Seakeeping ability or seaworthiness is a measure of how well-suited a
watercraft
Any vehicle used in or on water as well as underwater, including boats, ships, hovercraft and submarines, is a watercraft, also known as a water vessel or waterborne vessel. A watercraft usually has a propulsive capability (whether by sail, ...
is to conditions when underway. A
ship
A ship is a large watercraft that travels the world's oceans and other sufficiently deep waterways, carrying cargo or passengers, or in support of specialized missions, such as defense, research, and fishing. Ships are generally distinguished ...
or
boat
A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size, shape, cargo or passenger capacity, or its ability to carry boats.
Small boats are typically found on inl ...
which has good seakeeping ability is said to be very seaworthy and is able to operate effectively even in high
sea state
In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, ...
s.
Measure
In 1976, St. Denis suggested four principal terms needed to describe a seakeeping performance. These are:
*Mission: what the ship is intended to accomplish. The role of the ship while at sea.
*Environment: the conditions under which the ship is operating. This can be described as
sea state
In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, ...
, wind speed, geographic region or some combination thereof.
*Ship responses: the response of the ship to the environmental conditions. The responses are a function of the environment and the vessel characteristics.
*Seakeeping performance criteria: the established limits for the ship's responses. These are based on the
ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
Reference axes
The '' vertical/Z axis'', or ''yaw axis'', is an imaginary line running vertically through the ship and through its ...
and the accelerations experienced, and include comfort criteria such as noise, vibration and
sea sickness
Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, dizziness, tiredness, loss of appetite, and increased salivation. Complications may rarely include d ...
, performance based values such as involuntary speed reduction, and observable phenomena such as bow immersion.
A
drillship
A drillship is a merchant vessel designed for use in exploratory offshore drilling of new oil and gas wells or for scientific drilling purposes. In recent years the vessels have been used in deepwater and ultra-deepwater applications, equipped ...
and a
ferry
A ferry is a ship, watercraft or amphibious vehicle used to carry passengers, and sometimes vehicles and cargo, across a body of water. A passenger ferry with many stops, such as in Venice, Italy, is sometimes called a water bus or water taxi ...
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.
Background
![USS Waldron (DD-699) underway in heavy seas on 30 September 1953 (NH 96833)](https://upload.wikimedia.org/wikipedia/commons/e/e3/USS_Waldron_%28DD-699%29_underway_in_heavy_seas_on_30_September_1953_%28NH_96833%29.jpg)
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 function
In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a function (mathematics), mathematical function that mathematical model, theoretically models the system's output for ...
s called
Response Amplitude Operators (RAO). For a floating structure they will need to be calculated for all six
motions
In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and me ...
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 (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 sick
Motion sickness occurs due to a difference between actual and expected motion. Symptoms commonly include nausea, vomiting, cold sweat, headache, dizziness, tiredness, loss of appetite, and increased salivation. Complications may rarely include de ...
ness 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
[O’Hanlon and Mc Cauley, 1974] (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.
Factors
A number of factors affect seakeeping or how correctly the ship responds.
*Size: A larger ship will generally have lower motions than a smaller one. This is because the relative size of the waves is lower.
*
Displacement
Displacement may refer to:
Physical sciences
Mathematics and Physics
* Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...
: A heavier ship will generally have lower motions than a lighter one. Given that the wave energy is the same for each vessel and provides the exciting force, the one with the greater mass will have the lower accelerations.
*
Stability
Stability may refer to:
Mathematics
*Stability theory, the study of the stability of solutions to differential equations and dynamical systems
** Asymptotic stability
** Linear stability
** Lyapunov stability
** Orbital stability
** Structural sta ...
: A stable ship will tend to follow the wave profile more closely than a less stable one. This means that a more stable ship will generally have higher accelerations but lower amplitudes of motion.
*
Freeboard
In sailing and boating, a vessel's freeboard
is the distance from the waterline to the upper deck level, measured at the lowest point of sheer where water can enter the boat or ship. In commercial vessels, the latter criterion measured relativ ...
: The greater a vessel's freeboard the less likely it is to immerse the deck. Deck immersion is often a seakeeping criterion, as it affects mission capability in a number of ships.
*
Human factor: Often the most critical factors in seakeeping, especially in small vessels, are the experience and skills of the crew in extreme situations. Allied to these are avoiding seasickness, getting sufficient sleep, food and drink and staying injury-free for the duration of an extreme event.
*
Sea state
In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, ...
,
wave spectrum
In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction o ...
and relative direction. There may be undesirable resonances at some frequencies. Pitch and roll may be excited to dangerous amplitudes if the natural frequency of the hull coincides with incidence of waves and there is insufficient damping. Ship heading and speed have a large influence on the frequency of incidence of waves.
*
Damping
Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples in ...
of pitch and roll counteract resonant motions. Sometimes a change to the moment of inertia by shifting ballast can detune resonance.
*
Directional stability
Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation ...
, or yaw stability, which affects course-keeping, particularly in bow and quartering seas, and in overtaking seas, when flow velocity past the rudder may drop significantly, or become negative, while bow immersion may simultaneously increase, moving the centre of lateral area forward.
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.
See also
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Related subjects
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References
Bibliography
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Further reading
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External links
Factors of seaworthiness
{{Authority control
Nautical terminology
Transport law
Water transport