Rudders should be "Toe in" or closer together in front by 1/8-1/4"
You can mount the rudders on the incorrect hull which would cause them to be much farther apart at the rear than at the leading edges. Check the tiller arms as they leave the rudder... they should bow inboards.
Here is a detailed FAQ area and FAQ on rudders:
https://www.hobie.com/forums/viewforum.php?f=18Rudders:
https://www.hobie.com/forums/viewtopic.php?f=18&t=467Rudder Hummm:
https://www.hobie.com/forums/viewtopic.php?f=18&t=12859