Paper describes the development of plastic deformation on the lead keel of a yacht when the keel hits a rock. The size and depth of observed indentations depend on the speed of the yacht at the time of impact. Once the plastic properties of the lead have been determined by compression tests in laboratory, this inverse solid mechanics problem may be solved so that the speed can be calculated. Contact force at impact may become very high and cause secondary damage to the keel and hull. Such structural damage is often hidden and the method described may therefore become useful in estimating the seriousness of the damage suffered. The marks on the surface of the lead serve as a fingerprint which reveals the conditions of impact.
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