Seakeeping ability or seaworthiness is a measure of how well-suited a watercraft is to conditions when underway. A ship or boat which has good seakeeping ability is said to be very seaworthy and is able to operate effectively even in high sea states.
In 1976, St. Denis [1] suggested four principal terms needed to describe a seakeeping performance. These are:
A drillship and a ferry have different missions and operate in different environments. The performance criteria will be different as well. Both may be considered seaworthy, although for different reasons based on different criteria.
In ship design it is important to pre-determine the behavior of the ship or floating structure when it is subjected to waves. This can be calculated, found through physical model testing and ultimately measured on board the vessel. Calculations can be performed analytically for simple shapes like rectangular barges, but need to be calculated by computer for any realistic shaped ship. The results of some of these calculations or model tests are transfer functions called response amplitude operators (RAOs). For a floating structure they will need to be calculated for all six motions and for all relative wave headings.
Ship motions are important for determining dynamic loading on the crew, passengers, ship system components, secured cargo, and structural elements. Excessive ship motions may hinder the vessel's ability to complete its mission such as the deployment and recovery of small boats or aircraft. A measure of an individual's ability to complete a specific task while on board a moving ship is the motion-induced interruptions [2] (MII). It gives an indication of the number of events in which a standing person will look for support in order to maintain balance. MII is measured in occurrences per hour.
Ship motions have physiological effects on ship passengers and crew. The magnitudes and accelerations of ship motions, (particularly heave, roll and pitch) have adverse effects on passengers and shipboard personnel. Sea sickness will have negative effects on the ability of crew to accomplish tasks and maintain alertness and will obviously distress passengers. An important metric in evaluating sea sickness is the motion sickness incidence [3] (MSI). The most important study on MSI was published in Aerospace Medicine by O'Hanlon and McCauley in 1974, which established common subjective thresholds of MSI tolerance. MSI is measured in percentage of people who experience sea sickness during a given amount of exposure time. A commonly accepted limit of MSI is 20% occurrence of sea sickness over a four-hour exposure period. A small percentage of people are very susceptible to sea sickness and become ill even in the slightest conditions, while other people rarely get sea sick despite severe conditions. It has also been shown that most people acclimate to ship motions within a period of about four days, but some never acclimate at all.
Seakeeping directly impacts the design of a vessel. Ship motions are considered when determining the principal dimensions of the ship and in developing the general arrangements of the ship's internal spaces. For example, in most vessels the far forward parts of the ship experience the worst ship motions and are commonly unacceptable for berthing passengers or crew. In exceptional cases where ship motions pose a threat to crew, structure or machinery, or when ship motions interfere with the ability of the ship to accomplish its mission, then the design must be modified so that ship motions are reduced.
A number of factors affect seakeeping or how correctly the ship responds.
Response to given sea conditions by a given hull may vary considerably depending on loading, free-surface of tanks, weight distribution, speed, and direction of travel.
Naval architecture, or naval engineering, is an engineering discipline incorporating elements of mechanical, electrical, electronic, software and safety engineering as applied to the engineering design process, shipbuilding, maintenance, and operation of marine vessels and structures. Naval architecture involves basic and applied research, design, development, design evaluation (classification) and calculations during all stages of the life of a marine vehicle. Preliminary design of the vessel, its detailed design, construction, trials, operation and maintenance, launching and dry-docking are the main activities involved. Ship design calculations are also required for ships being modified. Naval architecture also involves formulation of safety regulations and damage-control rules and the approval and certification of ship designs to meet statutory and non-statutory requirements.
The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre. A larger metacentric height implies greater initial stability against overturning. The metacentric height also influences the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll which are uncomfortable for passengers. Hence, a sufficiently, but not excessively, high metacentric height is considered ideal for passenger ships.
A ship model basin is a basin or tank used to carry out hydrodynamic tests with ship models, for the purpose of designing a new ship, or refining the design of a ship to improve the ship's performance at sea. It can also refer to the organization that owns and operates such a facility.
Capsizing or keeling over occurs when a boat or ship is rolled on its side or further by wave action, instability or wind force beyond the angle of positive static stability or it is upside down in the water. The act of recovering a vessel from a capsize is called righting. Capsize may result from broaching, knockdown, loss of stability due to cargo shifting or flooding, or in high speed boats, from turning too fast.
The waterline is the line where the hull of a ship meets the surface of the water. Specifically, it is also the name of a special marking, also known as an international load line, Plimsoll line and water line, that indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves that may arise. Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater with a similar lessening effect upon buoyancy.
A chief mate (C/M) or chief officer, usually also synonymous with the first mate or first officer, is a licensed mariner and head of the deck department of a merchant ship. The chief mate is customarily a watchstander and is in charge of the ship's cargo and deck crew. The actual title used will vary by ship's employment, by type of ship, by nationality, and by trade: for instance, chief mate is not usually used in the Commonwealth, although chief officer and first mate are; on passenger ships, the first officer may be a separate position from that of the chief officer that is junior to the latter.
A second mate or second officer (2/O) is a licensed member of the deck department of a merchant ship holding a Second Mates Certificate of Competency, which is issued by the administration. The second mate is the third in command and a watchkeeping officer, customarily the ship's navigator. Other duties vary, but the second mate is often the medical officer and in charge of maintaining distress signaling equipment. On oil tankers, the second mate usually assists the chief mate with the cargo operations.
A third mate (3/M) or third officer is a licensed member of the deck department of a merchant ship. The third mate is a watchstander and customarily the ship's safety officer and fourth-in-command. The position is junior to a second mate. Other duties vary depending on the type of ship, its crewing, and other factors.
A motion simulator or motion platform is a mechanism that creates the feelings of being in a real motion environment. In a simulator, the movement is synchronised with a visual display of the outside world (OTW) scene. Motion platforms can provide movement in all of the six degrees of freedom (DOF) that can be experienced by an object that is free to move, such as an aircraft or spacecraft:. These are the three rotational degrees of freedom and three translational or linear degrees of freedom.
In the field of ship design and design of other floating structures, a response amplitude operator (RAO) is an engineering statistic, or set of such statistics, that are used to determine the likely behavior of a ship when operating at sea. Known by the acronym of RAO, response amplitude operators are usually obtained from models of proposed ship designs tested in a model basin, or from running specialized CFD computer programs, often both. RAOs are usually calculated for all ship motions and for all wave headings.
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
A following sea refers to a wave direction that is similar to the heading of a waterborne vessel under way. The word "sea" in this context refers to open water wind waves.
A sea trial is the testing phase of a watercraft. It is also referred to as a "shakedown cruise" by many naval personnel. It is usually the last phase of construction and takes place on open water, and it can last from a few hours to many days.
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 centers of gravity, centers of buoyancy, the metacenters of vessels, and on how these interact.
The sopite syndrome is a neurological disorder that relates symptoms of fatigue, drowsiness, and mood changes to prolonged periods of motion. The sopite syndrome has been attributed to motion-induced drowsiness such as that experienced by a baby when rocked. Researchers Ashton Graybiel and James Knepton at the Naval Aerospace Medical Research Laboratory first used the term "the sopite syndrome", in 1976, to refer to the sometimes sole manifestation of motion sickness, though other researchers have referred to it as "Sopite syndrome."
Noise and vibration on maritime vessels are not the same but they have the same origin and come in many forms. The methods to handle the related problems are similar, to a certain level, where most shipboard noise problems are reduced by controlling vibration.
Seaworthiness is a concept that runs through maritime law in at least four contractual relationships. In a marine insurance voyage policy, the assured warrants that the vessel is seaworthy. A carrier of goods by sea owes a duty to a shipper of cargo that the vessel is seaworthy at the start of the voyage. A shipowner warrants to a charterer that the vessel under charter is seaworthy; and similarly, a shipbuilder warrants that the vessel under construction will be seaworthy.
Dismasting, also spelled demasting, occurs to a sailing ship when one or more of the masts responsible for hoisting the sails that propel the vessel breaks. Dismasting usually occurs as the result of high winds during a storm acting upon masts, sails, rigging, and spars. Over compression of the mast owing to tightening the rigger too much and g-forces as a consequence of wave action and the boat swinging back and forth can also be result in a dismasting. Dismasting does not necessarily impair the vessel's ability to stay afloat, but rather its ability to move under sail power. Frequently, the hull of the vessel remains intact, upright and seaworthy.
The Terrible class was a group of four ironclad barbette ships built for the French Navy in the late 1870s and early 1880s. The class consisted of Terrible, Indomptable, Caïman, and Requin, and is sometimes referred to as the Indomptable class. They were built as part of a fleet plan started in 1872 after the Franco-Prussian War and were designed in response to the German Sachsen class of barbette ships. The Terribles were scaled down versions of the Amiral Baudin class, with one less main gun, though they were of significantly larger caliber. Because the ships were intended for operations against the German fleet in the shallow Baltic Sea, they had a low draft and freeboard, which greatly hampered their seakeeping and thus reduced their ability to be usefully employed after entering service. Armament consisted of a pair of 420 mm (16.5 in) guns in individual barbettes, which were the largest guns ever mounted on a French capital ship.
This glossary of nautical terms is an alphabetical listing of terms and expressions connected with ships, shipping, seamanship and navigation on water. Some remain current, while many date from the 17th to 19th centuries. The word nautical derives from the Latin nauticus, from Greek nautikos, from nautēs: "sailor", from naus: "ship".
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