I have a friend that did this successfully on a 12' Avon. Here is how he mounted two downriggers. First, he cut a good solid 3/4" plywood platform, say 10" wide, that spanned the width of his boat, less a few inches. The ends of the wood have to sit outside of the middle of the tubes, on top of the tubes. To the underside of the plywood, he attached a loop of PVC fabric similar to what the boat was made of. You can buy large pieces of this material at Industrial Plastics and Paint in Victoria. Here is the key... The loop of fabric is as wide as the plywood, and long enough that it can go along the insides of tubes, under the floor, and back up the inside of the other tube. He then attached this strip of fabric to the underside of the plywood with PL Premium and screwed a second piece of wood to the bottom. The first piece creating a sort of sandwich of wood, fabric, wood. The second piece of wood needs to fit between the tubes without causing chaffing. After you are done, you have a plank with a big loop of fabric hanging off the bottom.
Mouth the DRs to the plank. When you set up the boat, the loop of fabric goes under the floor and kind of presses along the insides of the tubes. It is tensioned by inflating the boat. This takes a
Little fiddling around to get the loop length just right, but once you do, you permanently attach it with the second board screwed and glued to the first one. It provides an incredibly stable and tight platform. I was very impressed.
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