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Stability conditions (watercraft) is the term used to describe the various standard loading configurations to which a ship, boat, or offshore platform may be subjected. They are recognized by classing societies such as Lloyd's Register, American Bureau of Shipping (ABS) and Det Norske Veritas. Classing societies follow rules and guidelines laid down by SOLAS conventions, the International Maritime Organization and laws of the country under which the vessel is flagged, such as the Code of Federal Regulations. Stability conditions describe the displacement or weight of the vessel, the loading distribution, extent of damage and where the damage may be located, and floodable spaces.

Stability is normally broken into two distinct types: Intact and Damaged

Naval vessels must meet established military standards (ex: MIL-STD, CFTO) while commercial vessels are regulated by the government of the flag they sail under and the governments of the ports they operate under.

Contents 1 Intact Stability 2 Intact Conditions 3 Damaged Stability 4 Effects on Stability 4.1 Transverse stability 4.2 Free surface effect 4.3 Ship's trim and freeboard 5 Development of Modern Stability Criteria 6 See also 7 References 8 External links

The vessel is in normal operational configuration. The hull is not breached in any compartment. The vessel will be expected to meet various stability criteria such as GMt, area under the Gz curve, range of stability, trim, etc. while subjected to cargo loading, wave spectrum, sustained winds, maneouvering, etc.

• Lightship : The vessel is complete and ready for service in every respect, including permanent ballast. All flooded spaces such as sea chests are considered but the vessel has no cargo, crew, provisions, ammunition or any other load item.

• Light Operating or Light Displacement : Along with all the Lightship loads, the vessel has all systems charged meaning that all fresh water, cooling, lubricating, hydraulic and fuel service header tanks, piping and equipment systems are filled with their normal operating fluids. Crew and effects are at their normal values. Consumables (provisions, potable water and fuel) are at 10% full load. Ammunition and/or cargo is not included (0% of full load).

• Full Load Departure or Full Displacement : Along with all the Lightship loads, the vessel has all systems charged meaning that all fresh water, cooling, lubricating, hydraulic and fuel service header tanks, piping and equipment systems are filled with their normal operating fluids. Crew and effects are at their normal values. Consumables (provisions, potable water and fuel) are at 100% capacity. Ammunition and/or cargo is at maximum capacity. The vessel is at its limiting draft or legal load line.

• Standard Condition (Only for military vessels) : Along with all the Lightship loads, the vessel has all systems charged meaning that all fresh water, cooling, lubricating, hydraulic and fuel service header tanks, piping and equipment systems are filled with their normal operating fluids. Crew and effects are at their normal values. Consumables (provisions, potable water and fuel) are at 50% capacity. Ammunition and/or cargo is at 100% capacity. This condition is normally used for range and speed calculations.

• Light Arrival : Along with all the Lightship loads, the vessel has all systems charged meaning that all fresh water, cooling, lubricating, hydraulic and fuel service header tanks, piping and equipment systems are filled with their normal operating fluids. Crew and effects are at their normal values. Consumables (provisions, potable water and fuel) are at 10% full load. Ammunition and/or cargo is at 100% capacity.

• Damaged : The vessel is analytically damaged by opening various combinations of watertight compartments to the sea. The number of compartments and their location are dictated by IMO regulations, SOLAS conventions, or other applicable rules. The vessel will be expected to meet various stability criteria such as freeboard, trim, list, GMt, etc. The allowed freeboard, trim, and list for a damaged vessel is much higher than an undamaged one.

The loss of stability from flooding may be due in part to the free surface effect. Water accumulating in the hull usually drains to the bilges, lowering the centre of gravity and increasing the metacentric height. This assumes the ship remains completely stationary and upright. However, once the ship is inclined to any degree (a wave strikes it for example), the fluid in the bilge moves to the low side. This results in a list.

Stability is also lost due to flooding when an tank is holed and filled with seawater. If it is empty, or the contents were lighter than seawater, the lost buoyancy of the tank results in that section of the ship lowering into the water slightly. This creates a list unless the tank is on the centerline of the vessel. If the tank or cargo hold contained materials heavier than seawater, the flooded section becomes more buoyant than it was before.

The Chief Mate is the maritime officer in charge of stability on a merchant ship. He must take into consideration that a ship is balanced precariously under the best of conditions upon the water and is subject to a number of forces, such as wind, swells, and storms which could capsize it. The cargo officer uses tools like ballasting and load balancing to optimize the ship's performance for the type of environment expected to be encountered.

The four main objectives are to cause the ship to have good:

1. transverse stability, which moderates rolling
2. trim, which adjusts the depth of the bow and stern
3. freeboard, which is usually related to stability depending on the waterline profile

Further information: Metacentric height

Transverse stability describes the set of forces that act to right a vessel that is heeling over to one side. This system creates the familiar side-to-side rolling that often causes seasickness. One of the chief mate's main responsibilities is loading cargo in such a way as to maximize transverse stability without causing undesirable side effects.

One of these forces is called the righting arm. It comes from gravity pulling down on the hull, effectively acting on its center of gravity.

The second force, called buoyancy, pushes the hull upwards; effectively acting along the vertical line passing through the center of buoyancy and the metacenter above it.

These two forces together create a torque which rotates the hull upright again and is proportional to the horizontal distance between the center of gravity and the metacenter.

The distance from point "G" in the diagram to point "M" is called the metacentric height or "GM". It is important because the righting force is proportional to the metacentric height times the sine of the angle of heel.

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 - a low GM increases the risk of capsizing in rough weather (see HMS Captain) and the likelihood of developing "synchronized rolling". It also puts the vessel at risk for large angles of heel if the cargo or ballast shifts (see Cougar Ace). The metacentric height will be reduced further and make it even less stable if a ship with low GM is damaged and partially flooded.

On the other hand, a metacentric height that is too large can cause a vessel to be overly "stiff." An overly stiff vessel rolls with a short period and high amplitude. Such excessive stability is uncomfortable for passengers and crew because it quickly snaps back upright after a wave or wind gust which heeled it over has passed. This can lead to damage to the ship and cause cargo to break loose or shift. A passenger ship will typically have a long rolling period for comfort, perhaps 28 seconds while a tanker or freighter might have a rolling period of 13 to 15 seconds.

Further information: Free surface effect

One of the bigger challenges a Chief Mate faces is offsetting the damage that the free surface effect can have on the ship's stability.

In tanks or spaces that are partially filled with a fluid,^[1] the surface of the liquid stays level as the tank is inclined. This causes a displacement of the centre of gravity of the tank or space. The effect is similar to that of carrying a large flat tray of water that tips. The water rushes to one side, which exacerbates the tip even further.

The significance of this effect is proportional to the square of the width of the tank or compartment. Two baffles separating the area into thirds will reduce the displacement of the centre of gravity of the fluid by a factor of 9. This is always of significance in ship fuel tanks or ballast tanks, tanker cargo tanks, and in flooded or partially flooded compartments of damaged ships. Another concern with free surface effect is that a positive feedback loop can be established. This occurs when the duration of the roll is equal or almost equal to the period of the motion of the centre of gravity in the fluid, resulting in each roll increasing in magnitude until the loop is broken or the ship capsizes.

For more details on this topic, see Waterline.

Another challenge faced while loading the ship with cargo is maintaining proper trim and freeboard.

A ship's trim is the relation of the depth of the bow of the ship to the depth of the stern. A ship that has too much weight concentrated forward of its center of buoyancy is "trimmed by the head" and will move through the water much less efficiently and is at greater risk for taking on water. Additionally, handling of the ship may be poor due to less than ideal flow of water around the propellor and rudder.

Freeboard is a measure of a ship's reserve buoyancy, or how much weight can be added to the ship before it sinks. International regulations call for a load line or Plimsoll line to be painted on each side of ship over a certain tonnage. This symbol marks the level to which the ship can be safely loaded. The ship floats lower and the symbol descends farther into the water as cargo is brought on board. This symbol is separate from tick marks marking the ship's draft

It has been suggested that Ship stability be merged into this article or section. (Discuss)

• Naval architecture
• Hull (watercraft)
• SNAME
• Instantaneous stability

1. ^ Free surface effect can occur with semi-fluid materials, such as fish, ice or grain.

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