bradsc wrote:
You could put a rod holder vertically on the railing and then have a downrigger base go into the rod holder. Look at my personal page, under the 15 Sport, how I had the rod holders mounted. I would think it would hold a small downrigger and a light ball. If you want something more heavy duty you will have to design/make a bracket. We kingfish some and I had to make a downrigger bracket and mounts. We use Penn swivel mounts and 8# balls. Look at my personal page.

Edit: look at the 3rd and 4th pictures. Those are the downrigger brackets. The down rigger bases go inside the brackets (pipe) and stick up just over the railing. I had a buddy cut out the stainless steel parts on his water jet, TIG welded them to schedule 40 stainless steel pipe, and then powder coated.