Take a look at the FAQ on the Hobie Cat forums about rudder cams first. It is possible that the rudders are just not staying locked down properly. http://www.hobiecat.com/community/viewtopic.php?t=467

Also check rudder alignment in the direction of travel. Proper rudder alignment would be 1/8" to 1/4" toe-in on a 14 or 16. This is measured with the rudders down. Measure leading edge to leading edge and then trailing edge to trailing edge. The front (toe) should be closer together than the trailing edge. Measure at about the little hump on the leading edge and straight back from there for the trailing edge. It is always possible that the rudders are actually mounted on the reverse hulls. That causes extreme toe-in problems. Look at the upper casting / arms to be sure they angle in towards the boats centerline when the rudders are in alignment with the hulls.

If the rudders are locked and staying locked while sailing, you may have to re-drill the rudders to set the rudder rake properly. There is a Hobie rudder drilling template that might help. Otherwise the basic rudder rake procedure is as follows.

Start off with the rudders in the locked down position. Measure down the blade 12” from the bottom of the lower casting. Mark the rudder with a line 2” long perpendicular to the 12” point. Using a very thin piece of line, starting at the top of the rudder pin, align the "line" with the rudder pin top to bottom (on the pins center line / axis) and check where the line intersects the mark on the rudder. Mark the point where your rudder pin axis crosses the mark you made 12" down the blade. Measure in from the front of the rudder. A larger measurement means that the rudder is raked forward more. More forward rake is less weather helm. Optimally, you should have 1 5/8” to 1 3/4” for a measurement. To achieve the 1 3/4” number, you might have to re-drill the front hole on the rudder or file away at the front of the rudder where it hits the casting. To much forward rake can cause lee helm and THAT is WORSE and even dangerous. The boats sail better and are safer if the boat rounds up slowly when the tiller is released.

You can use the above technique to measure and re-drill a blade. Mount the blade on the lower casting. Make all of the above measurements and set (and hold) the rudder in the desired rudder rake position. Lock the upper casting onto the rudder and lower casting. Push the upper assembly up into the cam to seat it tightly (that holds the rudder in the raked position once the rudder system is fully set up). Align the tiller arm angle (vertical position) with the rudder assembly on the opposite side of the boat. You can mark the bolt hole position or drill right through using the casting as a drill guide. You must hold the upper arm in position while drilling this way. I angle the drill slightly forward rather than straight through. This forces the upper casting forward more as the drill reaches the opposite side of the casting after going through the rudder blade. If the hole is too far aft, the rudder will slip aft a bit when under sailing loads.