Ship stability

The term stability refers to shipbuilding and nautical the property of a floating body, such as a vessel to maintain an upright swimming position or become self erect in response to a heeling moment again.

A ship is considered in the physical sense as stable if a positive force must be expended to dive deeper or the ship to rotate it around its longitudinal or transverse axis. The reaction forces and moments of the body counteract.

  • 2.1 Weight stability
  • 2.2 Dimensional stability
  • 2.3 capsizing angle

Stability of ships

Factors Affecting

The following factors determine the individual stability of a ship:

  • Size and shape of the hull
  • Mass and mass distribution of the hull
  • Charge weight and charge distribution ( trim )
  • Behavior of the load ( eg, any movement of bulk goods or passengers )
  • Dynamic behavior of the vessel eg rate changes at high speed
  • Free surfaces (cash or charge spreading / contents of partially filled tanks)
  • Crane loads

More to be drawn into consideration operating conditions are:

  • Sea
  • Wind
  • Current
  • Icing danger of over- water ship ( ice load )
  • Waterproof (salt water / fresh water)

Physical Basics

The basic parameters of the stability of a ship are the center of gravity and the center of lift (also shaped or center of buoyancy ), as well as the resulting from them metacentric height. In the center of gravity can be thought of the entire downward force of gravity of the vessel focused on a point. In a heeling of the ship center of gravity maintains its position within the ship, as long as all masses remain in the ship in place ( for example, if merges charge, this also changes the center of gravity ). In the center of lift can be thought of the whole push-up weight of the displaced water. It is equal to the total weight of the ship and change its position at a heeling.

In the upright position of the vessel center of gravity and center of lift are stacked vertically. If the vessel is heeled by an external influence, the center of gravity remains on the ship, although related in its place, but overall moves to the side of the heel from. The center of lift moves out to the same side, namely the center of the now displaced water. If center of gravity and center of lift now no more about each other and are perpendicular to the center of gravity is below the Anfangsmetazentrums of the ship, creating a so-called " righting lever " which returns to the ship upon release of the heeling influence in its original position.

Identification and Evaluation

The relevant parameters for evaluating the stability of a ship are the initial metacentric height, the range of stability and the area under the righting lever. The metacentric height is a parameter for the righting lever arm. The range of stability refers to the computational heeling of the ship in angular degrees to the point of capsizing and righting the is a graphical representation of the respective righting lever over the full Krängungsbereich to the point of capsizing. The lever arm grows with increasing angle of heel at first slightly, then more and is smaller again, until it finally reaches the capsizing point when the center of gravity to travel beyond the center of lift with an even more heeling. The area under the righting lever curve can not only meet the minimum stability show but also prove an unintentionally great stability.

Regulatory

Decisive for the stability of ships are several IMO resolutions. The most important of this are the resolutions A.749 (18) and MSC.267 (85 ) (2008 IS Code) for the intact stability of ships or in accordance with SOLAS regulation for passenger ships. Even if the claims formulated therein are not binding, many flag states and eg the EU have adopted the provisions of the IMO in its own stability requirements. Merchant ships flying the German flag, however, must in this respect also meet the stringent requirements of maritime trade association.

Typical stability requirements are, for example:

  • Minimum distance of center of gravity and metacentric height.
  • Area under the righting lever curve.
  • Angle of the maximum of the righting.
  • Righting moment at a defined load.

The stability is taken into account already in the design phase of a ship and, inter alia, examined on the basis of predetermined standard loading conditions. The proof of stability is nowadays usually means an onboard computer, the pre-calculated all charge and stability criteria. The lightship weight of the load cases underlying is determined experimentally in an inclining experiment. The accounts are audited by an authorized by the flag State classification society and purpose shall be recognized when all stability requirements applicable to the ship concerned are respected. The audited stability documents are among the documents on board.

Practical Considerations

The rolling behavior of ships with a large righting lever arm is called rigid, over which vessels of a small righting lever arm is referred to as soft and a ship with only a very small righting lever arm is called rank.

Have types of ships such as container ships or ferries, due to loading and type, often an undesirably high center of gravity, which would have a too low stability. To ensure sufficient stability, a high loading on deck is therefore with great ballast water capacity, mainly in double bottom tanks, balanced. The opposite situation is found for example in Erzschiffen, which have an extremely low-lying center of gravity is usually in the loaded state denenen. A ship with undesirably high stability has a very short roll period with small roll angles, which favor the high accelerations occurring transitioning the charge or personal injury and the vessel's structure would be very onerous. Here the center of gravity is moved by the uptake of ballast water in deep tanks up to improve this behavior.

The stability assessment of a ship refers not only to the pure hull alone, but also to different and varying operating conditions. This includes mainly the loading of the ship, at the instance of the particular requirements for grain cargo ( which easily slide around ) or small angle of heel at heavy lift needs to be taken on deck consideration. In addition to changing the operating conditions must be calculated in advance especially by consumption of bunker supplies and fresh water, as well as by changing the quantities of ballast water from the beginning to the end of the trip. The influence of different external operating conditions, such as wind pressure, sea state, water absorption of the deck cargo and water storage on deck, or freezing in cold regions must also be included in the considerations. Last but not least on internal influences, such as the laying of hard rowing at full speed, or to the possible situation that go all the passengers on one side of the passenger ship, consideration will be taken.

Through wind and wave loads more so-called dynamic stability may arise during a trip. In the main, these are for the effects of strong wind gusts, the seakeeping of the vessel to sea and swell and roll period occurring resonances. Since these phenomena are not to be taken on the basis of the underlying highly complex energy balances readily formulas whose judgment is still left largely to the nautical experience of navigation. In the case of leakages weight distribution as well as buoyancy can significantly change, so capsize a ship, though it is still fully buoyant. From all the foregoing it follows that the evaluation of the stability of ships is more difficult, the more complex it is built and the more variable are the operating conditions.

Ship stabilizers

For larger vessels, especially in passenger ships, add-on systems are often used with which a dampening influence the movement of a ship on the longitudinal axis, or such as with fin stabilizers, which can actively control.

Recreational marine environment

Unlike ships of commercial shipping and marine pleasure craft are constructed often easier. Often they consist essentially of a hollow trunk, where appropriate, with mast and sail. In practice, it is therefore sufficient consideration less aspects: medium -body fuselage, center of gravity and / or an additional stabilizing weight.

The Krängungsverhalten a sailboat largely depends on the hull shape and weight distribution of the boat (including crew). There are two components through which a heel can be compensated. Except in a few special cases (pure rigid boats), the stability always is composed of two components righting:

  • Weight stability - a low-lying ballast keel forces the boat back in the upright position ( tumbler principle).
  • Dimensional stability - the shape of the hull favors a return to the starting position.

Weight stability

With predominantly weight- stable boats, mostly yachts, the keel often contributes about 35-50 % to the total weight of the boat. He puts the heel against a righting force. Thus, a capsize is possible only under very heavy wind and sea conditions.

The picture to G is the center of gravity ( center of gravity of the boat) and A is the geometric center of gravity ( center of gravity of the displaced water mass ). On these points, one can imagine combines the weight and buoyancy forces. For weight stability, the position of G is decisive: With increasing heeling center of gravity shifts outwards, thus increasing the righting torque.

In addition, with increasing heel reduces the wind pressure on the sail.

Dimensional stability

In contrast to keel yachts most dinghies are predominantly dimensionally stable. The (mostly folding ) light sword a dinghy has no appreciable effect righting. And catamarans have a high dimensional stability due to its width.

The picture to G is the center of gravity ( center of gravity of the boat) and A is the geometric center of gravity ( center of gravity of the displaced water mass ). On these points, one can imagine combines the weight and buoyancy forces. For the dimensional stability of the position of A is crucial.

In the upright position of the boat is displaced on both sides of the fuselage same amount of water. A is then the center of the fuselage cross-section, there will be no torque. With increasing angle of heel (see picture) is displaced especially on one side of the hull water. This moves A to the outside, it creates a torque. The wider the boat, the more moves A to the outside and the stronger the righting torque. If the heeling angle is too large, the torque increases, however, from the back, because the width of the hull is then tilted and A is closer to the center again. A slight heel is therefore compensated by the strong righting torque ( " water resistance " ), whereas an excessive heeling leads to capsize the boat.

In addition, with increasing heel reduces the wind pressure on the sail. The crew can counteract the heeling by shifting their weight to windward.

There are even examples of completely rigid boat types with negative initial stability. These have no upright swimming position at rest.

Capsizing angle

Both in form as well as in weight- stable boats there is a certain angle of heel, the capsizing angle at which the weight of the keel and the crew provide reinforcement of the heel, so that the boat capsizes. For weight- stable keel yachts capsizing angle is usually between 110 ° and 160 °, with sword dinghies other hand, generally below 90 °. Depending on how the behavior of a particular boat at different heel angles, it is called high initial or final stability. While keel yachts keel top layer is difficult to achieve and is usually finished by sea quickly, dinghies capsize by light and lie with the sword upwards stable in the water.

Navy Dinghy sailing
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