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Onboard System

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Sea State

The knowledge of the sea state encountered during navigation is of particular interest in relation to: power performance of the ship with particular reference to consumption, induced structural stresses phenomena to prevent fatigue phenomena and conditions of comfort on board, topic of primary importance in case of cruise ships.

The system for determining the state of the sea encountered during navigation is based on the measurement of the angular velocity of the ship pitching and rolling that can be provided by a two axis gyro.

The fact of having chosen this type of measurement is very advantageous not only for the limited dimensions of the instrumentation dedicated to it, but also because the velocity measurements are independent of the angular position of the measuring instrument which can therefore be installed in any place that is easily accessible and at the same time not strategic for the activities necessary for the life on board. Furthermore, the two motions chosen (pitch and roll) are significantly sensitive to the variation of the angle of encounter and have their maximum phase difference of 90 degrees in terms of this variable that is one of the quantities that the system is able to determine. The measures of the two angular velocities are recorded at an appropriate sampling rate so that they can also determine sea states characterized by short wavelengths, and from the forward quadrant of the vessel even when it sails at high speed.

The method of determining the sea state, which is identified by four parameters (significant height, peak period, source angle, exponent of spreading), is based on finding the minimum deviation between the power spectra of the two measurements and those obtained theoretically assuming certain spectral distributions of wave motion. Of course the theoretical estimation requires knowledge of the response operators (RAO) of the two measured variables, which are known as to be function of the encounter angle between the direction of propagation of the wave motion and the bow ship,and ship speed and load condition.

All data determined by the system can be displayed on the panel or should be made available for monitoring systems such as PEM++ (Propulsion Efficiency Monitor) and Fleet Survey++.