ON a cloudless summer day, a 65-foot sailboat slips out of downtown Chicago's Monroe Harbor.The vessel's distinctive A-frame mast is visible to advertising men and commodity traders watching from their Loop skyscrapers. Dockside gawkers note the gracefully curved boom, which needs neither a vang for tightening the sail when under way nor a topping lift to support it when the sail is furled. Out of human eyesight is the keel, capable of changing position to increase the boat's speed. Christened the Amoco Procyon (PRO-see-on), the experimental craft is soon skimming over Lake Michigan at a pleasant speed of 7 knots, despite the light air. Now if it can just get the becalmed United States sailing industry cruising again... . In 1980, US builders produced 4,000 sailboats longer than 18 feet. Last year they built half as many, says Olaf Harken, the man who initiated Procyon's creation. As co-owner of Harken Inc., the Wisconsin-based company that is the world's third-largest supplier of sailboat hardware, Mr. Harken was concerned about that "pretty severe drop-off." Harken found that longer-working consumers were abandoning time-intensive leisure activities such as sailing, horseback riding, and flying airplanes. US boatmakers, meanwhile, were too busy trying to pay their bills to think about styling or technical innovation, Harken says. The new sailboats they were building were not improved, compared with the less-expensive, ever-increasing supply of older ones. "Fiberglass is a material that just doesn't wear out," sailing author and TV commentator Gary Jobson says. "Used boats have come onto the market in droves," adds Doug Logan, a consulting editor for Sailing World magazine. The recession and the 10 percent tax on boats costing over $100,000 have also squashed new boat sales, Mr. Logan says. "We were not capturing new people into sailing very well, and we were losing some of the old ones," Harken summarizes. "We had to try to address how we get more people into it. What could we do to compress those time factors and make it more pleasurable and easy and more comfortable?" Something new was needed, Harken decided. So four years ago, he formed a team of designers, naval architects, and manufacturers to develop a boat that would incorporate new technology and modern looks. Britt Chance, who was on the last two America's Cup design teams, served as the project's chief naval architect. "He had also gone on the edge a little bit in the past," Harken says. "He had some failures but he also has a lot of successes. I wasn't looking for somebody that had only had successes. Sometimes you need people who have failed in some of their designs. They've learned a lot more through that." Diane Atwood, Procyon's interior designer, is recognized for her "modern, futuristic" work with power boats. Eric and Ben Hall of Bristol, R. I., had done leading-edge work with carbon fiber. They built Procyon's mast. David Greeley, the keel designer, is a leader in the field of modeling underwater appendages, having done such work for the US Navy and the America's Cup team. "It's not often that you're given a blank piece of paper and told, 'Hey, think of the boat of the future, comments Lisa Gosselin, senior editor of Yachting magazine. Ideally, the new vessel would require a smaller crew, a rigging time of only five minutes, and be adjustable to provide comfort, speed, or both. "That has all come to pass," says Logan, who sailed on Procyon in Lake Michigan. Recalls Ms. Gosselin: "We were out in New York harbor with pretty gusty winds, 20 people on board, and a fashion shoot going on. I can think of few other sailboats you could do that on and be comfortable." "You're up high, close to the bow, you can see all around you," she says. "One person can very easily sail that boat." In some cases, the Procyon team turned to designs, materials, and technology that were already in use elswhere, like roller-furling sails for quickness in getting under way, a forward cockpit for visibility, water-ballast tanks for stability, and twin, side-by-side helms so the boat can be steered from whichever is highest when Procyon heels (leans with the wind). OTHER ideas, like its bipod mast and canting-wing keel, had been tried in some fashion in the past but are not in current use. "Each idea is fairly radical compared to 'normal' sailboats," Logan says. "There wasn't very much about this project that was easy," Harken recalls. Consider the canting-wing keel - a sort of swivel-mounted, 12,000-pound steel centerboard that can be angled 25 degrees off vertical to windward. The extra weight on that side reduces heeling, which makes the ride faster and more comfortable. The trick was to figure out how to move the keel, yet preserve the boat's balance. "That little problem took a month," Harken remembers. Mr. Jobson, the sailing writer, says he found the boat to be fast and well-balanced when guest-skippering Procyon in last month's Chicago-Mackinac Island race. Procyon's spinnaker blew apart - that happens, Jobson says - but it still finished 13th out of 274 entries. As for the bipod mast, efforts to calculate the stresses it would encounter proved impossible. "We said that the computer model is going to be too complicated," Harken recalls. "Let's just build one and we'll find out." While conventional masts descend through a hole in the deck to the cabin below, the bipod mast is hinged at the deck. It can be winched up or down, giving Procyon easy access to inland waterways blocked by low bridges. The luff, or leading edge, of the mainsail is attached to a cable running from the deck to the top of the mast. When Procyon is sailing on the wind, the cable disturbs the air flow less than a mast would. "It's an appreciable advantage," says Knowles Pittman, a sailing industry journalist who tried out the Procyon in Annapolis, Md. Along for that cruise was Robert A. Mosbacher, whom Mr. Pittman calls "an Olympic-quality sailor." The US commerce secretary "was on the helm almost the whole time, he was so intrigued," Pittman says. Making the mast of carbon fiber saved 400 pounds. That enables Procyon to carry 18 percent more sail, which adds speed. "You save a ton of weight, and save it where it counts," since a boat that is less top-heavy gets by with a lighter keel, Gosselin says. Procyon has proved to be just as fast or up to 20 percent faster than comparable boats, depending on the angle of the wind. Seed money to build Procyon came from Amoco Corporation, whose petrochemicals are the building blocks for half of Procyon's materials by weight. Its Thornel T650-35 carbon fiber was used in the mast, boom, rudder, keel, and hull. Its Torlon thermoplastic appears in Procyon's lubrication-free fittings and bearings. Purified terephthalic acid goes into the sails and polypropylene into the carpeting, upholstery, and wall coverings in the luxurious quarters below deck. Amoco even makes the solar panels that trickle-charge Procyon's batteries. Named after the brightest star near the sailing constellations, Procyon was launched last April at Largo, Fla. It made appearances in Annapolis, Md., and New York, then entered the Erie Canal and Great Lakes. Procyon will visit the Newport International Boat Show in Rhode Island Sept. 5-9 and the North Atlantic Boat Show in Stamford, Conn., Sept. 19-22. "The big show," Harken says, is the United States Sailboat Show in Annapolis Oct. 11-14. "The true test will be - do any builders start to use these ideas? We're talking about an incredibly conservative industry," Harken says. Whatever happens, sailing will retain its appeal as a "clean, ecological, environmentally sound" sport, Harken says. "Sailing is not ever going to go away."