Dynamic knockdown of canting keel yachts in breaking waves

Yacht canting keel configurations have been developed to maximise the available righting moment by rotating their keel bulb to windward. Regulatory authorities have been required to establish rules covering the design and operation of such systems, however significant dynamic investigations into their behaviour have not been performed. This paper presents results from a theoretical and experimental investigation into the dynamic stability of canting keel sailing yachts when experiencing a knockdown by large breaking waves. Towing tank experiments were conducted on a yacht model with a fixed keel and two canted keel configurations, beam-on to large solitary breaking waves. The motion of the model during the tests was recorded by four video cameras and analysed using photogrammetry. A comparison of the hydrostatic predictions and experimental results indicate that the traditional GZ approach of assessing the energy required to heel may not be appropriate for yachts with canting keels. In addition a correlation of the experimental results with the ISO stability index highlights that this index may be unsuitable for categorising yachts with canting keels with respect to their ability to recover after a knockdown event.