.
- As a result, the ship weight acting down and the force of buoyance acting up do not act in the same line.
- This produces a turning force, a bit like pushing and pulling a steering wheel when turning a corner.
- The effect is to return the ship to the upright
Explain metrocentric height (GM)
First, in a heeling ship, when the new force of buoyancy acting upwards intersects with the CL, that point of intersection is the Metacentre. The distance between point G and M is the metacentric height. When M > G = ship stable.
Explain the righting lever (GZ)
The perpendicular distance between the two forces (G and B) is called the righting lever.
The lower the centre of G, the larger the GZ and the more stable the ship. But will come upright very quickly and violently.
The higher the centre of G, the smaller the GZ and the more unstable the ship. It will be coming upright slowly.
The higher the G, the higher the chance of capsizing, because then you might have ZG - a capsizing lever - which will make the ship incline further.
What is the behaviour of tender ships (small GM)
Tender ships tend to heel quickly and reach a large angle of heel due to smaller GZ and have bigger rolling motion. Therefore they arel less stable in rough see conditions.
What is the behaviour of stiff ships (big GM)
Stiff ships tend to heel at a slower pace and take longer to reach an angle of heel. Therefore ships with a bigger metacetric height are more stable.
What can decrease the GM of a ship?
- Added weight at the top (for example the danger of overloading) because gravity moves towards the added weight, therefore if added at the top, G moves closer to M.
- Free Surface Effect - ex. when a liquid cargo in partially filled tanks is allowed to move freely, it creates a sloshing effect - this means that when the ship heels, G moves away from the CL, which means that the GM and the GZ decreases.
What is the FSE?
The reduction in stability or GM as a result of the free movement of liquid cargo that creates a sloshing effect during heeling.
How does FSE affect weight?
Weight, which is determined by the displaced liquid, experiences changes in distribution as the free surface of the liquid moves (sloshing). This movement can lead to shifts in the center of gravity and affect the overall stability of the object.
How does FSE affect buoyancy?
Simultaneously, the buoyant force, acting opposite to gravity, is influenced by variations in the displaced volume of fluid caused by free surface movement.
How does an angle of loll can develop and the behaviour of a ship rolling at an angle of loll?
When a ship has positive stability (positive GM, where M is higher than G), it means that if inclined by an external force, the ship will naturally return to an upright position. This is because the center of gravity (G) is below the metacenter (M), creating a restoring moment that helps the ship regain its stability.
Conversely, when a vessel has a negative GM or an initial metacentric height less than zero, any external force can cause the ship to incline. In such cases, the ship may continue to incline further due to the development of a capsizing lever. Alternatively, if the negative GM is only slight, the ship may stabilize in an inclined position, referred to as an “angle of loll.” This inclined position occurs when the center of gravity and metacenter align, resulting in a lack of a righting moment and compromising the ship’s stability.
This happens when:
When those onboard have assumed that the ship has positive stability, but the height of G has been raised higher above the keel due to:
- Consumption of fuel from low down tanks (removal of weight)
- Ballast is discharged from low down tanks (removal of weight)
- Snow or ice on top of deck cargo (addition of weight)
Behavior of ship rolling at angle of loll:
Stable or Unstable Rolling
If the angle of loll is not too severe, the ship may oscillate or roll back and forth around the inclined position. This is a stable rolling behavior.
If the angle of loll is more extreme, the ship may become unstable, and the rolling motion can intensify.
Capsizing Risk:
Depending on the severity of the angle of loll, there may be a risk of further inclination leading to capsizing. This is especially true if external forces continue to act on the ship, pushing it beyond its stable limits.
Restoration to Upright Position:
Restoring the ship to an upright position becomes challenging in this state since the righting moment (the force trying to return the ship to an upright position) is minimal or absent due to the alignment of the center of gravity and metacenter.
Why is ship lying at angle of loll at risk of capsizing?
Lack of Righting Moment:
In an angle of loll, the center of gravity and the metacenter align vertically, resulting in a minimal or absent righting moment.
The righting moment is the force that naturally tries to bring a ship back to an upright position when inclined. Without a sufficient righting moment, the ship lacks the inherent stability needed to counteract the inclination.
Capsizing Lever Effect:
External forces, such as waves, wind, or other dynamic factors, can exert a capsizing lever on the ship. The capsizing lever is the distance between the center of gravity and the center of buoyancy. In an angle of loll, this lever may become significant, making it easier for external forces to cause further inclination.
Increased Vulnerability to Waves:
Waves hitting the ship’s side can exacerbate the inclination. The ship, already in an unstable position, becomes more vulnerable to wave-induced rolling, potentially leading to a loss of stability and capsize.
Ineffective Righting Actions:
Traditional righting actions, like ballasting or shifting weights, may be less effective in correcting the angle of loll because the ship is already in a compromised state.
