It's always better to through-bolt marine accessories such as grab rails and rod holders, but bulkheads, decks and rail caps with no access from behind pose a problem. One solution is the stainless-steel Garelick Toggler toggle-bolt anchors (about $34 for a pack of six). Push the toggle into the mounting hole, pull it back up flat against the inner wall with the two nylon arms, then slide the retaining collar down, and snip off what remains above deck. Now you can thread the bolt into the hole and the threaded toggle. Garelick stainless-steel toggles are designed to fit ¼-inch 20-threads-per-inch bolts.