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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.

6/28/2010

Correcting unstable and neutral equilibrium

Correcting unstable and neutral equilibrium
1/ Weights already in the ship may be lowered.
2/ Weights may be loaded below the centre of gravity of the ship.
3/ Weights may be discharged from positions above the CG or
4/ Free surfaces within the ship may be removed.

The GM values
The values for GM for several ship types all at Full-loaded drafts are;
Ship type GM at full loaded condition
General cargo ships 0.30 – 0.50 m.
Oil tankers to VLCCs 0.30 – 1.00 m.
Container ships 1.50 m. (approximately)
Ro-Ro vessels 1.50 m. (approximately)
Bulk ore carriers 2 – 3 m.
Noted: the GM must never be less than 0.15 m.
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