The faster we go the more the wind appears to come from in front of us, so the lower we have to head. Apparent wind is a fairly simple concept, but it gets more interesting than that. If we could sail upwind at a boat speed of say15 knots, and the true wind was only 10 knots we wouldn't be able to point very high at all because the apparent wind would be so far forward. To be brief hull speed is like a natural speed limit. The hull speed of any boat is affected by many things, but most profoundly by the length of it's waterline.
Boats create a wave as they move.
Since longer wavelengths tend to travel at higher speeds longer waterlines create a higher hull speed because they make waves with longer wavelengths. It sounds complicated and it is, but I visualize it as though I were sailing around in the trough of a big wave. If I want to go faster I would have to start sailing up hill to get over the wave in front of me. Occasionally downwind we gain enough power to actually sail up hill and break through the wave. Well, I like to think of a boat as traveling in one of three different modes.