Seakeeping

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.

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.

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, 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 and the accelerations experienced, and include comfort criteria such as noise, vibration and sea sickness, performance based values such as involuntary speed reduction, and observable phenomena such as bow immersion. 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. \eta(x,t)=a\cos(kx-\omega t) where a is the wave amplitude, k is the wavenumber, and \omega is the angular frequency. In finite water depth h , these quantities satisfy the dispersion relation \omega^2 = gk\tanh(kh) The vessel response is commonly represented by six generalized coordinates corresponding to surge, sway, heave, roll, pitch and yaw. In the frequency domain, the linear equations of motion can be written in matrix form as \sum_{\ell=1}^{6}\left[-\omega^2\left(M_{j\ell}+A_{j\ell}(\omega)\right)+i\omega B_{j\ell}(\omega)+C_{j\ell}\right]\hat{\zeta}_{\ell}=\hat{F}_{Ej},\qquad j=1,\ldots,6 Here M is the generalized mass matrix, A is the added-mass matrix, B is the radiation-damping matrix, C is the hydrostatic restoring matrix, and \hat{F}_{E} is the complex wave excitation force or moment. The corresponding RAO is the complex ratio between a vessel motion amplitude and the incident wave amplitude, and is used to estimate motions, accelerations and loads in specified sea states.

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: 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: 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: 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, wave spectrum 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 of pitch and roll counteract resonant motions. Sometimes a change to the moment of inertia by shifting ballast can detune resonance. Directional stability, 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

Related subjects

Bibliography

Further reading

External links

Factors of seaworthiness

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