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. 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. 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.