Rudder Lack of Stability when locked down

Hello. This is my first time posting but am hopeful someone can give me some guidance. I own two adventure islands, one with the old and one with the new rudder system. On the island with the old rudder system, when I set the rudder with the rudder control, it stays where I set it, no matter the wind or heading. On the island with the new rudder system, I have to continually fight the control in high winds and it doesn't stay where I set it much at all. I've read through the posts I can find thus far so I have locked the down rudder line and, from the aft of the boat on land, have tried to move the rudder up and down from the locked position. There is resistance, but I can move the rudder up and down so I have to assume that in strong winds, it is just not holding course and is moving. Is this normal or is there something I could/should do to lock the rudder into the down position more completely? Thanks in advance for any insight. DMM

Your post is a bit confusing... Is the unwanted rudder movement back and forward, or side to side?

If you are talking about the rudder not staying locked fully down, this can lead to the blade being all behind the rudder pin line, so the forces are magnified (without the counterbalance normally in front of the pin)

If this is your issue, you might like to do what I did. I removed the pulley from under the deck near the "foothold sides", adding a swivel to the rudder down line, plus extra line to reach the new location of the return pulley. Now when I pull the rudder down line, there is a large clunk at the stern, and the steering counterbalances beautifully, with fingertip control

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Tony Stott
2012 Tandem Island "SIC EM" with Hobie spinnaker