Above: a model of ‘wall stress’ or surface friction


seen in one horizontal slice taken from a RANS analysis of an A-sail, showing


areas of flow separation. Left: here’s one that Tom Whidden did earlier… Perfect sail twist, as


demonstrated by the 1987 America’s Cup winner


difficult to follow at one end and too simplistic and therefore potentially inaccurate on the other. Tom and Michael worked hard to marry this controversial topic – for example, did you know the flow through the ‘slot’ of a headsail and main is far slower than the onset flow – with the practicalities of trimming. And the issue of why sailmakers add leech twist to a sail is not met with


the normal answer: ‘because it works’. The authors logically explain the aero dynamics in comprehensive but surprisingly clear detail. Throughout this book theory and practice flow back and forth, leaving the reader with a knowledge foundation of exactly why correctly trimmed sails work best. Over the years many excellent books have been written on sails


and aerodynamics. Perhaps Manfred Curry was the first to attempt to explain in 1948. CA Marchaj wrote his treatise in 1964, which is still seen on the shelves of many sail and yacht designers – and to this day referred to as ‘the bible’ on sails. The Art and Science of Sails Revised Edition in no way attempts to usurp the achievements of these groundbreaking authors. However, what it does do is make every attempt to render the complication of sails an approachable topic that is the key to successfully under- standing how and why a sail performs. And, as you’d expect, inter woven with a review of today’s state-of-the-art is an accessible update on the still fast increasing role of computers in the analysis of sail design and performance. www.seapointbooks.com


q SEAHORSE 47


KOS


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