Measure of Seakeeping Ability
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.
Clearly, 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.
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