Places often change rapidly right after a leeward-mark rounding. The other night we rounded in the middle of a clump of boats, took a clearing tack, then tacked back toward the shore for relief from the current. As the boat we'd tacked away from initially began lifting up in front of us again, it hit me: "We just made the same mistake Stu Walker did against Don Cohan!" When the water in which you're racing is moving, it's as if you're sailing on a conveyor belt. If the racecourse is near shore, however, it's more like you're sailing across a number of conveyor belts, each moving at different speeds and sometimes in different directions. Perceiving how the water is flowing and then dealing with the implications adds a level of complexity to a race that's as challenging as figuring out the variations in the wind.Contributing Editor Walker's story this issue tackles one aspect of sailing in current, but it's part of a subject that's as rich and fascinating as any other in competitive sailing. If you race in an area with tidal currents, you can predict the direction and speed of the current by reading local tide tables and tide charts. You can also predict where the flow will be the strongest by looking at the shape and depth of the bottom--current usually runs strongest in the deep water. But weather systems (wind, rain, and high and low pressure) cause variations in tidal flows and can create currents on any body of water, so it's vital to learn to look for the signs of current when you get to the racecourse. On our boat we're always on the lookout to see which way the water's moving past a nearby mooring, fishing buoy, government mark, or race committee mark. (Check for a wake, down-current of the buoy, or weed, caught on the line.) We look at how the committee boat is lying on its anchor, and as we tune up for a race, we look for changes in the water texture; bigger waves often signal more current flowing against the wind. Have you noticed your boat sometimes sailing across distinct lines in the water? That's often a sign that you're sailing from one conveyor belt to another, in effect, two separate rivers of current moving at different speeds. During the race, look to see if boats on the other side of one of these tide lines are suddenly going faster or slower than you. With practice, you can tell when there's a significant change in the current.Perceiving what the current is doing on any given day at any given moment takes years to truly master, but if there's current on the course, you can't ignore it and expect to place well regularly. Especially considering, as Walker writes, the current flow and velocity also affects the wind in your sails.This isn't an easy topic to fathom, but it's great mental exercise, and we've added several diagrams to help. The story will probably challenge and possibly frustrate you in some measure. But don't worry if the concept of "current wind" doesn't immediately become second nature. Start by reading the box in Walker's story ("The Wind You Feel"), and refer back to the story after your next race in current.Think in terms of the conveyor belt analogy, and remember that even if you have the current wired, you can still lose big-time in the next windshift. When racing in current, I find my best assets are a keen curiosity and a good sense of humor.