The linear seakeeping code FATIMA is being

seakeeping assessments O

riginally developed in 1999 at the Delft University of Technology by Dr. Tim Bunnik, the linear sea-

used more and more at MARIN. Here, we explain why this code is so useful.

keeping code FATIMA is proving increasingly relevant in MARIN’s seakeeping assessments. Just like strip theory and more ‘conventional’ 3D panel codes using zero-speed Green’s functions, the Rankine source code FATIMA is based on linear theory, which does not account for motion induced geometrical changes in the immersed volume and other non-linear effects. The most important differences with the above mentioned methods are, amongst others, the account for aftward dispersion of the reflected and radiated waves (by means of the Rankine sources in the free surface) and the lineari- sation with respect to the actual mean hull immersion. This is in contrast to the zero-speed calm water surface used in conventional methods. The actual steady flow, estimated by means of the non-linear potential flow code RAPID, is used to obtain a more detailed account of the effects of forward speed on the wave and motion induced pressures, the relative wave elevation and the restoring terms in heave, roll and pitch.

Reint Dalling 18 report

One of the practical consequences of the above differences is an improvement in the

prediction of the added resistance, which is strongly affected by the relatively high relative wave elevation in the forward part of the waterline. Another consequence is an adequate prediction of the effect of forward speed on the wave-making damping com- ponent in roll, which improves the prediction of roll motions for ships in transit.

Improved resistance predictions Being based on potential flow theory the method does not cover the effects of flow separation around the bilges and in the stern area (giving rise to the ‘lift’ damping components), the effects of wave breaking in the diverging flow at the bow and the breaking of steep incident waves ahead of a blunt bow. The consequences of these limitations vary between ship types.

Validation of the method to predict added resistance of ships in waves proved less straightforward than anticipated. The reasons appear to be threefold. First of all the calcu- lation of one of the computed components in the added resistance seems not entirely robust for some ship types. Secondly, the accuracy of experiments is an issue. But the largest problem seems to be that the theoretical quadratic relation between the

FATIMA takes on more prominent role in

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