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Ship's Stabily

Ship stability is a complicated aspect of naval architecture which has existed in some form or another for hundreds of years. Historically, 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 1860 and the 1880s, anti-collision bulkheads having been made compulsory in British steam merchant ships prior to 1860[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. Today, most ships have means to equalize the water in sections port and starboard (cross flooding), which helps to limit the stresses experienced by the structure, and also alter the heel and/or trim of the ship.

References

1. ^ Ship Stability. Kemp & Young. ISBN 0853090424

2. ^ a b c d Comstock, John (1967). Principles of Naval Architecture. New York: Society of Naval Architects and Marine Engineers. pp. 827. ISBN 670020738.

3. ^ a b Harland, John (1984). Seamanship in the age of sail. London: Conway Maritime Press. pp. 43. ISBN 0851771793.

4. ^ U.S. Coast Guard Technical computer program support accessed 20 December 2006.

6/25/2010

Different centers



Different centers
The center of buoyancy, is the center of the volume of water which the hull displaces. This point is referred to as B in naval architecture. The center of gravity of the ship itself is known as G in naval architecture. When a ship is stable, the center of buoyancy is vertically in-line with the center of gravity of the ship.[2]

The metacenter is the point where the lines intersect (at angle φ) of the upward force of buoyancy of φ ± dφ. When the ship is vertical it lies above the center of gravity and so moves in the opposite direction of heel as the ship rolls. The metacenter is known as M in naval architecture.

The distance between the center of gravity and the metacenter is called the metacentric height, and is usually between one and two meters. This distance is also abbreviated as GM. As the ship heels over, the center of gravity generally remains fixed with respect to the ship because it just depends upon position of the ship's weight and cargo, but the surface area increases, increasing BMφ. The metacenter, Mφ, moves up and sideways in the opposite direction in which the ship has rolled and is no longer directly over the center of gravity.[3]

The righting force on the ship is then caused by gravity pulling down on the hull, effectively acting on its center of gravity, and the buoyancy pushing the hull upwards; effectively acting along the vertical line passing through the center of buoyancy and the metacenter above it. This creates a torque which rotates the hull upright again and is proportional to the horizontal distance between the center of gravity and the metacenter. The metacentric height is important because the righting force is proportional to the metacentric height times the sine of the angle of heel.

When setting a common reference for the centers, the molded (within the plate or planking) line of the keel (K) is generally chosen; thus, the reference heights are:
KB - Center of Buoyancy
KG - Center of Gravity
KM - Metacenter


Righting arm
Distance GZ is the righting arm: a notional lever through which the force of buoyancy acts.
Sailing vessels are designed to operate with a higher degree of heel than motorized vessels and the righting torque at extreme angles is of high importance. This is expressed as the righting arm (known also as GZ — see diagram): the horizontal distance between the center of buoyancy and the center of gravity.GZ = GM sin φ
Monohulled sailing vessels are designed to have a positive righting arm (the limit of positive stability) at anything up to 120º of heel, although as little as 90º (masts flat to the surface) is acceptable. As the displacement of the hull at any particular degree of list is not proportional, calculations can be difficult and the concept was not introduced formally into naval architecture until about 1970.[4]

References

1. ^ Ship Stability. Kemp & Young. ISBN 0853090424
2. ^ a b c d Comstock, John (1967). Principles of Naval Architecture. New York: Society of Naval Architects and Marine Engineers. pp. 827. ISBN 670020738.
3. ^ a b Harland, John (1984). Seamanship in the age of sail. London: Conway Maritime Press. pp. 43. ISBN 0851771793.
4. ^ U.S. Coast Guard Technical computer program support accessed 20 December 2006.
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