Jake,

Now you've started a more interesting discussion. If you are making significant leeway, that is sliding sideways while sailing downwind, then you are not going downwind as fast as you could.

I would postulate that you need LESS daggerboard while sailing downwind than upwind but that you still do not want to be making significant leeway. You need less daggerboard because of the relative vectors of the energy (lift) you are extracting from the two fluids moving relatively to one another. And from the relative speeds with which your foils are moving through those fluids.

For simplicity lets assume that your sails and boards are flat. Let's also assume that the lift off of each foil is perpendicular to the plane of the foil. While sailing upwind the forces are almost directly opposed. While sailing downwind they may be approaching 90 degrees to one another.

Also your apparent windspeed goes down, while your apparent waterspeed goes up. Thus, you need more sail area, and less board area. Upwind your apparent windspeed is up, and your apparent waterspeed is down. Thus for that reason you also need less sail area and more daggerboard area (relatively)

A couple of emperical ways to look at and think about the problem:

1.) If leeway were really good why would we reach downwind instead of just running dead downwind?

2.) Take the boards out of your 6.0. In shallow water push the boat sideways. The push it forward. It is much easier to push it forward. In the simple argument the wind is pushing it downwind. If it were faster to make leeway, that is to go slightly sideways, then wouldn't it be even faster to point your boat the way you want it to go, since there is less friction on the boat, even with a little board down going forward, than there is on the boat with it going sideways?

Another principle that you can take from this discussion and apply to racing. You need more board the lighter the conditions. In very light wind I leave my boards all the way down all the time. Ony when it blows harder do I start to bring them part way up for sailing downwind. If it's really nuken you don't even need them all the way down for upwind.