Reinforcing bolt holes.
Whenever installing stressed bolts in cored fiberglass (and that's what youre' doing here), this is what I do. 1.) Determine the exact location for the holes. In other words, just drill the standard size holes where they belong using the metal part as a guide. 2.) Drill about half of the holes in the fiber glass twice oversize. If a part has two or three 1/4" mounting holes, drill one of them in the fiberglass out to 1/2". Don't at any point drill oversize the holes that are in the part you're attaching! That's not the goal here! 3.) Coat the inside of the oversized holes through the core material with epoxy resin (correctly mixed with hardener of course, but without any thickening powders added). 4.) Fill the oversized hole(s) with reinforced epoxy. Tape over one side of the enlarged hole, lay the rudder that side down, pour in a mixture of epoxy and high strength additive like milled fibers. 5.) After the epoxy sets, redrill the filled holes to finished size using the unmodified holes to locate the redrilled holes. (Temporarily install the part using the other holes, drill through the part into the center of the epoxy plugs.) 6.) Using the reinforced holes as a reference pattern, repeat the drill/fill/drill steps for the remaining holes. This process creates a compression bushing through the soft core around each fastener to prevent crushing the core and skins under load. It also seals the core from water intrusion in the event that the polysulfide bedding compound leaks. This may sound like a lot of effort if you've never done it before, but with practice, it becomes routine. I've done this to every stressed fastener through the cored areas of my Catalina 25 deck and transom. The only ones I skipped were the teak mounting screws. Anyone who has had to deal with water damaged and/or delaminated core will realize that this work is the lesser of two evils. Good luck! -- Leon Sisson
