The Straight Dope - How can racing yachts sail faster than the wind?
How can those America's Cup yachts sail faster than the wind? Here's what the San Francisco Chronicle has to say: "It's physics." Come on, Cecil, I know you can do better.
— P. McCartney, Oak Harbor, Wash.
I should hope. To give the Chronicle some credit, though, "it's physics" was preceded by an accurate if somewhat murky explanation that, unfortunately, you didn't get. Let's try again.
The America's Cup, for those whose taste in sports runs more to bowling, is the most prestigious prize in sailing. First awarded in 1851 and later named after the first ship to win the race, the ornate silver cup is pursued by sailing fanatics racing the most high-tech yachts in the world. The boats used in the original America's Cup races were wood-and-canvas schooners, but over time they've evolved into computer-designed craft made of carbon fiber and epoxy. One thing hasn't changed, though: All that makes them go is the wind.
For the 33rd America's Cup race, to be held in February, the defender is the Swiss yacht Alinghi 5, sailed by the Société Nautique de Genève, which won the cup in 2007 despite the handicap of having no local ocean access. (They train in Spain.) The challenger is the American yacht BOR 90, sailed by BMW Oracle Racing. The team's 2007 effort cost $200 million. BMW charges a lot for its cars, and Oracle charges a lot for its software. I can’t imagine this project is helping them hold costs down.
The BOR 90 is a three-hulled yacht covering an area about the size of two basketball courts. The mast is 185 feet high and carries a half-acre of sail. Those numbers make the boat sound enormous, but to see it you'd swear there's more mass in a toothpick. The sails catch so much wind that capsizing is a serious risk, even with a hydraulic mast that can tilt to stay vertical as the boat leans. If any boat can sail faster than the wind, you have to figure, this is it.
It does, too — two to three times as fast. Not that I'd ever doubt anything I read in the Chronicle, but I thought it'd be smart to have my assistant Una check with BOR, specifically design director Mike Drummond. He confirmed two things familiar enough to sailors but mystifying to landlubbers:
• Yachts can't outrun the wind if it's behind them. As common sense suggests, they can go no faster than the wind can push them.
• However — here's where common sense hits a tree — when yachts sail at a 45-degree angle into the wind, they can in fact go faster than the breeze that powers them. Drummond offers two reasons for this: "less turbulence due to a better angle to the wind; and the oncoming air is faster due to the forward speed of the yacht combining with the wind."
Sound like doubletalk? It's not. A sail is an airfoil, like an airplane’s wing. Hold a slip of paper to your lips horizontally and blow over the top — it rises, like a wing. Now hold the paper vertically and blow to one side. It pulls to that side, like a sail — and the harder you blow, the more it pulls.
Now think about a sailboat in the water. The relative speed of the wind past the sail is a function of two things: first, the absolute speed of the wind (that is, relative to the ground) plus the speed of the boat moving forward.
In short, the airfoil combined with the boat's mobility acts as a multiplier. You can think of a boat sailing upwind as being in overdrive. Strictly as a matter of aerodynamics, there's no reason a close-hauled boat (one heading upwind with its sails trimmed tight) can't outrace the wind.
Few do, of course, because of another factor — the drag of the boat in the water. For traditional single-hull boats, which ride relatively low, drag is considerable. Still, with the right mix of design and determination, even a single-hull boat can outrun the wind. The first America's Cup entry to do it was the New Zealand in 1988.
Today's multihull boats, which skate on top of the water, can attain much higher speeds, and craft that don't need to contend with water at all can go faster still. According to the World Ice Racing Circuit, ice boats can sail four to five times wind speed. In March 2009, a land sailboat reached 126 miles per hour on a dry lake bed in the Mojave Desert. By comparison, the BOR 90 may sound positively pokey — Drummond says it's gotten close to 50 knots, or roughly 57 miles per hour. However, we're talking about a medium in which supertankers max out at 18 miles per hour and the fastest 19th-century clipper ship achieved just 25 miles per hour.
So, we clear on the concept now? If not, "it's physics" will have to do.
Comments, questions? Take it up with Cecil on the Straight Dope Message Board, straightdope.com, or write him at the Chicago Reader, 11 E. Illinois, Chicago 60611.