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[[Image:CougarAceListing24july2006-1.jpg|thumb|right|200px|The Chief Mate is generally responsible for the stability of the ship.]] As cargo officer, a chief mate oversees the loading, stowage, securing and unloading of cargoes. Moreover the chief mate is accountable for the care of cargo during the voyage. This includes a general responsibility for the stability of the ship and special care for cargoes that are dangerous, hazardous or harmful.
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 [[Sailing ballast|ballast]]ing and [[weight distribution|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:
# transverse stability, which moderates rolling
# trim, which adjusts the depth of the bow and stern
# [[freeboard]], which provides reserve buoyancy
# and hogging and sagging, which describe longitudinal stresses
===Transverse stability===
{{see|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.
[[Image:Righting arm.png|right|thumb|A number of forces affect a ship's transverse stability.]]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 (1869)|HMS ''Captain'']]) and more likely to develop "synchronized rolling". It also puts the vessel at risk of potential 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.
===Free surface effect===
{{see|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<ref>Free surface effect can occur with semi-fluid materials, such as fish, ice or grain.</ref> 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.
===Ship's trim and freeboard===
{{detail|Waterline}}
[[Image:Brosen plimsoll line1.jpg|right|thumb|Plimsoll line]]
Another challenge the Chief Mate faces while managing the ship's 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 nose-down," will move through the water much less efficiently and is at greater risk for taking on water.
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.
Before these symbols were made compulsory many ships were lost due to overloading.<ref>Sometimes ships were deliberately overloaded in the hope of collecting insurance money. Ships carrying emigrants from Europe to America were also lost.</ref> The British social reformer and politician [[Samuel Plimsoll]] advocated improved safety standards at sea. The Plimsoll line bears his name in his honour.
These marks are used to ensure adequate reserve buoyancy for the intended area and season of operation.<ref>The letters on the Load line marks have the following meanings: TF - Tropical Fresh Water, F - Fresh Water, T - Tropical Seawater, S - Summer Seawater, W - Winter Seawater, WNA - Winter North Atlantic. Letters may also appear to the sides of the mark indicating the [[classification society]] that has surveyed the vessel's load line. The initials used include AB for the [[American Bureau of Shipping]], LR for [[Lloyd's Register]], and NV for [[Det Norske Veritas]].</ref> For example, ships encounter rougher conditions in winter as opposed to summer, and in the North Atlantic as opposed to tropical waters. The load line has separate fresh water marks because ships float deeper in fresh water.
===Hogging and sagging===
{{see|Hogging (Naval)|Sagging (Naval)}}
[[Image:ShipSaggingHogging.png|thumb|right|300px|Diagram of ship hull (1) Sagging and (2) Hogging under loads. Bending is exaggerated for demonstration purposes.]] On cargo ships, hogging and sagging are the result of poor load management, and must be dealt with by the chief mate.
Hogging is the stress a ship's [[Hull (watercraft)|hull]] or [[keel]] experiences that causes the center of the keel to bend upward.
A cause of dynamic hogging is when a wave is the same length as the ship and the [[crest (physics)|crest]] of the wave is [[amidships]]. This causes the middle of the ship to bend up slightly, and depending of the level of bend, may cause the hull to snap or crack. This may have been what sank the ''[[Prestige oil spill|Prestige]]'' off [[Spain]] on [[19 November]] [[2002]].
Sagging is the stress a ship's [[Hull (watercraft)|hull]] or [[keel]] is placed under when a wave is the same length as the ship and the ship is in the trough of two waves. This causes the middle of the ship to bend down slightly, and depending of the level of bend, may cause the hull to snap or crack. This may have been what sank the ''[[Prestige oil spill|Prestige]]'' off [[Spain]] on [[19 November]] [[2002]].
===Damage Stability===
The loss of stability from flooding is due to the free surface effect. Subsequently, water accumulating in the hull will be in the bilges, lowering the centre of gravity and actually increasing the metacentric height. However once the ship is inclined, the centre of gravity of the fluid in the bilge moves to the low side, resulting in a list, which will reduce the righting lever (the tendency of the ship to right itself). The Chief Mate has advanced training in damage stability.
==Deck Department==
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