Basic elements of a performance system are:
- Propeller shaft including shaft power measurement
- Hull including wake and thrust deduction varying by speed, draught and trim
- Ship’s resistance including varying speed, draught and trim
- Wind resistance
- Wave resistance
If a fleet is using the performance system, the evaluation of the performance index has to include some sort of automation i.e. the system will generate an index and in cases where the index is off the expected value or exceeds some user defined boundaries, an alert will generated which will lead to a further investigation by the performance evaluation dept. To avoid ―false alarms the data fed to the system have to be free of noise and outliers generating off limit indexes.
Drift in timing of the signals also have to be avoided in order to have all parameters logged at the same timestamp. It is the intention to use the data to give an overview of the ship’s operational profile during the logging period. The data will be subject to a time series analysis in order to give an input to the performance system.
By implementing data from auto logging systems on board and by using a proper filtering method the index reliability is intended to be improved. The filtering methods are based on identifying data periods in which the behaviour of the ship is captured by the model. Constraints in the model are
linked to the elements describing the resistance during service and also linked to lack of ability to describe the dynamics of the ship.
By establishing an initial filter used on the logged speed on board the ship it is the intention to extend the parameters used to filter the data in order to achieve a reliable index. Improvement of the model leads to the range of data being excluded by the filtering can be reduced. This will lead to a larger amount of data to be used to evaluate the performance index reliability. A model can be improved by introducing models for rudder/stabiliser movements, variance in sea water temperature and sailing in shallow waters.
Various resistance models can be analysed and compared to find the most suitable model to describe the performance of the ship. The wave height is estimated from the wind force. By including the data logged by the motion sensor placed in the bow section it is the intention to use this data to estimate the wave height and thus get a more accurate estimate of the sea state.
The performance index can be given by the average hull roughness, the increase in power consumption and the speed loss compared to model values. Another approach to estimate performance of a ship is to use the propeller as performance index. The index is based upon characteristics of the propeller and the measured shaft torque and logged speed on board.