In looking at various designs its hard to see from the rocker line what makes one hull have no rooster tail and another have it. Does anyone know how you go about designing it out ?

I think its more down to how sharp and square your transom/hull corner is.

It's about how much the stern is depressed into the water when underway. If the hull makes a clean exist from the water on it's bottom using the rocker then there is no rooster tail. If the hull makes an exit from the water with the square stern, then there is a rooster tail. However, I wouldn't imagine that the presence of a rooster tail isn't necessarily a bad thing. As long as the water is leaving the hull cleanly (i.e., the boat has enough hull speed that the water isn't flowing backwards into the stern) the rooster tail is no real issue.

You also have to remember that things are a compromise between varying wind conditions. In light air, we move forward to keep the square vertical part of the stern out of the water (among other reasons). In heavier air there is much more pressure on the buoyancy of the hulls due to the leverage, we move our weight back on the boat to keep the bows at proper trim which pushes the sterns down...which isn't much of a problem because the sterns are exiting cleanly at that point.

Jake
There has to be some sort of energy expended/wasted in chucking an amount of water up in the air like that

I'm certainly no expert here...but my understanding is that to get the needed buoyancy to exit cleanly means you either need fatter or longer hulls. But you'll be displacing the same amount of water. I don't know, but I would imagine that increasing the amount of hull would cost more in drag than what little you lose with a rooster tail...but if the water is leaving the hull cleanly - does it matter what the water does after it is no longer in contact with the hull?

When I see the rooster tail (usually on a blast reach in good wind) it looks like the low side rudder is causing it more than the upwash off the hull transom. The water seems to run up and off the low side of the rudder up and into the air. Now, of course this water was -displaced- by the hull first, but it seems the side load on the rudder is what gets it to spiral up into the air.