ORC
This is what losing that ‘life-and-death’ struggle with drag looks like in practice. Even close to layline a duck might have been a better choice… With the starboard tack J Class already sailing fat to get over Barcelona’s difficult waves, tacking to leeward was unlikely to work out well (it didn’t). To get these deep and sticky 150-ton yachts back up to speed is really one for the diary rather than for a stopwatch – however enormous those sails might look
No escape
Heading for the Offshore Racing Congress and, first time in an A380, wondering why this thing is nearly grazing the trees that surround Queen Mary Reservoir. It’s a big heavy plane and needs enormous wings to produce enough lift to fly. Induced drag (the drag that accrues from generating lift on a wing of finite span) is proportional to Lift2/(V2
x wingspan2 ) so the A380
suffers a double whammy; it’s heavy, so it needs a lot of lift to fly, but it has the same wingspan as the lighter planes. I’m sure the designers would have liked a wider wingspan, but then of course it would be impossible to squeeze into the airport. So the plane takes off at such a shallow angle, in part due to its high induced drag… a heavy plane with a low wingspan. For a sailing yacht the best way to minimise induced drag is to
increase draft, but of course class rules and water depth restrict this. It was an MIT paper in 1978 describing force models built to simulate yacht performance that more widely introduced the concept of Effective Draft that can be used in lieu of the physical draft for calculating induced drag. The cause: the presence of the canoe body pulls the keel tip vortices towards the water surface and results in an effective draft that is always less than the physical draft. On a yacht you have a simple heeling force meter, it’s the heel
angle – no heeling force, no heel. So now going upwind at 20° of heel your induced drag fate is sealed. The heeling force is fixed, draft is fixed, the only thing in play now is your speed; the faster
34 SEAHORSE
you go the lower your induced drag, which is great, until you have to turn a corner. ‘Ready about, lee ho’: immediately speed drops, through the eye of the wind the speed is still falling, sheet in on the new tack, heel to 20°, but now your speed has dropped. If you went in at 10kt and came out at 7kt your induced drag is
100/49 (102/72) = 2, it’s twice as high! No problem, you will be back up to speed soon. Next time you look at your logs take a minute to check how long it takes to get back up to speed after a tack – every second you are below target you have more induced drag than your opponents. Even a 5% speed deficit is 10% more drag. You can’t get away from it. These thoughts came to mind at the J Class Worlds in Barcelona
just before the America's Cup. Sailing a J Class yacht you are involved in a life-and-death struggle with induced drag. The rig is enormous, the boats are heavy, the draft is relatively shallow. This means that the heeling force is very high, 25 tonnes or more, the speed is not fast and induced drag is 30% of the drag even when you are going flat chat. When tacking your minimum speed could be less than 7kt, so
getting back to speed is mission impossible; you have the induced drag holding you back and the apparent wind way down because the boat speed is down. The boat is trying to accelerate with 30% extra drag and 40% less driving force from the sails. So it can take a long time to get going… and in very light conditions you still may
JASON LUDLOW
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