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.
1. A ship arrives in Adumport which has displacement 7,500 tonnes and KG 7.5 m. She then discharge and loads the following quantities.
During the stay in Adum port 65 tonnes of ballast Water ( KG 2.5m.) are consumed. If the final KM is 9.7 m find the GM on departure.
2.The MV Taro has a displacement of 4500 tonnes (KG=4m,KM=6.8m) She then loads 6,300 tonnes of cargo (KG=6।4m) KG = 8.9m. Find howmuch deck Cargo which is loade at if the ship is to complete loading and a positive GM of .45 metres.
Step 1 : Write KBGM diagram At : 4,500 Tonnes
Step 2 : Write KBGM diagram At : 6,300 Tonnes
Step 3 :
Final KG = Final KM - Final GM = 6.8 - 0.45 = 6.35 how much cargo was load at KG =8.9 m. Let cargo was loaded = x tonnes
Effected of loading, shifting & discharging to CG
Conclusions
1/ CG will move directly towards the center of gravity of any weight added(Loaded)
2/ CG will move directly away from the center of gravity of any removed (discharged)
3/ CG will move parallel to the shift of the center of gravity of any weightmoved
The Load Lines Rules require minimum stability conditions as follows:
a) The area under the curve of Righting Levers (GZ curve) shall not be less than
(i) 0.055 metre-radians upto an angle of 30 degrees
(ii) 0.09 metre-radians upto angle either 40 degrees or the angle at which lower edges of any openings in the hull, superstructure or deckhouses, being openings which cannot be closed weathertight, are immersed if that angle be less
(iii) 0.03 metre-radians between the angles of heel 30 & 40 degrees of such lesser angles as referred to in (ii)
b) The Righting Lever (GZ) shall be at least 0.20 metres at an angle of heel equal to or greater than 30 degrees.
c) The maximum Righting Lever (GZ) shall occur at an angle of heel not less than 30 degrees.
d) The initial transverse metacentric height shall not be less than 0.15 metres. In the case of a ship carrying a timber deck cargo which complies with subparagraph (a) by taking into account the volume of timber deck cargo the iniial tranverse metacentric height shall not be less than 0.05 metres.
Statical Stability curve
เราสามารถอ่านข้อมูลจากกราฟลักษณะนี้ได้ดังต่อไปนี้
1/ The range of stability คือ ค่าของมุมเอียงทั้งหมดของเรือที่มีค่า GZ เป็นบวก
2/ The angle of vanishing stability คือ มุมที่ค่า GZ มีค่าเป็น ‘0’ ก่อนจะมีค่าติดลบ(พลิกคว่ำ)
3/ The maximum GZ คือ มุมที่ค่า GZ มีค่าสูงสุด (ยอดกราฟ) ต้องเกิดขึ้นระหว่างมุม 30 และมุม 40 องศา
4/ The point at which the deck edge immerses คือ มุมเอียงซึ่งดาดฟ้าเปิดของเรือเริ่มแตะผิวน้ำ (สังเกตจากเส้นกราฟเริ่มเปลี่ยนแปลงความชันเป็นเส้นโค้ง)
5/ The negative stability area บริเวณที่มุมเอียงทำให้เกิดการทรงตัวที่ไม่ดี
6/ The Approximate GM หาได้จากสูตร GM = GZ / Sin Sin θ (θ = 57.3 องศา)
7/ The dynamic stability หาได้จากการนำพื้นที่ใต้กราฟทั้งหมดไปคูณกับระวางขับน้ำของเรือ
*/ The initial metacentric height หาค่าได้จากการลากเส้นตรงเป็นมุมสัมผัสกับเส้นกราฟจากจุดเริ่มต้นไปตัดกับเส้นตรงซึ่งลากตั้งฉากกับแกนนอนของกราฟที่มุม 57.3 องศา
Step 3 :
