to matching the tank results.
To put this into perspective: induced drag is proportional to the square of the effective draft. This means that a 10% error in effective draft results in a 19% error in induced drag (0.9 x 0.9 = 0.81). Sailing upwind, particularly in light winds, induced drag is a very significant portion of the total drag.
Discussion
The study described in this report is the most comprehensive ever undertaken for publication in the public domain, and has met the goals that were set. The following broad conclusions may be drawn: A body of physical test data relating to a defined geometry is now published and available for validation of other data. Commercial CFD codes may be used to confidently predict the variation of the forces on a sailing yacht hull as speed, heel and leeway change. These studies do not need prohibitively large mesh density to achieve valid results. Less computationally heavy codes, for example FlowLogic, can produce data to capture the typical global behaviour of a sailing yacht hull. Yacht handicapping organisations can utilise correctly configured CFD studies to generate data for a wide variety of yacht hull shapes. Because there can be inconsistencies between tank results and CFD, as shown in this study, the burden on the handicapper is to merge the two sources of hydrodynamic predictions. For
‘The effective span discrepancies [tank vs CFD] are both troubling and interesting…’
example, it may be prudent to take the trends from CFD and use them to adjust the algorithms derived from tank testing.
Future applications in yacht handicapping
The purpose of Phase 2 of Wide and Light is to take what was learnt in Phase 1 and make use of it in a way that would support the handicapping of sailing yachts as well as those interested in the science of sailing yacht performance. Specifically, the plan is to take one or two of the promising CFD programs of Phase 1, provide them with the geometry of a small fleet of designs and let them evaluate those designs and the combinations of speed/heel/leeway that are of interest to handicap rulemakers. The fleet of boats would represent real- istic variations from a baseline design in the critical parameters that drive perfor- mance: displacement and beam for a fixed length are an obvious choice of variations. This programme has a precursor: the nine-model tank test programme con- ducted at NRC CNRC in Canada. That programme has been immensely useful in
providing the differences in hydrodynamics for a range of beam and displacement vari- ations that bracketed the fleet of boats racing worldwide in the 1990s. But there has been rapid growth in the number of new boats that have wider beam and lighter displacement since that series and these new boats show up at every major big boat regatta. If handicap rules are to treat them fairly, and to prop- erly rate them against the existing fleet of heavier and narrower designs, it is impera- tive that the developers of those rules have a database of their hydrodynamics. Neither tank testing nor CFD analysis at this stage model the complete ‘situation’ of sailing a boat on the ocean with waves and variable wind structure, and certainly do not include the interactions of boats racing each other. But both are very useful tools in guiding design, predicting perfor- mance and handicapping boats for racing.
SYRF
The Sailing Yacht Research Foundation (SYRF) was founded to help support the science of sailboat performance ‘for the benefit of all racing sailors’. Through a dis- tinguished and active board membership chaired by Steve Benjamin, and an advi- sory council of experts, SYRF has an ongoing and active role in soliciting and reviewing proposals for new research, as well as seeking the financial support needed from those parties interested in the growth and development of the sport. Jim Teeters and Dobbs Davis
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