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.
6/28/2010
Angle of loll
Angle of loll is a term used to describe the state of a ship which is unstable when upright (ie: has a negative metacentric height, GMt) and therefore takes on an angle of heel to either port or starboard.
When a vessel has negative GM i.e., is in unstable equilibrium, any external force, if applied the vessel, will cause it to start heeling. As the heels, its underwater volume increases, which increases the vessel's BM (distance from the center of buoyancy to the metacenter). Since there is no change in KB (distance from the keel to the center of buoyancy) of the vessel, the KM (distance from keel to the metacenter) of the vessel increases.
At some angle of heel (say 10°), KM will increase sufficiently equal to KG (distance from the keel to the center of gravity), thus making GM of vessel equal to zero. When this occurs, the vessel goes to neutral equibrium, and the angle of heel at which it happens is called angle of loll, In other words, when an unstable vessel heels over towards a progressively increasing angle of heel, at a certain angle of heel, the center of buoyancy (B) may fall vertically below the center of gravity (G). Note that Angle of List should not be confused with angle of loll. Angle of List is caused by unequal loading on either side of center line of vessel.
Although vessel at angle of loll does display features of stable equilibrium, it is an extremely dangerous situation, timely remedial action is required to prevent the vessel from capsizing.[1][2][3][4]
It is often caused by the influence of a large free surface or the loss of stability due to damaged compartments। It is different from list in that the vessel is not induced to heel to one side or the other by the distribution of weight, it is merely incapable of maintaining a zero heel attitude.
ขณะที่เรืออยู่ในสภาพการทรงตัวไม่ดี เรือจะเริ่มเอียงไปเรื่อยๆจนกระทั่ง ณ มุมเอียงมุมหนึ่งความกว้างของเรือที่ผิวน้ำเพิ่มขึ้นมากจนเป็นผลให้จุด M เคลื่อนตัวกลับไปอยู่ในตำแหน่งเดียวกันหรืออยู่เหนือตำแหน่งของจุด G อีกครั้ง แต่จุด M จะไม่อยู่บนเส้นกึ่งกลางลำอีกต่อไป มุมนี้เรียกว่า ‘Angle of Loll’ซึ่งเป็นสภาวะเริ่มต้นตำแหน่งใหม่ของสภาพการทรงตัวของเรือ เรือมักจะพบว่ามีอาการเอียงไปมาอย่างรวดเร็วใกล้ๆมุมดังกล่าวนี้
References
1. ^ Kemp, "The Oxford Companion to Ships and the Sea", 1976, p. 494
2. ^ "Stability Calculations - Estimating Center of Gravity". Maritime & Coastguard Agency. http://www.mcga.gov.uk/c4mca/mcga-safety_information/mcga-safety_alerts-tech/ssb_safety_alert-2.htm. Retrieved 2007-01-24.
3. ^ "Definition - angle of loll". Maritime Dictionary. http://www.m-i-link.com/dictionary/default.asp?term=angle+of+loll. Retrieved 2007-01-24.
4. ^ "Stability Definitions". MCA Orals. http://www.mcaorals.co.uk/Stability%20Definitions.htm. Retrieved 2007-01-24.
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