In the field of aeroelasticity, flutter is a well known instability phenomenon. Flutter is a synchronized vibration which takes place in a flexible structure moving through a fluid medium. It occurs when two regular, rhythmic motions coincide in such a way that one feeds the other, drawing additional energy from surrounding flow. A classic case of wing flutter might combine wing bending with either wing twisting.

This article explores the flutter phenomenon in water. An important difference from the flutter phenomenon in air is the fact that the flexible structure is evolving in heavy fluid; this implies in particular added mass effects and important fluid damping.

Flutter appeared for the first time on racing yacht keels with composite fins, so in water, in 2004:

• On the IMOCA 60 feet boat POUJOULAT-ARMORLUX of Bernard STAMM during the transatlantic race ‘The Transat’: he lost his keel and capsized.

• On the IMOCA 60 feet boat SILL Rolland JOURDAIN: the keel and the boat were saved.

Following these problems — particularly following the loss of the keel of Bernard STAMM sailboat, accident that could have dramatic consequences for the skipper — HDS company focused on the phenomenon.

Flutter has occurred only for canting keels with composite fins on IMOCA 60 feet and Volvo 70 feet racing yacht. The main questions asked are “Why are composite keels susceptible to flutter, and is it possible to predict and prevent this behaviour?”, then “Can a fair indication of the flutter critical speed of the keel be given at low cost?”.

This presentation will introduce the strategy of HDS faced to the problem and the analytical and numerical methods implemented to estimate the flutter critical speed. Our model is based on a truncated modal basis for the most energetic modes which are generally, for a bulb keel, the lateral bending predominant mode and the torsion predominant mode. One of our requirements was to make a simple model in order to integrate the calculation of the flutter critical speed in the first design loops of a composite or steel keel.

This model has worked well for the two cases of flutter appeared on IMOCA sailboat keels. Besides, to verify the quality of the model and to complete our analysis of flutter phenomenon on racing yacht keels, a 3 dimensional multi-physics simulation has been developed using the software ADINA.

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