There are possibly a couple of things going on here...
The first is the rudder alignment. "Extended the rudder tube" (tiller cross bar) to its maximum and the rudders are still not in alignment (perpendicular to the back of the boat) ?
The second is rudders that are not locking down correctly: http://www.hobiecat.com/community/viewtopic.php?t=467
In this case, I think the rudders may be mounted on the incorrect hulls (reversed). This can cause serious alignment problems that cannot be corrected with the tiller cross bar. Be sure the upper castings / tiller arms are angled inwards when the rudder is in alignment with the hull. Ultimately the rudders should be in a straight line with the track of the hulls through the water. We call that adjustment toe-in or toe-out. Lock the rudders down. Pull a tape measure across between the leading edges of the two rudders (just below the transom at the little forward hump on the blade). Then measure across between the two trailing edges. The difference between the two measurements? Shorter in the front is toe-in. Longer is toe out. You do not want toe-out. On an 18 it should be square or maybe 1/8" toe-in. Too much difference in the measurements will cause allot of cavitation of the blades as they travel through the water.