Instantaneous stability

Ship stability is an area of naval architecture and ship design that deals with how a ship behaves at sea, both in still water and in waves, whether intact or damaged. Stability calculations focus on the center of gravity, center of buoyancy, and metacenter of vessels and on how these interact.

Ship stability, as it pertains to naval architecture, has been taken into account 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 calculus-based methods of determining stability, particularly Pierre Bouguer's introduction of the concept of the metacenter in the 1740'S 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; by replicating a stable design 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 analytical design.

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. Before 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 to alter the heel and/or trim of the ship.

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