Bob Dill, Burlington VT
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THE 16th CHESAPEAKE SAILING YACHT SYMPOSIUM ANNAPOLIS, MARYLAND, MARCH 2003 Sailing Yacht Design for Maximum Speed Bob Dill, Burlington VT ABSTRACT appears that it is, currently, the fastest sailing yacht on any surface. Finally, the paper provides perspective on a This paper reviews the performance of ice and land variety of aspects of speed sailing on dirt and ice. yachts and the factors that determine their top speeds. The data clarify a century of misunderstanding about top iceboat speeds and serve as background for discussing the design and performance of the author’s land yacht, the NOTATION Iron Duck. This yacht was built as an amateur project and holds the a Angle of attack of the wing or sail world record for speed in a land yacht at 116.7 mph. It β Angle between the apparent wind and the yacht travel direction values, which made the prospect of going faster than 143 γ The angle between the true wind and the daunting. Efforts by iceboating authors Calhoun Smith1 yacht’s travel and Jack Andersen2 to provide a more realistic perspective Cd Coefficient of drag on top speeds has done little to dampen the acceptance of Cross Flow Drag the prevailing claims. The result of all this was that by Drag associated with sailing when β is 1990 no one had built a yacht that was truly optimized for higher than a few degrees. top speed. Drag: The forces operating opposite to the direction of the yacht’s travel. A LITTLE HISTORY Dirt Typically a dry lake surface suitable for sailing on wheels. Land yachts in particular have been around for a Drive The force from the sail or wing acting in long time. The remains of a sailing chariot were found in the direction of the yacht’s travel. the tomb of Egyptian king Amenenhat III (1844-1794 Effective Frontal Area BC)3. Around the turn of the century the sport gained The frontal area of the yacht, other than interest on beaches in Europe. Land yacht racing became the wing, normalized to a Cd of 1.0. organized in the United States in 19724. The open land FISLY Federation Internationale De Sand Et yacht classes essentially have no constraints on design. Land Yachting This has resulted in a particularly diverse range of NALSA North American Land Sailing solutions to getting around the racecourse. There has Association been considerable experimentation with wings and very Pilot Land sailor speak for skipper wide masts as well as a wide range of construction Speed Yacht A yacht designed primarily for top speed materials. Va Apparent wind velocity Vb Yacht velocity Ice yachting started in Holland in the mid 1600’s. VMG Velocity made good, up or down wind By the late 1800’s in North America, boats as long as 60 Vt True wind velocity feet with up to 1000 square feet of sail were sailing on the Wing Used instead of a soft sail on some Hudson River and elsewhere in the ice belt that runs yachts across the northern part of the US and southern Canada. Units Mph, ft, lbs As was typical of sailboats of the day, the yachts did not benefit from a proper understanding of aerodynamic drag. INTRODUCTION The sails were lateen or gaff rigged and the steering runner was at the stern. This configuration allowed the Ice and land yachts occupy one end of the sailing pitching moment of the sail to un-weight the steering yacht performance spectrum. Their very low runner as the yacht powered up. The angular momentum ‘hydrodynamic’ or rolling resistance while resisting the associated with the rear steering action further loads the considerable lateral force these yachts develop allows steering runner. This same action can be seen when them to sail at speeds well over twice that of the fastest driving a car backwards with some speed. The result was watercraft. Particularly on ice, their top speeds have been often a spin at near top speed called a ‘flicker.’ more a matter of speculation than science. There is a lot of folklore from this period. These While many sailors of these yachts have had ideas yachts were said to be the fastest means of conveyance of about setting speed records, most have focused their the day. Without an easy way to accurately measure considerable ingenuity on being fast on the racecourse. speed, exuberant speculation dominated speed Racing has a very objective measure of performance: discussions. Claims built on previous claims. One whoever gets to the finish line first is fastest. Racing hundred-plus mph was claimed by the late 1800’s. In allows sailing in a wide range of conditions. There is a 1908 the unremarkable lateen rigged ‘Carel’ claimed 140 reasonable ratio of sailing time to the amount of time a mph. A 1938 claim of 143 mph for the rear steering yacht sailor has to spend preparing to sail (building, ‘Debutant’ in the “Guinness Book of World Records” is maintaining, transporting and waiting for the right sailing the most well known. conditions). In 1931 Joys Brothers Sail makers (Milwaukee) Speed sailing, on the other hand, has a number of made a small front steering yacht that did not have the drawbacks. It has a particularly low ratio of sailing time flicker problem. With the steering runner in front, the to preparation time. It is common to wait weeks for the pitching moment increased steering control as the yacht right sailing conditions. Making accurate speed built speed. Over the next two years Wisconsin sailor measurements is a much more involved process than Walter Beauvois, working with Joys, developed a 13-foot setting two marks to race around. Finally, widely long, 9-foot wide, front steering yacht with a single 75 accepted speed claims on ice reached 143 mph in 1938. square feet sail. It was a faster race boat than all the much Any scientifically sound measurements gave much lower larger rear steering yachts. This design has evolved into ever-faster racing yachts on both ice and dirt. These yachts almost always sail in a ‘close hauled’ OVERVIEW OF MODERN RACING YACHTS condition, as ß (the angle between the apparent wind and the yacht travel direction) is rarely more than 20 degrees. The DN is the most popular iceboat in the world. It Iceboats race only on windward-leeward courses, is 8 feet wide, 12 feet long with a 67 square foot sail on a allowing them to use their closeness to the true wind to 16 foot mast. It weighs about 150 lb. It started as a home depower quickly in windy conditions (by heading up or building project in 1936 and has evolved into a well- down a few degrees, as appropriate). This allows them to balanced racing machine. It is surprisingly carry their largest sails in all wind conditions. undiscriminating about who is successful: light and heavy, young and old, all have had their day as champion Land yacht racecourses often have long reaching of the class. legs and the yachts have evolved a variety of ways to allow them to sail fast on beam reaches in high winds. These include the use of small sails for windy days, mainsheet travelers, wide masts with mast rotation DN controls, and the use of flapped wings instead of sails. A TOP SPEEDS OF RACING YACHTS Until the advent of the inexpensive GPS, iceboats have not had an easy way to measure their speeds (unlike land yachts that can easily incorporate a speedometer on a wheel). As previously mentioned, there has been no shortage of speculation on the matter of iceboat top speeds. To get a better understanding of true performance, in 1992 I bought a good radar gun and started taking it to iceboat regattas. The following graph is from many Fig 1 Side view of a Skeeter and DN iceboat hundreds of measurements of DNs. The Skeeter is the fastest iceboat. The class is based largely on a single rule: the sail triangle can be no larger DN Boat Speed = f(Wind Speed) than 75 square feet. This does not count the mast, boom, and roach. The maximum propulsive area is about 140 70 square feet. They are typically around 30 feet long, 25 60 wide and have a 27-foot mast. Weights are 500-600 lb. eed 50 There are presently two popular styles of Skeeters. The 40 ph) at Sp 30 ‘Bubble Boat’ design is shown in Fig. 1 with the skipper (m under a canopy in front of the mast. The design was 20 developed by Tom Nichols and Dan Clapp and is popular Max Bo 10 in the East. In the Midwest, the skipper usually sits 0 behind the boom in what is known as a ‘Rumble Seat’ 0102030 configuration. In most other iceboats the skipper sits or Wind Speed (mph) lies under the boom. Fig 3 GPS Track of a two lap DN race Ice yachts on good ice will not start in winds less than 2.5 mph. In very light winds they sail fastest up wind in a tight reach. From 4 to about 12 mph they sail Downwind well on any point of sail. Above 12 they are fully Speed: powered. By 20 mph they are overpowered and are 40 mph relegated to pointing and broad reaching to control the yacht. The flatness of the curve in winds above about 18 True Wind Upwind mph was somewhat of a surprise. Sailing a boat in 30 Speed: speed: feels much faster than in 18 as a result of the greater 10 mph 30 mph difficulty of controlling the yacht.