Alternative to the "bent nail" technique for holes in cored decks

[ramblinrod]

Originally Posted by GrahamHO

The "strength" is required to prevent compression of the core material when a bolt securing a fitting is tightened. If the core material is compressed the bolt cannot be tightened correctly and eventually water will penetrate the core and in the case of plywood, will rot it. It requires a reasonably wide column of epoxy filler to resist this compression, as well as the working load. Foam will compress. Plywood will compress. End grain balsa resists compression best but doesn't like getting wet. They all need the epoxy treatment. Personally I use a 20 mm dia column of epoxy (using a hole saw) for a 6.5 mm bolt hole. And of course countersink the top of the hole and use a marine sealant under the top of the bolt / fitting. In order to countersink the hole for better sealing there should be an adequate width of epoxy.

I don't think you understood my post, the prime thrust is that the bent nail trick (effort to remove core around the clearance hole, to fill with epoxy) does not really gain one anything. For a bolt that needs a 6 mm clearance hole, I simply drill a 12 mm hole right through the lay-up. Seal the bottom with tape, fill with epoxy (low viscosity to soak into the core fibres), and then redrill the 6 mm. By using a 40 mm O.D. fender washer backing, one has the compression resistance of the epoxy (lots) and original core beyond the 12 mm dia filled hole.

RE: epoxy / core bond. No worries, that is what the epoxy is designed to do, laminated wood is totally dependant on the epoxy/wood bond.

And the actual stress on the bond is very limited, because as you have correctly indicated, the stress is actually compression not tensile.

Anyone who wishes to do the bent nail trick is certainly free to do so, but in my humble opinion, it is totally unnecessary and takes more time and effort than it is worth.


Where I do use the bent nail trick (actually just gouge balsa out with the screwdriver) is when I blind mount a fitting exposed to shear stress, like a deck organizer. In this case, I drill a hole at least twice the size of the fastener, gouge out a bunch of core material (as far as I can reach) beyond that, wax the bolt, screw a nut and washer to the bottom, set the bolt in the hole with nut and washer at the bottom, and then pour in the epoxy. I then grind the mating surface of the organizer to key it, and bed it with 3M5200, screwing the mounting bolts through the organizer into the now deck captive nuts and washers.


The result is a tenacious bond that will never let go.


Beats drilling umpteen access holes through a beautiful pristine liner.

[GrahamHO]

Originally Posted by ramblinrod

I don't think you understood my post, the prime thrust is that the bent nail trick (effort to remove core around the clearance hole, to fill with epoxy) does not really gain one anything. For a bolt that needs a 6 mm clearance hole, I simply drill a 12 mm hole right through the lay-up. Seal the bottom with tape, fill with epoxy (low viscosity to soak into the core fibres), and then redrill the 6 mm. By using a 40 mm O.D. fender washer backing, one has the compression resistance of the epoxy (lots) and original core beyond the 12 mm dia filled hole.

RE: epoxy / core bond. No worries, that is what the epoxy is designed to do, laminated wood is totally dependant on the epoxy/wood bond.

And the actual stress on the bond is very limited, because as you have correctly indicated, the stress is actually compression not tensile.

Anyone who wishes to do the bent nail trick is certainly free to do so, but in my humble opinion, it is totally unnecessary and takes more time and effort than it is worth.


Where I do use the bent nail trick (actually just gouge balsa out with the screwdriver) is when I blind mount a fitting exposed to shear stress, like a deck organizer. In this case, I drill a hole at least twice the size of the fastener, gouge out a bunch of core material (as far as I can reach) beyond that, wax the bolt, screw a nut and washer to the bottom, set the bolt in the hole with nut and washer at the bottom, and then pour in the epoxy. I then grind the mating surface of the organizer to key it, and bed it with 3M5200, screwing the mounting bolts through the organizer into the now deck captive nuts and washers.


The result is a tenacious bond that will never let go.


Beats drilling umpteen access holes through a beautiful pristine liner.

Yes I understood your post. I may not be a genius but I was still able to understand what you wrote, even if from thousands of miles away you think otherwise. I wasn't criticising you. I was actually agreeing with you, that there needs to be adequate epoxy around a bolt hole in a cored layup.

The difference between what you do and what I do is that you drill all the way through with a 12 mm drill and I drill from below with a 20 mm hole saw but I leave the upper (outer) skin intact. Then I pop out the plug with a screwdriver before filling until epoxy comes out the pilot hole. I consider my way, on my boat, to be cosmetically better, and possibly stronger as the outer skin is mainly still in place. I have drilled seemingly hundreds of holes that way. Your way is what a "professional" might do to save time and money but is not necessarily "better".

I actually used something like your method last week when I modified my bow pulpit and it stretched slightly with distortion from welding. I had no suitable access from below for the hole saw. When I replaced it I had to drill right through from above with a 15 mm hole saw. The feet of the pulpit covered the hole in the outer skin.

I don't think either of us like the idea of a bent nail. I was simply providing details of "my way".

[cwyckham]

I think Mainesail has the definitive resource for the dremel tool method. It's a fully illustrated guide as all of his stuff is.