//SYS21///INTEGRA/BST/VOL2/REVISES 31-7-2001/BSTC12.3D ± 533 ± [473±538/66] 30.7.2001 3:51PM
Seakeeping 533 value in assessing prediction methods. The longer the time period the better the measure of a ship's `average' performance. Short duration trials are expensive and the opportunity is usually taken to record hull strains, motions, shaft torque and shaft thrust at the same time. Increasing attention is being paid to the performance of the people on board. The sea state itself must be recorded and this is usually by means of a wave recording buoy. For the second type of trial a simpler statistical motion recorder is used, often restricted to measurement of vertical acceleration. No wave measurements are made but sea states are observed. Statistical strain gauges may also be ®tted. Satellites can be used to measure the wave system in which the trial ship is operating and can help record the ship's path. Although various methods have been proposed for measuring a multidirectional wave system it is a very di cult task. Good correlation has been achieved between calculated and measured sea loadings in some trials by applying the cosine squared spreading function proposed by the ITTC (see Chapter 9) to a spectrum based on buoy measurements. In the earliest trials the waves were recorded by a shipborne wave recorder but nowadays a freely ¯oating buoy is used. Signals are transmitted to the trials ship over a radio link or recorded in the buoy for recovering at the end of the trial. Vertical motions of the buoy are recorded by an accelerometer and movement of the wave surface relative to the buoy by resistive probes. Roll, pitch and azimuth sensors monitor the attitude of the buoy. For studying complex wave systems in detail several buoys may be used. A typical sequence for a ship motion trial is to: (a) carry out measured mile runs at the start of the voyage to establish the ship's smooth water performance and to calibrate the log; (b) carry out service trials during passage to record sample ship motions and propulsive data under normal service conditions; (c) launch the recording buoy, record conditions and recover buoy, when conditions are considered suitable, i.e. waves appear to be su ciently longcrested; (d ) carry out a manoeuvre of the type shown in Fig. 12.35 recording motions and waves for each leg. Figures denote time in minutes spent on each leg. The accuracy of the analysis depends upon the number of oscillations recorded. For this reason, the legs running with the seas are longer than those with ship running head into the waves. The overall time on the manoeuvre has to be balanced against the possibility of the sea state changing during the trial. The two sets of buoy records and a comparison of the results from the initial and ®nal legs provides a guide to the stability of the trial conditions. The remaining steps of the sequence are: (e) (f) (g) (h)
launch buoy for second recording of waves; repeat (c), (d ) and (e) as conditions permit; carry out service trials on way back to port; carry out measured mile runs on return.