I'm two lengths ahead of another boat on the beat. Should I sit right on her wind to slow her down, or cover her loosely and let her breathe?

Usually cover loosely. A tight cover parks your dirty wind on her, so she tacks away, you tack on top of her again, and you are in a tacking duel where every tack costs you both. A loose cover keeps you between her and the next mark on the same tack, but off her wind. Each time she tacks, you tack too. She stays in clean air, so she has no reason to keep tacking, and because you are both sailing the same tack in the same wind, any shift that helps her helps you. The tight cover is right when she is the only boat you have to beat. Otherwise, the further ahead you are, the looser you can be.

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Next on this shelf — The Upwind Beat