When a floating body is given a small displacement it will rotate about a point, so the point at which the body rotates is called as the Metacenter. The distance between center of gravity of a floating body and Metacenter is called as Metacentric height. It is necessary for the stability of a floating body, If metacenter is above center of gravity body will be stable because the restoring couple produced will shift the body to its original position.

The point though which the force of buoyancy is supposed to pass is called as the center of buoyancy.

**Ship Stability, Part-1, F3**

First of all I adjust the movable weight along the vertical rod at a certain position and measured the distance of center of gravity by measuring tape. Then I brought the body in the water tube and changed the horizontal moving load distance first towards right. The piston tilted and suspended rod gave the angle of head, I noted the angle for respective displacements.

Cartoon cat gmodThen I took the body from water tube and find another center of gravity by changing the position of vertically moving load. I again brought the body in the water tube and find the angle of head by first keeping the movable load towards right and then towards left.

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Related Civil-Engg. Distance of movable mass at right of center mm. Metacentric height MH Y2. Distance of movable mass at left of center mm.But let us imagine we together built a ship. Or something looking like a ship. A smaller one may be. If I want to know the answers to these questions, one thing is for sure. We need to be able to understand the language of ship stability. But not if you know the basics of ship stability.

Once you know these basics, all other parts of ship stability will be as easy as eating a pancake. Why does a small metal ball sink in water but not ship? Probably the first question that a Pre-sea cadet is asked during his training. The answer lies in the Archimedes principle. A body wholly or partially immersed in a liquid is subject to an upthrust equal to the weight of the liquid displaced by the body. Try to force a ball down into the water. You will feel a force stopping you from doing that.

This is the upthrust we are talking about in Archimedes principle. This upthrust will be there on any object you place in water. When we place any object into the water, that object would displace some water. I bet everyone knows it because even the birds know it. Remember the story of thirsty crow? So if we drop a stone in a jar completely filled with water, some water would spill out of the jar because the stone has displaced some water.

Now let us see what Archimedes principle is trying to say? It is giving us a way to calculate the amount of upthrust that an object will feel when immersed in water or in any liquid. So if we have to make something float, all we have to do is to make sure that it displaces more water than its own weight. And we have a solid cube of T weight made of same material. As the weight of both is same, the downward gravitational force acting on both is same. But the upthrust acting on the ship will be more than that acting on the ball.

The upthrust acting on the steel cube will be T. As the weight of the cube downward force is T, the cube will continue to move downwards and will sink.Discussion in ' Boatbuilding ' started by ShidoranSep 21, Log in or Sign up.

Boat Design Net. Not sure if this is the right place to put a few questions, but I'm working on a project and am having trouble figuring out these calculations, or the method to do so. Any help is appreciated. Assuming the inclining weights used weighed 0. I also have a hydrostatic table to get the GM with the weights removed :? ShidoranSep 21, I'm not in the habit of doing people's course-work for them It should be able to tell you all you need to know Tim B.

Tim BSep 21, I can't see anything wrong with someone explaining what to do for the calculations instead of doing the calculations for me Thus why I'm asking. It's the theory I'm after preferably online if there is anything not someone to do it for me.

### What is Ship Stability?

ShidoranSep 22, You need displacement and KM to calculate GM from an inclining experiment. You get these from the hydrostatics tables for the vessel, based on the freeboard or draft. CDBarrySep 22, Draw a free-body diagram of the situation and everything will be clear. You've got way too much data so throw out the stuff you don't need out. And even if your lector and notes are crap, the basic texts aren't. You need to be able to "get" this concept or some of the later stuff i. Ok, so Alright, thanks for your input.

That makes sense. Well the drafts at the 4 corners are 0. Could you tell me if I'm approaching this right? I'm getting somewhere! Thanks for all your help I really do. ShidoranSep 23, Not to be harsh or anything, but have you considered another field of study?

Your posts show that you cannot or have not grasped the basic definitions and concepts and lack the problem solving ability necessary for simple hydrostatics. Alright, if it's gotta be that way Alright, here's the deal.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 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. Damaged stability, however, cannot be understood without a clear understanding of intact stability, and the interesting scenarios related to it.

Hence, we will first focus on intact stability from this article onward, leading to a discussion of cases where the application of concepts of intact stability come of use and then move on to damaged stability. Damage Stability Of Ships. 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.

Study the figure below. A stable equilibrium is achieved when the vertical position of G is lower than the position of transverse metacenter M. 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. The moment resulting in 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 Leverand abbreviated as GZ refer to the figure above. An important relation between metacentric height GM and righting lever GZ can also be obtained from the figure above. This is the most dangerous situation possible, for any surface ship, and all precautions must be taken to avoid it.

It occurs when the vertical position of CG coincides with the transverse metacentre M. As shown in the figure below, in such a condition, no righting lever is generated at any angle of heel. As a result, any heeling moment would not give rise to a righting moment, and the ship would remain in the heeled position as long as neutral stability prevails. The risk here is, at a larger angle of heel in a neutrally stable shift, an unwanted weight shift due to cargo shifting might give rise to a condition of unstable equilibrium.

An unstable equilibrium is caused when the vertical position of G is higher than the position of transverse metacenter M. 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.A Floating body displaces a volume of water equal to the weight of the body.

A Floating body will be buoyed up by a force equal to the weight of the water displaced. It is therefore equal to the Total weight of the vessel. The units are tons long. The center of Buoyancy B is a theoretical point though which the buoyant forces acting on the wetted surface of the hull act through. The center of Gravity is the theoretical point through which the summation of all the weights act through.

The position of the center of buoyancy changes depending on the attitude of the vessel in the water. As the vessel increases or reduces its draft so the center of buoyancy moves up or down respectively caused by increase in water displaced.

As the vessel lists the center of buoyancy moves in a direction governed by the changing shape of the submerged part of the hull. For small angles the tendency is for the center of buoyancy to move towards the side of the ships which is becoming more submerged.

## Ship Stability: Stiff and Tender Ship, Angle of Loll & Inclining Experiment

Affects of listing to larger angles or low freeboard. Note this is true for consideration of small angle stability and for vessels with sufficient freeboard.

In the example shown above when the water line reaches and moves above the main deck level a relatively smaller volume of the hull is submerged on the lower side for every centimeter movement as the water moves up the deck. The center buoyancy will now begin to move back towards centreline. The Metacenter M is a theoretical point through which the buoyant forces act and small angles of list.

At these small angles the center of buoyancy tends to follow an arc subtended by the metacentric radius BM which is the distance between the Metacenter and the center of buoyancy.

A the vessels draft changes so does the metacenter moving up with the center of buoyancy when the draft increases and vice versa when the draft decreases. For small angle stability it is assumed that the Metacenter does not move. When a vessel lists there center of buoyancy moves off centreline.

The center of gravityhowever, remains on centerline. For small angles up to 10 degrees depending on hull form the righting Arm GZ can be found by. The above examples all show the metacentre above the centre of gravity. This creates a righting arm at small angles always returning the vessel to the upright position. Where the metacentre is at or very near the centre of gravity then it is possible for the vessel to have a permanent list due to the lack of an adequate righting arm.

Note that this may occur during loading operations and it is often the case that once the small angle restrictions are passed the metacentric height increases and a righting arm prevents further listing. In a worst case the metacentre may be substantially below the center of Gravity. The Draft diagram is a simple and quick method of determining the following. A line is drawn joining the ford and aft draft marks. Blue Line. The Displacement can be read directly off.

A horizontal line is drawn passing through the intersection of the blue line onto the displacement curve Red Line.Subscribe in a reader. Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years.

Dra2 books pdfHistorically, ship stability calculations for ships relied on rule-of-thumb calculations, often tied to a specific system of measurement. Some of these very old equations continue to be used in naval architecture books today, however the advent of the ship model basin allows much more complex analysis. Master shipbuilders of the past used a system of adaptive and variant design.

Ships were often copied from one generation to the next with only minor changes being made, and by doing this, serious problems were not often encountered. Ships today still use the process of adaptation and variation that has been used for hundreds of years, however computational fluid dynamics, ship model testing and a better overall understanding of fluid and ship motions has allowed much more in-depth analysis.

Transverse and longitudinal waterproof bulkheads were introduced in ironclad designs between and the s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to [1]. Prior to this, a hull breach in any part of a vessel could flood the entire length of the ship. Transverse bulkheads, while expensive, increase the likelihood of ship survival in the event of damage to the hull, by limiting flooding to breached compartments separated by bulkheads from undamaged ones.

Longitudinal bulkheads have a similar purpose, but damaged stability effects must be taken into account to eliminate excessive heeling. ISBN Principles of Naval Architecture. Seamanship in the age of sail. London: Conway Maritime Press. Coast Guard Technical computer program support accessed 20 December Figure shows the effect of changing the cg fluid movement back and forth within the tank, which contained no full Affect the CG location and the ship.

Cloud cover and overlap parameterizationsReducing the impact of the Free Surface. To reduce the impact. Listing of the ship due to the action of the force within the ship. Caused by movement of the CG centre-line from the vertical to the stable equilibrium official position of the new boat. Where the balance of forces acting on the weight and buoyancy CG made through the CB.

From Figure 1 is the state of the vessels during the match, a state of balance of the boat at the beginning Initial positionbut when moving weight within the boat, resulting in the CG moved from its original position on the line centre.

Figure 2 line is the result of the ship tilted to the side that moves the CG away Figure 3. From Figure 3 that when the ship tilted to continue until the CB moves from its original position on the vertical centre line due to the change of volume under the water line up.

The Methoding to solve the problem about the boat List. To calculate the Final KG changing the vertical vertical moved. List when suspended weight. When The Ship carry goods on a tool or device is installed on the hull.

Komatsu pc01 reviewsWhile the weight of the goods that have not been placed on the hull. We must consider cg position of the object at the end position of the tool lifting of the boat compared to the height of K from KG and compared with the distance from the vertical centre line distance d then calculated as usual.

Correction to list. Editing Angle of List, it will lead to conditions of stability back to the boat upright again Uprightso it is the duty of the cadet officer will must calculated and adjust weight within the boat. Therefore We must to bring balance to the Listing moment to solve problems that occur.

Ship's Stabily Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. References 1.The metacentric height GM is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre.

A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers.

Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships. When a ship heels rolls sidewaysthe centre of buoyancy of the ship moves laterally. It might also move up or down with respect to the water line. The point at which a vertical line through the heeled centre of buoyancy crosses the line through the original, vertical centre of buoyancy is the metacentre.

The metacentre remains directly above the centre of buoyancy by definition. In the diagram, the two Bs show the centres of buoyancy of a ship in the upright and heeled conditions, and M is the metacentre.

The metacentre is considered to be fixed relative to the ship for small angles of heel; however, at larger angles of heel, the metacentre can no longer be considered fixed, and its actual location must be found to calculate the ship's stability.

The metacentre can be calculated using the formulae:. Where KB is the centre of buoyancy height above the keelI is the second moment of area of the waterplane in metres 4 and V is the volume of displacement in metres 3. KM is the distance from the keel to the metacentre. Stable floating objects have a natural rolling frequency, just like a weight on a spring, where the frequency is increased as the spring gets stiffer.

In a boat, the equivalent of the spring stiffness is the distance called "GM" or "metacentric height", being the distance between two points: "G" the centre of gravity of the boat and "M", which is a point called the metacentre.

Metacentre is determined by the ratio between the inertia resistance of the boat and the volume of the boat. The inertia resistance is a quantified description of how the waterline width of the boat resists overturning. Wide and shallow or narrow and deep hulls have high transverse metacenters relative to the keeland the opposite have low metacenters; the extreme opposite is shaped like a log or round bottomed boat.

Ignoring the ballastwide and shallow or narrow and deep means that the ship is very quick to roll and very hard to overturn and is stiff.

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