Hi all,
Thanks for the responses. As far back as possible it will be.
The links have given me some more ideas to ponder.
My present design is one where the rudder normally resides in the kayak and is dropped straight down when needed. It will be raised and dropped by means of a foot pedal as well as steered. The shape of the rudder is the now usual foil design but it will be longer in the chord than deep, almost square. I figure that since most turning force is generated at the intersection of the rudder and the hull, where the water is trapped between two surfaces the loss of depth won't have a greatly adverse effect on its turning power. At the bottom of the rudder I will have a horisontal piece the same shape as the hole from which it drops. I hope that this will create a similar form of water compression at the bottom of the rudder and so create more turning force. It will also plug the hole in the hull when up. I figure this won't cause any jamming since there will be space above it for sand to escape. As for water entering via the assembly, it's top will be above the water line and a piece of bike tube and a couple of hose clamps as back up will take care of that.
When I'm done I'll post some pics. That at least was the intention. Now to the new ideas :-)
John