The dynamical stability of a ship at a given angle of heel is defined as the work done in heeling the ship to that angle very slowly and at constant displacement, i.e., ignoring any work done against air or water resistance. What is Ship Stability
Ship stability
Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on the center of gravity, center of buoyancy, and metacenter of vessels and on how these interact.
How to track the stability of a ship for heel angles?
Since the stability of a ship can be directly commented on by the nature and value of its metacentric height (GM), a direct method to track the stability of a ship for a range of heel angles would be, to generate a curve that relates this parameter to the angle of heel.
What is a ship's heel?
Ship motions. The rolling motion towards a steady state (or list) angle due to the ship's own weight distribution is referred in marine engineering as heel. List normally refers to an unintentional or unexpected offset, as caused by flooding, battle damage, shifting cargo, etc.
What is a heeling moment?
A heeling moment exists at any angle of inclination where the forces of weight and buoyancy act to move the ship away from the upright position. The center of buoyancy of a ship or a surfaced submarine moves with respect to the ship, as the ship is inclined, in a manner that depends upon the shape of the ship in the vicinity of the waterline.
What is stability of a ship?
What is stability? Ans. Stability is the ability of vessel to return to the up-right equilibrium. In case of a listed vessel or the vessel at angle of loll it would be the ability to return to the respective angles of equilibrium.
What is ships heel?
HEEL - A heel is a "temporary" inclination of a ship, caused by outside forces such as winds, waves, or during a ship's turn.
What is heel stability?
The dynamical stability of a ship at a given angle of heel is defined as the work done in heeling the ship to that angle very slowly and at constant displacement, i.e., ignoring any work done against air or water resistance. Consider a ship with a righting moment curve as shown in Fig.
What is heel and trim ship?
importance in ship design float at unwanted angles of heel (sideways inclination) and trim (endwise inclination). Nonzero trim angles may lift the tips of propeller blades above the surface, or they may increase the possibility that the bow will slam into waves during heavy weather.
What is heel angle of ship?
At some angle of heel (say 10°), KM will increase sufficiently equal to KG (distance from the keel to the centre of gravity), thus making GM of vessel equal to zero. When this occurs, the vessel goes to neutral equilibrium, and the angle of heel at which it happens is called angle of loll.
What is heeling moment of a ship?
Heeling moments can be caused by wind, by the centrifugal force in turning, by crowding of passengers on one side, by towing, or by the tension in the cable that links two vessels during operations at sea.
What is GZ in ship stability?
The vessel's centre of gravity (G) has a distinct effect on the righting lever (GZ) and consequently the ability of a vessel to return to the upright position. The lower the centre of gravity (G), the bigger is the righting lever (GZ).
What is the difference between heel and roll?
Heel describes a temporary inclination generally involving motion, while roll indicates periodic inclination from side to side. For example, a ship rolls in a seaway, lists due to a side damage, and heels in a turn.
Why do ships turn heels?
The underwater form of the ship and centrifugal force on it cause the ship to heel to starboard. These two forces produce a couple which tends to heel the ship away from the centre of the turn.
What is trim and list?
LIST is defined as transverse inclination of vessel due to weight of cargo on ship( generally uneven distribution of cargo in ship's cargo hold) either direction. This is transverse inclination of the vessel. Whereas TRIM is the transverse inclination of the vessel due to wind, Current and other natural factors.
When can a vessel heels?
The ship will heel to a point at which the new centre of buoyancy (B1) is at such a position such that weight and buoyancy act through the same line. This also happens when ballast water is transferred from one side to another, or when ballast water is taken into only one side of a tank.
What is range of stability?
the angle to the perpendicular through which a vessel may be heeled without losing the ability to right itself.
What is a GZ curve?
The curve of statical stability, or GZ curve as it is most commonly referred to, is a graphical representation of the ship's transverse statical stability.
What does stability do in a shoe?
A stability shoe, or a shoe with stability features, is a running shoe designed to help athletes who overpronate.It supports the arch of the foot and provides greater support through the midsole and into the heel. Stability shoes are usually more rigid than neutral running shoes.
What kind of feet need stability shoes?
Stability shoes are best for runners with normal arches and only mild control problems. The extra stability these shoes offer comes from extra arch-side supports and high-density foam.
What is heel counter used for?
The heel counter is responsible for rearfoot stability. This is located in the rear of the shoe under the foxing, which extends laterally and medially to the corresponding longitudinal arch. It is typically made of cardboard or a type of plastic.
What is initial stability of a ship?
Initial stability or primary stability is the resistance of a boat to small changes in the difference between the vertical forces applied on its two sides.
What is the metacentre of a curve?
With this definition we also state that the metacentre is the centre of curvature of the curve of centres of buoyancy, i.e. the B-curve.
Why did grain ship lose?
In the nineteenth century, when loading was not supervised by port surveyors, grain ships were lost because voids were left under the decks. Grain and similar cargoes may shift when the ship rolls at sea, providing an upsetting moment which, if greater than the ship's maximum righting moment, causes it to capsize; if it is less, the ship heels, which may cause a further shift of the grain, causing a larger angle of heel, which may immerse openings or capsize the ship. Protection for deep sea ships is now provided as follows:
What are some cargoes that may provide an intrinsic hazard to a ship and for which a large?
Grain, oil, explosive, hygroscopic materials, liquid gas and radioactive materials are some of the cargoes which may provide an intrinsic hazard to a ship and for which a large network of rules exists. These cannot be examined in a book of this size beyond a brief mention of their existence.
What is the role of point P2?
In other words, the point P2 plays the role of metacentre. From Figure 6.17 and Eq. (6.57) we see that pulling the ship to the left increases the distance ¯ GP2, while pulling the ship to the right reduces it.
What is the point of the keel in Figure 6.17?
Figure 6.17 shows a ship grounded on one point of the keel; let this point be P0. We draw a horizontal line through P0; let P1 be its intersection with the transverse plane passing through the centre of gravity, G, and P3, the intersection with the transverse plane passing through the centre of buoyancy, B, and the metacentre, M. Taking moments about the line P0P3 we write
How to calculate limits of a denominator?
We divide both nominator and denominator by h and calculate their limits as P1 → P. For the denominator we obtain
What happens if a ship is grounded on the whole length of the keel?
Figure 6.15 shows a ship grounded on the whole length of the keel. If local tide lowers the sea level, at a certain draught the ship will loose stability and capsize. To plan the necessary actions, the ship master must know how much time remains until reaching the critical draught. A similar situation occurs when a ship is laid in a floating dock. While ballast water is pumped out of the dock, the draught of the ship decreases. Props must be fully in place before the critical draught is reached.
What causes an unstable equilibrium?
An unstable equilibrium is caused when the vertical position of G is higher than the position of transverse metacenter (M). So, when the ship heels to an angle (say theta- Ɵ), the center of buoyancy (B) now shifts to B1. But the righting lever is now negative, or in other words, the moment created would result in creating further heel until a condition of stable equilibrium is reached. If the condition of stable equilibrium is not reached by the time the deck is not immersed, the ship is said to capsize.
What is the righting moment of a ship?
The moment resulting in the uprighting of the ship to its original orientation is called Righting Moment. The lever that causes the righting of a ship is the separation between the vertical lines passing through G and B1. This is called the Righting Lever, and abbreviated as GZ (refer to the figure above).
How much is a transverse GM?
The important thing to note here is that the values of longitudinal GM usually range from 100 to 110 times the value of the transverse GM. And since the values of transverse GM of all ship types vary from 0.2 to 0.5, it implies that GM in the longitudinal direction is usually as high as 100 metres or above. It is due to this, ships are inherently highly stable in the longitudinal direction, and hence, most studies of ship stability are focused on the transverse stability of the ship.
When is the ship in stable equilibrium?
Study the figure below. A stable equilibrium is achieved when the vertical position of G is lower than the position of the transverse metacentre (M). So, when the ship heels to an angle (say theta- Ɵ), the centre of buoyancy (B) now shifts to B1. The lateral distance or lever between the weight and buoyancy in this condition results in a moment that brings the ship back to its original upright position.
What is the concept of intact stability?
The fundamental concept behind the understanding of intact stability of a floating body is that of Equilibrium. There are three types of equilibrium conditions that can occur, for a floating ship, depending on the relation between the positions of centre of gravity and centre of buoyancy. 1. Stable Equilibrium:
What is the study of damaged stability of a surface ship?
The study of damaged stability of a surface ship includes the identification of compartments or tanks that are subjected to damage and flooded by seawater, followed by a prediction of resulting trim and draft conditions.
What is the stability of a surface ship?
The understanding of a surface ship’s stability can be divided into two parts. First, Intact Stability. This field of study deals with the stability of a surface ship when the intactness of its hull is maintained, and no compartment or watertight tank is damaged or freely flooded by seawater. Secondly, Damaged Stability.
THE CONCEPT OF INTACT STABILITY
In 1993, IMO developed the intact stability criteria for various types of ships. The code includes various fundamentals of general precautions against, capsizing, weather conditions, the effect of free surface and icing and watertight integrity.
DAMAGE STABILITY
CHANGE OF DRAFT: When a ship compartment is damaged, water is likely to enter in if flooded. This will hence create a difference in draft, where displacement of intact part will be equal to displacement of the ship before damage less the weight of ship after water seeped in.
How does lowering the center of gravity affect the stability of a ship?
When a righting moment exists, lowering the center of gravity along the centerline increases the separation of the forces of weight and buoyancy and increases the righting moment . When a heeling moment exists, sufficient lowering of the center of gravity along the centerline would change the heeling moment to a righting moment. Similarly, sufficient lowering of the center of gravity along the centerline could change the initial stability in the upright position from negative to positive.
How is the attitude of a floating object determined?
The attitude of a floating object is determined by the interaction of the forces of weight and buoyancy. If no other forces are acting, it will settle until the force of buoyancy equals the weight, and will rotate until two conditions are satisfied:
What happens when a floating body is disturbed?
If a floating body, initially at equilibrium, is disturbed by an external moment, there will be a change in its angular attitude. If upon removal of the external moment, the body returns to its original position, it is said to have been in stable equilibrium and to have positive stability.
How many positions can an object rest in?
A submerged object that is clear of the bottom can come to rest in only one position. It will rotate until the center of gravity is directly below the center of buoyancy. If its center of gravity coincides with its center of buoyancy, as in the case of a solid body of homogeneous material, the object would remain in any position in which it is placed.
What is the state of a floating body that assumes a displaced inclination because of an external moment?
If, on the other hand, a floating body that assumes a displaced inclination because of an external moment remains in that displaced position when the external moment is removed, the body is said to have been in neutral equilibrium and has neutral stability.
What happens when a floating body is disturbed by an external moment?
If a floating body, initially at equilibrium, is disturbed by an external moment, there will be a change in its angular attitude. If upon removal of the external moment, the body returns to its original position, it is said to have been in stable equilibrium and to have positive stability. (B) Neutral equilibrium.
How to determine the weight of a ship?
The total weight, or displacement, of a ship can be determined from the draft marks and Curves of Form . The position of the center of gravity may be either calculated or determined experimentally. Both methods are used when dealing with ships. The weight and center of gravity of a ship that has not yet been launched can be established only by a weight estimate, which is a summation of the estimated weights and moments of all the various items that make up the ship.
What is the metacentre of a curve?from sciencedirect.com
With this definition we also state that the metacentre is the centre of curvature of the curve of centres of buoyancy, i.e. the B-curve.
What is Figure 5.3?from sciencedirect.com
Figure 5.3. An ellipse and two of its osculating circles
What happens to a heeled ship in smooth water?from sciencedirect.com
However, due to its kinetic energy, the ship will roll beyond the upright position to a heel angle on the other side and from there back, etc. In the absence of damping, this oscillatory motion would continue forever. For small roll angles, the roll motion of such an undamped free roll motion in calm water is characterized by:
What is the turning rotation of a vessel about its vertical/Z axis?from en.wikipedia.org
The turning rotation of a vessel about its vertical/Z axis. An offset or deviation from normal on this axis is referred to as deviation or set. This is referred to as the heading of the boat relative to a magnetic compass (or true heading if referenced to the true north pole); it also affects the bearing.
How is M′ determined?from sciencedirect.com
In the equations for X1 etc., m′ is determined taking into account the heel angle. This leads to larger m′ values in the midship range due to increase of draft with heel for the full midship sections. Capturing the influence of heel in the computations of m′ is straightforward in CFD computations, but also a Lewis transformation approach can be extended to include heel (Söding 1984).
What are passive and active motion stabilization?from en.wikipedia.org
They include static hull features such as skegs and bilge keels, or active mechanical devices like counterweights, antiroll tanks and stabilizers .
What is wall sided hull?from sciencedirect.com
In Section 2.8.1 we defined as wall-sided those floating bodies whose hull surface is a cylinder with generators perpendicular to the waterplane. The assumption of a wall-sided hull can be a good approximation for many vessels, at small heel angles. In this section we are going to see that this approximation leads to a very interesting result in the case of an initial negative metacentric height.
What happens when the heeling moment does not generate a righting moment?
This means that the heeling moment does not generate a righting moment to bring back the ship to its original upright position. In this case, the ship is in a neutral equilibrium.
What happens when the center of gravity is not perpendicular to the center of buoyancy?
This righting lever creates a moment, known as the righting moment, which returns the ship to its original position. If the ship is able to return to an upright position, it is said to be in stable equilibrium.
What happens to the center of gravity when a ship is loaded?
When the ship is loaded, the center of gravity retains its position within the ship, as long as all the masses in the ship remain in the same place. But if some cargo on board shifts, due to rough seas for example, its motion and new location may change the ship’s center of gravity.
Where does the center of buoyancy move?
The metacenter is where the two lines of the original center of buoyancy and the newly shifted center of buoyancy intersect, as pictured in the image below. As the center of buoyancy moves laterally when the ship heels, the metacenter typically stays directly above it.
When a ship is upright, what is the center of gravity?
When the ship is in an upright position, the center of gravity and the center of buoyancy are aligned vertically above one another. If the ship heels to one side due to an external force, the center of gravity remains in its place relative to the ship, but it migrates to the side of the heel as a whole. The center of buoyancy migrates ...
What is the force that a ship displaces?
The weight of water a ship displaces gives it buoyancy – a force acting against gravity on a body immersed in liquids or gases. This force is caused by the displacement, or pushing aside, of the surrounding medium (water) and corresponds to the weight of the deformed fluid. This relation between an object and the fluid it is immersed in is known as ...
Why do ships have a low density?
If the total volume of a ship is large enough in relation to its weight, it has a low density. Simply put, because of all the air spaces inside, a ship has a lower average density than water, despite the heavy materials it is made of. The weight of water a ship displaces gives it buoyancy – a force acting against gravity on a body immersed in ...
What is the turning rotation of a vessel about its vertical/Z axis?
The turning rotation of a vessel about its vertical/Z axis. An offset or deviation from normal on this axis is referred to as deviation or set. This is referred to as the heading of the boat relative to a magnetic compass (or true heading if referenced to the true north pole); it also affects the bearing.
What are passive and active motion stabilization?
They include static hull features such as skegs and bilge keels, or active mechanical devices like counterweights, antiroll tanks and stabilizers .
What is the yaw axis?
The vertical/Z axis, or yaw axis, is an imaginary line running vertically through the ship and through its centre of mass . A yaw motion is a side-to side movement of the bow and stern of the ship. The transverse/Y axis, lateral axis, or pitch axis is an imaginary line running horizontally across the ship and through the centre of mass.
What are the three axes of a ship called?
There are three special axes in any ship, called longitudinal, transverse and vertical axes . The movements around them are known as roll, pitch, and yaw respectively. Pitch. The up/down rotation of a vessel about its transverse/Y (side-to-side or port-starboard) axis.
What is heel in sailing?
Heel refers to an offset that is intentional or expected, as caused by wind pressure on sails, turning, or other crew actions. The rolling motion towards a steady state (or list) angle due to the ship's own weight distribution is referred in marine engineering as heel.
What is the motion of a ship?
The linear transverse (side-to-side or port-starboard) motion. This motion is generated directly either by the water and wind currents exerting forces against the hull or by the ship's own propulsion; or indirectly by the inertia of the ship while turning. This movement can be compared to the vessel's drift from its course.
What is pitch motion?
A pitch motion is an up-or-down movement of the bow and stern of the ship. The longitudinal/X axis, or roll axis, is an imaginary line running horizontally through the length of the ship, through its centre of mass, and parallel to the waterline. A roll motion is a side-to-side or port-starboard tilting motion of the superstructure around this axis.
