Vanadams":2vih9psg said:
I know there are multiple discussions on here about overdrilling and filling for hardware installations on a cored gunnel. The link below offers some interesting test results that show the failure of all fillers. Leads me to think drilling holes correct size and painting the core with neat epoxy and slow hardener may be the way to go for through bolt installations. Avoiding the use of all fillers. this sounds like heresy based on previous discussions here but the article below is worth a read.
Thank you for the Link and it is a very interesting article. It is a bit different than my experience.
A lot will depend on what you are trying to do. For the study that was "attaching light fittings to a deck". In the case of most places (such as screwing down a hatch), we know that just sealant leads in almost 100% of the cases failure. A thru bolt is entirely different that what was done in the study--the study only dealt with #8 sheet metal screws. Specifically the author noted that this was where the back side was not accessible.
Also they drilled a 7/16" hole, and then only removed 5/32 of core. (slightly > 1/8"). I try and drill a smaller hole, and cut back more core (finding that a Dremel cutter works best.), closer to 1/4".
I use a mixture of cabosil and high density filler--I have never done any destructive testing, since I have not put any upward force on what I have screwed down. As for bolts, the plug is far better, because it gives a solid core for the bolt and
back up fender washer to pull against, and avoid crushing the core. If the fitting is going to be subject to a high load (cleat, fishing rod holder, down rigger etc, then bolt, but put in enough epoxy structure to avoid crushing the core.
I don't know what the mode of failure was in the solid glass--if it was weakened by attempting to put the screw in--I have had #8 sheet metal screws shear off the head when attempting to put into solid glass laminate. Also I am a bit puzzled by the "solid glass" laminate--what was that composed of? It is not unusual for high end builders to insert solid materials, such as Coosa board into a laminate where there will be a fitting, such as a cleat. Even putting plywood in is better than the balsa or foam core for bolting purposes. I am not at all surprised with the failure of wood flour or micro balloons. I have always advocated to avoid those materials. There are many ways to insert glass into plugs, one is to use Milled or short fibers, another is to get fiberglass rope and use the strands, (I have done both multiple times) where re-inforcement was desirable.
Again, thanks for the thought producing article link!
