Sails for the America's Cup
In the doldrums for 6000 years, sailing technology is gaining a second wind from an unlikely source.
IN ONE OF HIS TEXTS the sailmaker Tom Whidden takes a moment from the details of construction and upkeep to list the reasons why people sail. He mentions the paradoxical balance of the speed rush with the glow of serenity; the thrill of moving inside the body of the wind, of merging with a force that can only be felt, not seen; the historical resonance in a technology that is 6,000 years old. But what Whidden seems to treasure most is knowing that every moment is a corner around which lies a new tangle of forces, a situation never encountered before, either by the sailor or possibly by anyone. For the master sailmaker it is the sense of feeling each moment flower and renew itself that makes sailing so absorbing. Every voyage is a great novel with no last page.
However, it is a fair wind indeed that blows no ill, and another consequence of this fathomless complexity is that it has forced the science and (therefore) the engineering of sailing to develop very slowly. Doing science means controlling all the factors but one, manipulating that one in a controlled fashion, and then reading the results. Controlling all the variables in sailing the pitch of the waves, the character and conditions of the sail fabric, the flexions of the spars, the circulating and intermeshing patterns of turbulence in air and water, to mention a few, is hard enough to manage in tow tanks and wind tunnels, let alone tow tanks in wind tunnels; that level of control is a sheer fantasy out on the ocean itself. This is why sailboat designers still find themselves blindsided from time to time by radical innovations, like Australia II's winged keel.
Horst Richter, professor at the Thayer School of Engineering, is exploring a second path into the field: virtual boats that sail through cyberspace. In theory, in a computer every detail can be controlled and every change read. All the forces and geometries that are invisible in the real world the streamlines and vortexes, the points of flow separation and the regions of turbulence, the pressure differentials and velocity gradients can be made as visible as the boat itself. Once made visible, they become grist to the extraordinary pattern-finding and recognizing powers of the human brain. Again in theory, such illumination should accelerate the science and engineering of sailing prodigiously; the downside is that the computational resources required lie somewhere between enormous and astronomical, and for all the progress our machines have made they might not be up to this yet.
A specialist in thermodynamics and fluid flow, Richter started vectoring in on this subject in 1992, when he was volunteered to teach a course in the science of sailing. After surmounting a few obstacles nothing on the topic had appeared since the 1930s that the librarians at Baker deemed worth collecting ENGS 2, "The Technology of Sailing," was launched and Richter started assigning research projects to students. Examples might be the 1993 study by David Tabors '93 ontheloss of boat lengths while tacking, or the computer program written by Alex Goldenberg '95 that modeled the whole profile of velocity changes during a tack.
Computer modeling has been done on hull architecture for some time, but less often on sails, which are more complicated by orders of magnitude. Apparently the first use of sail simulators was in the early 19705, in a collaboration between sailmaker Lowell North and fluid dynamics specialist Heiner Melder. In 1996 Peter Neiman TH'96 joined this small circle by writing a simulator and comparing the results with a suite of tests run in wind tunnels using real sails. A good overlap in results would have suggested that simulators could be taken seriously as an instrument in sail science, but the information from the tunnel tests was too limited for conclusions to be drawn either way.
In 1997 Richter and Jeffrey Shoreman '97, then captain of the Dartmouth sailing team, acquired better data and ran a more detailed comparison. These results agreed to within five percent, which was interesting enough to attract the attention and then the sponsorship of Young America, one of the consortia building a challenger for the next America's Cup Challenge, in the year 2000.
Young America was wrestling with the chronic problem facing racing sailboat designers: lots of open questions and no good way to test the possibilities. "We have two boats we can race side-by-side," says Duncan Mac Lane, project manager of design technologies for Young America, "but if you run them too close they interfere with each other and if you run them too far apart the sailing conditions are no longer the same." Mac Lane says there are no settled answers as to the right camber and twist (three-dimensional structure) to give sails, or the optimal arrangement of the rigging, or the design of the deck surfaces. In theory the tenth of a knot that would decide where the Cup sits in the opening years of the next century might lurk in any of these, or (more likely) in some combination of them. The consortium hopes to use the Richter project to drive the fog out of these questions.
To do that Richter, yacht designer Olin Stephens, and Thayer graduate students Joe Mclnerney and Solomon Marini have built a quasi-realistic sailing environment in software. (Quasi-in that the sea is flat; contemporary computers are not fast enough for wave action or boat pitch to be incorporated in the models. "Maybe in ten years," Richter speculates dreamily.) There is much to do even so. Perhaps because so few learn sailing in a classroom, even skilled sailors often misunderstand how complicated sailing is. Sails are like wings only on the most abstract level; they do generate lift, but in nothing like the same way.
Wind blowing into the belly of a sail compresses itself. If the boat carrying the sail were lying on the beach, the compressed air would spill around on all sides of the sail into the lee side opposite the wind, equalizing the pressures. A moving boat puts a cork in this process. The air flowing around the sails on a sailboat moving into the wind finds it hard to move up around the leading, forward, edge of the sail, since that means pushing against the currents flowing toward the stern. The air can spill around the rear, trailing, edge easily enough, but once it has, it has much the same problem in flowing upstream into the (relatively) low pressure area—it has to do so against the air moving down over the lee of the sail. This latter current forces the air moving around the rear edge back upon itself, creating what is sometimes called the "starting" vortex. (While this cannot be seen in the atmosphere, an analogous process happens with the keel, and starting vortexes are routinely visible in the water as they shed off the boat's track.)
Blockaded from taking the shortest route, the air flowing around the leeward side shifts to plan B, creating a second vortex (the "bound" vortex) that goes around the sail in the other direction. The two vortexes fit together like gears in a gear train. (Rotational energy is one of the features nature likes to conserve, as if one clockspring could only be used to wind another.) If we imagine a boat sailing from right to left with the wind blowing up from the bottom, then the starting vortexes will flow counter-clockwise and the bound vortexes clockwise. The flow from the bound vortex flows into the low-pressure area on the lee and the pressure difference between the leeward and windward sides of the sails creates a forward (driving) force and a side (heeling) force. The keel steers these two forces in a single direction and keeps the boat on course.
The task of virtual sailing is to enter this story into fluid modeling software (Richter uses a program from Fluent, Inc., one of New Hampshire's rising technology concerns) and then run a specific sail design inside it. The simulation is built up out of cells, computational entities that capture the smallest possible unit of relevant physical change. (Each cell has a size, since changes take place in space, but that size changes with the importance of the change it contains. A cell representing the flow of air some distance from the boat might be a cubic meter; a cell capturing the flow of air around the edge of a sail might be a cubic millimeter.) Each cell calculates its own characteristics (mass, flow velocity, pressure, momentum, and so on) from equations that combine the innate physical properties of the fluid and the interaction with the neighboring cells or walls such as the sail, rigging, hold, or ocean surfaces. When the computation is finished, each cell sends the results to its immediate neighbors, which they use to figure out their condition during the next tick of time. Richter works with hundreds of thousands of these cells and would very much like to work with millions.
Every day Richter's team enters a sailing "situation" into the computers and then waits a day or so for a statement to emerge representing one second or less of actual sailing. Sometimes these situations come from the client and sometimes from each other, from what Richter calls the "crackpot ideas" that come in the shower. (He may call them crackpot but he refuses to discuss them till the race is over. Then, he says, he will have a lot to write about.)
Whatever the importance of sail simulators in a Cup race, they might have profound effect on sailing itself. Eventually personal computers will get powerful enough to run these programs right from the desktop (or a company might sell access to a mainframe running a sailing simulator over the Internet). When that happens, it is only a step to let everyday sailors import digitizations of their own boats into these simulations, enter their own local sailing conditions, and then, for the first time, they will be able to see what really happens when they sail. They will be able to watch the streamlines and vortexes and regions of turbulence. They will not only be able to optimize the organization of their boat; they will be able to learn a lot about sailing that up till now has been locked in the intuitions of its few real masters.
For instance, many sailors believe that the flow of air speeds up between the main and the jib, creating lift and generating power. This causes them to yell at crew for "standing in the slot." In reality, the "slot effect" is a myth; the two circulation fields cause the flow in the slot to slow down. This is instantly obvious in a computer simulation. No sailor worth his salt who sees this simulation once (and who believes it) will shout quite so loudly again about blocking the flow through the slot.
The outcome of races depends on the design of the boat, the skill of the skipper, and luck what the winds give or withhold. Simulators will narrow the gap between boats and the gap between skippers, thereby increasing the proportionate role of the random accidents of wind and wave. You might think sailors would object to this, but the results of an informal and completely unscientific poll I have been taking over the past few days are to the contrary. Perhaps sailors are so egotistical that they think they will win anyway, or perhaps they feel that accepting the favors of nature, whatever they are, is really the essence of the craft.
Propelled by extraordinarily complex forces, two U.S. entries cross-tack during a '92 America's Cup race.
A sailor himself, Horst Richter hopes his computer simulations will short-cut the expensive trial-and-error process of designing sails. A mainsail and jib for an America's Cup boat can cost up to $100,000 he says and each entry is allowed 15 full sets of sails at the finals.
Landlubber Fred Hapgood writes about science and technology for Smithsonian, Wired, and other magazines in between.
