Maintaining your lead on an upwind leg is usually not too hard. Once you get ahead, you simply have to stay between your competitors and the windward mark. If you're skillful, you can even put them in your dirty air and actually increase your lead.
When sailing downwind, however, staying ahead isn't quite so easy. Here the boat that's behind is the one who can use her wind shadow as a tactical weapon. That's why I like runs so much -- they give the boats behind a great chance to catch up. But they also mean the leaders must be extra sharp in order to stay in front.
When You're Ahead
Pretend you've just had a great weather leg, and you round the top mark with the rest of the fleet behind, but close on your heels. Obviously, you can't rest on your laurels. Those other boats will come at you with tons of dirty air, so this is not the time to pull out your cooler and sandwiches. What's required to stay ahead is some cool thinking and close adherence to a few basic rules of thumb.
Cardinal Rule #1: Keep sailing your boat as fast as possible. Again, the best defense is a good offense. If you keep playing the windshifts well and pushing your boat to the max, there is no way anyone will catch you from behind. This applies to downwind legs just as much as upwind.
On a run, sailing your boat fast means doing two important things. First, sail your optimum apparent wind angle. Don't let the boats behind force you to sail higher or lower than your optimum. The second way to go fast is to take advantage of changes in the breeze. Jibe on the lifts so you stay on the headed tack, and head up in lulls and off in puffs and headers. Use the boats behind to predict what the wind will do, but don't cover their moves unless the wind is very steady.
Cardinal Rule #2: Stay between your opponent(s) and the next mark. This is the most fundamental rule about how to stay ahead on any leg. Draw a mental line between your competition and the mark ahead, then try to stay near that line. If you wander off the line and "split" from your competition, you risk losing when the wind shifts.
If other boats are close behind, another important consideration is protecting the inside. You want to stay between those boats and the mark, but you should favor the side of the course that will put you on the inside as you approach the next mark (so you'll have buoy room).
Cardinal Rule #3: Keep your air clear. Whether you're racing upwind or downwind, you have to stay out of your competitors' wind shadows. If you're trying to stay between an opponent and the leeward mark, however, this can be tough, especially if the boats are within four or five boatlengths.
The key is to know where your apparent wind is coming from, since this (not the true wind) determines where a boat's wind shadow will be. Unfortunately, it's harder to keep track of apparent wind on a run, for several reasons. First, the apparent wind is less in velocity because the boat is moving with the true wind. For example, if the wind is blowing 15, you may have only 7 or 8 knots of apparent wind. This makes it hard to feel. Second, the apparent wind is more divergent from the true wind on a run, which means water ripples and other clues won't help. And third, your apparent wind fluctuates over a wider angle. So even if you figure out where the wind is at one moment, it will probably change soon.
Fortunately, there are a few good ways to judge your apparent wind direction. First, use the telltales in your rigging. If your masthead fly is pointing toward another boat, for example, there's a good chance you're in bad air. I usually go by the feel of the wind on my head and arms; anyone who has a lot of sailing experience should be able to detect small shifts this way. Another person who's in a good position to feel the wind is the spinnaker trimmer. By watching the action of the sail and feeling the pressure on the sheet, he or she will often be first to sense the effects of another boat's bad air.
If you're able to follow all three of these rules -- sail fast, stay between your opponent(s) and the mark, and keep clear air -- there's no way that anyone will catch you. Just remember that if you start to slow down, head up a little for clear air and speed. If in doubt, keep your boat going fast through the water.
When You're Behind
We've just described all the tricks a boat can use to stay ahead, which might lead one to think it's tough to be behind. But this isn't the case. In fact, I love the challenge of being close behind on a run. With a few good tactical moves, it often doesn't take too long to get by the boat(s) ahead. Here are a couple thoughts.
Idea #1: Sail your own race. Many people sail poorly on runs, and the easiest way to gain big distance is simply by being smart and fast. The object is to gain as much distance on the fleet as possible, and put yourself in a position to attack later on. So unless it's near the end of a race, don't get mixed up with just one or two boats.
The key is to be patient. Follow Cardinal Rule #1 above, and try to gain bit by bit on the other boats. One advantage you have is that you'll get the puffs first. These will often bring you right up to the boats ahead.
Idea #2: Use your wind shadow to attack other boats. When you get near the finish, or there are only a couple of boats you're worried about, you can get more tactically aggressive. Position your boat so your wind shadow will hit the competitor(s) ahead. This means that you must be within four or five boatlengths of the other boat, and in line with her apparent wind.
The most common mistake made by boats behind is not getting far enough forward to really hurt the other boat. You almost always have to be farther forward than you think. Don't worry about getting ahead of the other boat's breeze. You can always head off a little, if necessary, to slide back onto her wind. When possible, go for the inside position at the mark and use your wind shadow to force boats ahead toward the outside.
Downwind legs are fun because they tend to bring the fleet closer together, rather than telescope it apart. If you're fortunate enough to be ahead, be thankful and keep pushing. If you're behind, remember that this is your best chance to catch up.