Research on lift-carry-over in hull-keel-centre board configurations, to improve the performance prediction of sailing yachts at Dykstra Naval Architects

Abstract

Dykstra Naval Architects are regularly designing large classic sailing yachts or motor-sailors, of which the draft is an important design restriction.The sailing performance is increased by adding lift-generating appendages,without increasing the draft. An example is a retractable centre board, havinga big influence on the sailing properties. Predicting the performance of the centre board contribution in a hull-keel-centre board configuration is the subject of this research. When predicting the performance of large sailing yachts, Dykstra wants to keep the ability to superpose an appendage to the data of a hull-keel configuration obtained from towing tank experiments or CFD simulations. This means that a method must be developed to estimate the contribution of the centre board, in terms of side force, resistance and centre of effort. Both towing tank experiments and CFD simulations are conducted for this research. The Maltese Falcon is used as the 'case ship'. A towing tank model of the Maltese Falcon was already made and tested at the TU delft in 2002. This model is again subjected to towing tank experiments, with a new keel and two new centre boards, resulting in 9 different hull-keel-centre board configurations. The main focus was on the towing tank experiments, executed in the Delft Hydromechanics Laboratory. CFD simulations were done to validate the results of the towing tank experiments and to gain visual insight in the flow around the vessel. The lift-carry-over on the keel and hull above the centre board can clearly be seen, as well as the influence of the centre board on the circulation in the flow around the underwater body of the yacht. After post-processing all experimental data, the results of the towing tank experiments are used to develop formulations to predict the performance contribution of the centre board. The measured lift of the centre board contribution was roughly a factor 2 higher than the centre would generate according to Wicker & Fehlner theory. Additionally, it was found that the lift-carry-over does not only have a positive effect on the generated side force, but also on the resistance. Furthermore, it was found that heeling the Maltese Falcon model by 15 degrees, yields the same magnitude oflift-carry-over as for the upright conditions. This resulted in the conclusion that heeling the yacht has no influence on the lift-carry-over from centre board to keel-hull. The new prediction methods, derived from the towing tank experiment data, are validated on YACHT1 and Adela. These are existing yachts with hull-keel-centre board configurations, but both very different. This enabled an interesting examination on the influence of certain aspects of the configuration on the performance of the centre board contribution. All in all, it was found that the predicted centre board contribution corresponded really well to the measured data of YACHT1 and Adela. This provides enough trust to implement the new centre board performance prediction methods in the Dykstra performance prediction tool. Every new design cycle of a yacht with hull-keel-centre board configuration will serve as a validation of the derived performance prediction methods.