Handy, attractive, quick-tightening knobs for mounting brackets, leeboards, kick-up rudders, and so on.  We use these in the Eastport Nesting Pram and the Take-Apart Passagemaker Dinghy.  Even cold fingers can spin these on and off.  Measure  3" in diameter,  3" deep, and have 5/16-18 threads.  Marine grade;  won't corrode.