Mark, I think a retractable rudder is a good idea. I've never used one. The problem of sand and gravel mucking up the box seems problematical just as in skegs. I saw a production kayak, German ?, that had a retractable rudder. I did not get to see inside of it but from the outside it looked like the rudder pivoted down from a slotted shaft at the front of the rudder box. The shaft was about the diameter of a paddle shaft. The rudder was located about 1-2 ft. from the stern. When I built a Patuxent three years ago (There's a photo of me in it inside the back cover of Sea Kayaker,4/97?, on a snow bank)I made a regular rudder out of the hardware and fin that come with a Necky Gannet. The rudder mounted on the stern but didn't pivot onto the deck. The rudder had about half the area of a regular rudder. It's pivot is a couple inches lower to the water, so it can be a shorter with no difference in the water. The reason I bring this up is that I found out that a small rudder is pretty neat. One doesn't get locked into paddling on a rail with the rudder in use. It doesn't take a big rudder to assist keeping a course. I found the whole issue of having the rudder come out of the water on waves a bit of a none issue. Most of the time it is in the water, ergo most of the time it's working. I think when one paddles on waves there's a lot of of steering strokes and partial bracing strokes anyway so rail like tracking from a big rudder in the water feels a little confining to me when the water is moving all around. I think the biggest reason for a hidden rudder is so that one is less likely to get whacked, cut , bashed by all that hardware. It seems that when rescues occur in big waves everybody stays far away from rudder, this unnecessarily constricts ones options for rescue. There are lots of accidents from people grabbing rudder cables in the surf-ouch. While not retractable, the rudder on a Tsunami Ranger kayak is inspirational and worth looking at,in so far as the hull design is integral to the rudder, unlike most sea kayak rudders. I haven't come up with any prototypes. Everytime I think of a design, the simplicity of a skeg looks better, at least for boats that weather cock. I sometimes wonder if a rudder made out of a stiff 3x5 piece of tupperware would work. A short plastic rudder that pivoted at or just above the waterline at the stern might be a good compromise for utility/damage potential. not much help here, just ideas, have fun.