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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

Stability

GM and rolling period

GM has a direct relationship with a ship's rolling period. A ship with a small GM will be "tender" - have a long roll period - an excessively low or negative GM increases the risk of a ship capsizing in rough weather (see HMS Captain or the Vasa). It also puts the vessel at risk of potential for large angles of heel if the cargo or ballast shifts (see Cougar Ace). A ship with low GM is less safe if damaged and partially flooded because the lower metacentric height leaves less safety margin. For this reason, maritime regulatory agencies such as the IMO specify minimum safety margins for sea-going vessels. A larger metacentric height, on the other hand can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew. This is because the stiff vessel quickly responds to the sea as it attempts to assume the slope of the wave. An overly stiff vessel rolls with a short period and high amplitude which results in high angular acceleration. This increases the risk of damage to the ship as well as the risk cargo may break loose or shift. In contrast a "tender" ship lags behind the motion of the waves and tends to roll at lesser amplitudes. A passenger ship will typically have a long rolling period for comfort, perhaps 12 seconds while a tanker or freighter might have a rolling period of 6 to 8 seconds.

The period of roll can be estimated from the following equation[2]

T =\frac{2 \pi\, k}{\sqrt{g \overline{GM}}}\

Where g is the gravitational constant, k is the radius of gyration about the longitudinal axis through the center of gravity and \overline{GM} is the stability index.

Damaged Stability

If a ship floods, the loss of stability is due to the increase in B, the Center of Buoyancy, and the loss of waterplane area - thus a loss of the waterplane moment of inertia - which decreases the metacentric height.[2] This additional mass will also reduce freeboard (distance from water to the deck) and the ship's angle of down flooding (minimum angle of heel at which water will be able to flow into the hull). The range of positive stability will be reduced to the angle of down flooding resulting in a reduced righting lever. When the vessel is inclined, the fluid in the flooded volume will move to the lower side, shifting its center of gravity toward the list, further extending the heeling force. This is known as the free surface effect (see below)

Referrence

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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1 comment:

  1. Jaya Prentice7 November 2019 at 01:13

    I read your blog on daily basis. This is really great and informative post. Thanks for sharing.
    Surface Effect Ship

    ReplyDelete

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