I raced a Hobie 18mag around the country one summer a few years back. Ran into the same problem, but I remembered something I read about the old America's Cup boats.

I read something about flossing the keel to keep it clean under sail. So I made two "flossing sticks". I used 2-3' pieces of PVC tubing. Drilled holes in them so that water would run right through. I unbraided some old mainsheet line and attached it to the ends of each small pole.

They looked like little mops, but boy did they clear weeds out of the rudders fast. Just floss the daggers and rudders real quick and they were clean. Cost $2 to make.