Open ocean swell can be one of the biggest advantages or disadvantages to your drag racing success. People who have a lot of experience in swelly conditions are inevitably way faster in those conditions than those who usually sail on flat water. This is because they are able to look way ahead on the ocean and pick themselves a good racing line through the swell; kind of like a mogul skier picking his line down the mountain. You need to look ahead and upwind of you to see where the swell is coming from, which direction it is heading in and where you want to be in relation to the person you are trying to overtake.
If you can get on a swell that your opponent hasn’t seen, you can gain a huge advantage over them. If you miss one however, and end up on the back of a swell while your opponent manages to sneak onto the front of it, then you are doomed. There is almost nothing you can do to get back ahead, even if you have better board speed.
This swell timing is a very useful but very difficult part of race tactics to learn. To get over the back of a swell quickly, you need to bear off downwind and have as much power in your sail as possible. The last thing you want to do is spend a lot of time effectively sailing uphill on the back of a swell while trying to make it onto the front of it. Once on the front of the swell, you can then head upwind again, trying to ride the swell for as long as possible. You will be surprised how long you can stay on a swell for when you really think about trying to milk that extra speed out of it. Then, if you time it right, you should be able to sail quickly from swell to swell, ducking upwind and downwind to take full advantage of the lumps of water.
Of course, if someone is close behind you then you need to gauge when they are getting onto the front of a swell or when they are going to be held up by the back of another one. You also need to be aware that if you are slightly in front, you can hold up the person behind you by giving them dirty-wind (the trail of turbulence left behind you by your sail, more about this in the next paragraph), so if you know that they are a little faster than you, then your time might be better spent keeping an eye on them and making sure that they do not get far enough upwind of you or downwind of you to have clean air.
Your ‘dirty wind’ zone is a triangle of turbulent air behind you; it extends about one metre behind and upwind of you, straight back along your wake for a good 10 metres or so, and then away from you downwind at about 45 degrees, probably for at least 20 metres. Thinking about this and keeping someone in your dirty air is a great way of maintaining a lead.
You really need to be within about one or two metres of someone to give them dirty wind if they are on the upwind side of you. If you let them get further upwind of you than that, then you have no chance of holding him or her off with your dirty air. If they are downwind of you, then you have a slightly bigger area to play with, but you still have to keep a really close eye on them as they could dart away downwind and clear out of the dirty-wind triangle on the other side, before you have had the chance to redirect yourself downwind.