Hi Mary,
Oh I wish you hadn't asked that question - it's the biggest source of debate amongst all sailors - cat, mono or otherwise.
First of all, if you are floating in the water which is moving, it doesn't matter which way your boat is facing, how far in or out the boards are, etc. Picture two miniature boats (say, from a Gulliver's Travels scene) racing each other in a bucket of water. Now a boy picks up the bucket and starts walking with it. The effect is EXACTLY the same as current. All the boats are in the same bucket, which is moving. BUT, there is one VERY big difference - that is the apparent wind. The current effects your apparent wind. This is why nearly ALL sailors believe (but for the wrong reasons) that it is always better to have your boat nose-to-current, if possible. They THINK it's because there is less drag on the boat, since a boat sideways to the current must get swept further, right? - NONSENSE! All the boats are in the same moving bucket of water, so it doesn't matter which way they are facing, EXCEPT, that those nose-to-current are getting lifted by the apparent wind generated by the "motion of the bucket" or current, and those sideways to it are getting headed. (If sailing upwind, of course). I have a full detailed explanation of this with diagrams, vectors, etc, if you want. I could E-mail it or try to attach it. I've attached a figure that shows two boats on opposite tacks trying to go upwind in 12 knots of true wind with 0 current and again with 3 kts of current. If you can read this figure you'll see that the boat nose to current gets lifted, while the boat more sideways to it gets headed. It has NOTHING (have I said this enough?) to do with the drag of the underwtaer portions thorugh the water...