HULL SPEED AN INSIGHT INTO THE PRINCIPAL BARRIER TO HIGH SPEED UNDER SAIL ALAN SKINNER HULL SPEED AN INSIGHT INTO THE PRINCIPAL BARRIER TO HIGH SPEED UNDER SAIL ALAN SKINNER PREFACE Sailboat design is an art and, as for all art, success demands not only the development of natural talent but also an intimate knowledge of the subject. For sailboat design, the accumulation of that knowledge has been a lengthy, evolutionary process, beginning in prehistory and achieving a significant level of advancement by the time of man's earliest records. The proas and double canoes of Oceania, the junks of Asia and the Viking ships of Europe typify the extraordinary refinement and diversification of evolutionary design evident throughout the history of the maritime world. Although aided by increasingly sophisticated technology, modern sailboat design is essentially an extension of that evolutionary process. Today, the theory of all boat design is regarded as an engineering science, whereby the behaviour of a vessel is explained by the application of mathematics. However, some factors that determine the performance of a boat, especially under sail, are still seemingly impossible to resolve other than by empirical methods. Consequently, the theory of certain aspects of sailboat design remains limited to broad principles, the interpretations of which are dependent on the adeptness of the designer. Shaping a hull to reduce resistance is the most fundamental, most studied but generally the least understood feature of design for all types of vessels. Surface waves caused by the forward motion of a hull represent a major component of a vessel‘s total resistance. But, despite being the most visible form of resistance and the principal barrier to high speed under sail, the causes and effects of wave-making are commonly misunderstood. Although detailed analyses of wave-making resistance are to be found in advanced texts on hydrodynamics and undoubtedly have their elaborate mathematical solutions in use in today's proliferation of hull design software, very little of that technical knowledge seems to have filtered through to the general world of boating by way of sensible, logical explanation. Hull Speed is not intended as an expert analysis of the resistance caused by the formation of waves but is the product of one layman‘s endeavour to gain an insight into what has always been a most challenging aspect of sailboat design, shaping a hull to reduce wave-making resistance. Presented in two parts, the first, From Theory to Evolution, is introductory, an historical prelude to A Component Waveform Theory, tendered as a fresh, unique and relatively uncomplicated interpretation of wave-making to demonstrate the effect that wave-making has on the performance of a vessel, how wave-making creates a barrier to high speed and, finally, how the resistance due to wave-making might be minimised. At the outset, I readily admit to being unqualified to attempt the analysis of a topic of such complexity, my interest in sailboat design simply stems from an inexplicable lifelong obsession with small boats and sailing. Inevitably, the use of a novel ‗component waveform theory‘ to simplify the technicalities of wave-making is likely to be dismissed as being a very naive approach to an exceptionally complex mathematical problem. It is, but the concept, no matter how elementary or flawed, does help to clarify for laymen an otherwise extremely vague area of hull design. Essentially, Hull Speed is the outcome of a project that began in the early 1970s as an attempt, by me, to design an NS14, an Australian small sailing dinghy capable of efficient planing. Being a novice to sailboat design, my original intention was merely to gain an understanding of the hull design process but almost immediately, because of an apparent lack of useful theoretical design information available, the exercise became, instead, a personal quest to derive a mathematical method for determining the optimum ‗curve of areas‘ for the immersed sections of a hull travelling at any speed, including speeds above ‗hull speed‘. To my knowledge, which remains very wanting in these matters, no one had previously achieved such an objective. Adding to my uncertainty at the time, the science of planing hulls was depicted in the boating publications of the day as almost a distinct discipline, somewhat detached from the traditional approach to the design of displacement hulls. Although that point of view had always seemed strange to me, young and practically ignorant of the history and technicalities of design, there appeared to be unanimous agreement amongst experienced designers of a virtual collapse of traditional design theories once planing began, an opinion that still seems to hold sway today, some four decades later. Nevertheless, having already witnessed first-hand the smooth transition of NS14 sailing dinghies from displacement to planing speeds and convinced that the supposed discontinuity in design philosophy that occurred at ‗hull speed‘ was implausible and seemed to lack a solid scientific basis, I persisted, developing my own lines of thought. During the next couple of years, while struggling to untangle the complexity of wave-making, I inadvertently developed the Component Waveform Theory, a logical approach to minimising the wave-making resistance of a hull at any practical speed, an approach that is straightforward and understandable, in the spirit of the early ‗amateur‘ theorists such as John Scott Russell and Colin Archer, whose theories on how to minimise wave-making resistance are included in the prelude. Subsequent investigation has led to my realisation that all of the know-how necessary to develop a component waveform theory was already common knowledge to designers when boats capable of planing were in the early stage of development, more than one hundred years ago, in the first decade of the 20th century. However, since independently developing the component waveform concept intuitively almost four decades ago, I have yet to encounter a more persuasive account of ‗hull speed‘ or of wave-making resistance generally in any boating publication, old or new. My ongoing exploration of the past and present ‗state-of-the-art‘ of boat design has, if anything, increased my confidence in the Component Waveform Theory which, for laymen at least, has the potential, I believe, to rationalize the conventional approach to sailboat design, hence the compulsion to subject the theory to criticism in the public arena. Alan Skinner Saratoga NSW Australia September, 2014 INDEX FROM THEORY TO EVOLUTION The Age of Sail A New Beginning The Trochoidal Wave Theory A Waveline Theory The Wake A Waveform Theory A Return to Evolution A COMPONENT WAVEFORM THEORY The Component Wave Hull Speed Below Hull Speed Semi-planing and Planing The Theory Applied Fact or Fiction? APPENDIX A Design for a Dinghy FROM THEORY TO EVOLUTION THE AGE OF SAIL A sailboat is unique. Travelling at the interface of two media, a sailboat is supported by the water, continually pitching, rolling, yawing, surging, heaving and heeling on that fluid‘s unpredictable surface while gaining propulsion from the air above, equally unpredictable. Attempts to study sailboat design, even at an elementary level, reveal a complex and indefinite topic. Design possibilities are infinite, exemplified in a broad sense by the multiplicity of traditional and commercial sailing craft that have evolved worldwide over countless generations of designers, builders and sailors. The modern sailboat, having its infancy in the decline of commercial sail during the nineteenth century, is a product of that evolutionary process. The origins of sail precede recorded history, possibly by many thousands of years. Archaeological evidence suggests that as early as the 4th millennium BC, commercial sailing vessels, as distinct from more ancient traditional craft, were in use on the Nile River in Egypt. Similar but unrecorded developments were undoubtedly occurring independently throughout Asia. From that period in the late Stone Age, until the Industrial Revolution more than five thousand years later, the sailing ship evolved at the forefront of man‘s technological achievements. Influenced by the demands of trade, exploration and warfare, development was generally cautious and deliberate, interspersed with the occasional revolutionary innovation and constantly reflecting the ingenuity of the societies involved. Constructed of bundles of reeds, lashed together and bent to form the shape of the hull, the first Egyptian ships were rowed with oars and steered with oars at the stern, a square sail being used for running downwind. These vessels were a development of smaller traditional craft that had been in use by Neolithic man on the Nile for thousands of years. Such had been the impact of reed boats on society that apart from weapons, they are reputed to be often the only manufactured product depicted in early Egyptian art. Although seemingly primitive, reed construction enabled vessels to be quickly and easily built from readily available materials and by using the simplest of tools. Testimony to the adequacy of reed construction and to the refinement of design possible within a stone-age culture is still to be found in several parts of the world and as remote from Egypt as Lake Titicaca, the world‘s highest navigable waterway, shared by Bolivia and Peru in South America. Located on a high plateau in the Andes Mountains, the area surrounding Lake Titicaca is bleak and treeless. Totora reeds growing along the shoreline have long provided the indigenous population with its only source of local boatbuilding material and, even today, using techniques strikingly similar to those of the ancient Egyptians, the reeds are meticulously fashioned into small traditional boats that are not only superbly functional works of art but have the advantage of being inherently buoyant. Usually poled through the shallows, the boats can be sailed downwind like their Egyptian counterparts, using a sail woven from the reeds.