The figure compares an estimate from roll decay tests (extrapolated to a low roll amplitude) with the FATIMA prediction. The results show that the effect of forward speed on the roll damping accounts for a substantial part of the total damping. This effect is particularly important for conventional hulls with a high centre of gravity, such as cruise and containerships in full load.

added resistance and the wave amplitude often does not materialise in practice; for several ship types the values in lower waves (which are extremely hard to measure) show a relative increase compared to the values in higher waves.

The figure below shows that there was good agreement obtained for a rather hypothetical case - a Wigley hull form1

in head waves. The

effects of wave amplitude in the measured results (25% at the peak) illustrate the relative decrease in higher waves.

Roll damping At modest roll amplitudes, the roll damping of ships in transit is domi- nated by wave-making and ‘lift’ effects.

1 Journee, J.M.J., Experiments and calculations on 4 Wigley hull forms in head waves, Report 0909, May 1992, Delft University of Technology

report 19

Running the code for many normal hull forms has shown that it yields good results in low waves at low and moderate speeds over a broad range of ship types. At higher speeds, such as those for naval hull forms, ferries and cruise ships the linear solution seems to overestimate the relative wave elevation and consequently, the added resistance.

The fact that FATIMA offers a more complete description of the wave systems surrounding a ship in waves offers an improved prediction of details like the relative wave elevation, the related added resistance and the effect of forward speed on roll damping. Several problems with potential flow predictions that were in the past attributed to ‘non-linear’ or ‘viscous’ effects seem to be covered quite well by means of this method. Because of these advantages, the code is certain to play an even broader role in MARIN’s services in the future.

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