MAX EBB — "N
eed a hand with that?" I asked when I saw Lee struggling with large and unusual objects in the parking lot. Lee Helm had just exited a ride-share car after I had parked. I assumed she was there for the same event that I was attending, a lunch lecture at one of the more prestigious yacht clubs on the Cityfront about the proposed new Sailing Science Center, a hands-on, Exploratorium-like museum with a sailing theme. I'm not a regular at St. Francis, but they do put out a good lunch spread on Wednesdays, and the lecture series has sometimes been called "the TED talks of sailing." Unlike some other functions at fancy clubs, there's no extra charge for non-members who crash the event — guests pay the same $18 as members (although it shows up as $26.37 on the credit card after they add tax and a very generous tip). In fact, the club has gone out of its way to invite sailors from any recognized yacht club, even if they arrive by land. It's a good deal considering the quality of the food, the service, the speakers and the dessert table. "What in the world is that thing?" I asked as I tried to decide how to best pick up this large object that resembled a table top with a rotating mainsheet traveler bolted to it, along with aluminum tubing bolted to the table at odd angles, and a small wooden plate cut out in the recognizable shape of a sailboat deck, with an aluminum angle on top. One of the aluminum tubes, free to slide in and out of another tube, had a furniture caster wheel at the end. "It's like, one of our first portable exhibits," Lee explained. "Are you in on this Sailing Science The Upwind Intuition manipulative exhibit. Push in the direction the wind is blowing, and see the model boat move upwind!
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• January, 2019
Center project?" I asked. "Collaborating on some of the demos," she said. "And I get a totally awesome free lunch! This is one of our first portable manipulative exhibits. Can you guess what it does?" I worked the moving parts in and out, but still had no clue. "Upwind Intuition," she grinned. "This is a hands-on device to address the first fundamental intuitive leap that new sailors have to take. After they play with this for a few minutes, they will understand, at least sort of, how upwind sailing works and, like, why it's even possible." She moved the little boat model to the middle of the traveler track, adjusted the sliding aluminum tube to line up with it, and pushed on another tube — the one with the wheel at the end — to engage the angled plate on top of the boat model. "See? I'm pushing away and in a direction that represents the wind direction, but the boat moves toward me! It's at an angle, but still. I can rotate the traveler and sail on the other tack to bring the model right upwind, and always by only pushing on it downwind." "Isn't that kind of obvious?" I said. "Max, you'd be amazed how many new sailors — even some student engineering geeks — totally can't understand how a sailboat can move upwind. Even when I have them out sailing and they see it for themselves. Like, total cognitive dissonance. I had one very smart biochemist sitting on a boat sailing to windward, declaring that what he was seeing was impossible." "It's just like the pumpkin seed analogy," I suggested. "Squeeze the seed, and it squirts out from between your fingers in a different direction." "For sure, Max, but like, that's only a beam reach. The seed pops out at right angles to the force. With this rig, the motion is 45 degrees into the wind, or even closer if the friction is low. There's a quantitative part too: I can show that the pointing angle is the sum of the two drag angles, just like on a real boat." "Drag angles?" I asked hesitantly, not really sure if I wanted a trigo-
nometry lesson out there in the parking lot. "Drag angle is another way of representing the lift-drag ratio. It's the arc tangent of the drag divided by the lift. A perfect foil, with no drag, would produce a force perpendicular to the flow, and the drag angle would be zero. A totally inefficient object with all drag and no lift would have a drag angle of 90 degrees." "But you don't have any foils on this demo," I noted. "The traveler acts like a foil in the water — actually like the keel. It prevents sideways motion, only allowing the boat to move forward or aft. But it does have some drag, and you can see the drag angle by holding the track sideways and seeing what angle you have to tip the traveler to get the car to slide downhill under its own weight. That angle shows the ratio of longitudinal force to transverse force, analogous to lift and drag if this were a keel resisting side force." "Let's get all this gear inside," I said. "They start serving lunch at 11:30." Lee picked up the rest of her equipment while I took the Upwind Intuition demo apparatus. As we walked toward the building, Lee explained that the push rod with the wheel at the end represents
