[quote=Dogboy]Boat props cavitate and pump impellers cavitate. Sailboat foils do not cavitate - they ventillate and/or stall.
If you're sailing the boat hard with the daggers half way up, that could cause a stall (especially if there are weeds in the water). But that would only happen when sailing upwind, certainly not when sailing offwind, since we always pull the boards up when sailing downwind anyway. Ventillated or stalled rudders really wouldn't cause the rudders to "lock up" though. A stalled rudder is just unresponsive. I've had this happen numerous times on my H17 when pushing the boat hard upwind in a medium breeze- the rudder will just let go and the boat will be going along sideways. You have to sheet out several feet, bear off, and re-establish flow. Then you can sheet back in and get going again. In windsurfing it's referred to as "spinning out."
If you're experiencing a situation in which the tiller physically won't turn under load, then I would look at the rudder mechanisms - lower castings, gudgeons, pins, bushings, and tiller connectors. It sounds like something may be binding up.
You mentioned burying the bow downwind. The 18's bows are so broad and deep that the boat can be quite difficult to steer if they are buried or nearly buried. I still wouldn't describe the rudders as being "locked up" in this condition however. And once the bow pops out, the steering returns to normal.
One thing you didn't mention is what do you have to do to make the "locked up" condition go away?
Also, what type of rudder blades are you running?
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