On my 13' whaler I use a piece of 3" PVC that just lays across the back from gunwale to gunwale and hangs off the side between 18-24" and doesn't hang down at all. This gives me the flexibility to dip to either side of the line. RARELY will you be able to dip from the same side all of the time as the boat will be at different angles, the line can drift, if using snoods the crabs can swim to the opposite of where you want them.
I use a 2"x4" across the back of my 19' bowrider with galvanized pipe mouted to it. Once at the end, it does dip down because on this boat the gunwales are higher. Still sits off the side of the boat to allow dipping from either side of the line.
No matter what, it will need to be adjusted until you find what works for you on your boat.
Chris
