Drilling holes in cored material

mazzy · #1 (**permalink**) 09-20-2008

I have to mount new gudgeons on my new rudder, which is fiberglass with a foam core. The standard procedure, from what I've read with West Systems, Dan Casey etc, is that

the holes should be first drilled oversize
some core removed, wilt an "L" shaped bit in the drill
everything filled with epoxy
then the hole is drilled to the true size

My question: what's the advantage of drilling oversize? Why not drill final size, but follow the other steps? The core is still going to be protected, but a cleaner, smaller hole is drilled thru the fiberglass.
Mike

sailingdog · #2 (**permalink**) 09-20-2008

If you drlll the hole the final size, you can't get a bit in to ream out the core material. BTW, you need to use a THICKENED EPOXY... unthickened epoxy is rather brittle and tends to chip and crack.

brak · #3 (**permalink**) 09-20-2008

i just did the same thing yesterday (on deck, for a new frame support). you don't have to drill oversize, really - but often it is difficult or impossible to remove core (especially wood core) and drilling oversize guarantees that at least some layer of epoxy will remain around the fitting once the hole is redrilled.

I usually drill with appropriately sized bit and just widen the hole a bit by turning the bit around a few times.

mazzy · #4 (**permalink**) 09-20-2008

sailingdog,
thanks for the insight. I am thickening with West 406 (I think it's 406)
brak, the core's foam. Fastener's are 1/4" so I'll drill 3/8 first. I happen to have the bit with me, so it'll have to do!
Mike

brak · #5 (**permalink**) 09-20-2008

i prefer unthickened epoxy for a job like that. to get a better structural property out of epoxy it has to be thickened quite a bit, which (depending on size and type of holes and their position) will make it difficult to achieve a uniform and complete fill. Arguably voids are not very desirable in this sort of installation.

sailingdog · #6 (**permalink**) 09-20-2008

voids are pretty easy to avoid if you use a syringe and go slowly. However, pre-coating the core with some unthickened epoxy is a very good idea.

Quote:

Originally Posted by brak

i prefer unthickened epoxy for a job like that. to get a better structural property out of epoxy it has to be thickened quite a bit, which (depending on size and type of holes and their position) will make it difficult to achieve a uniform and complete fill. Arguably voids are not very desirable in this sort of installation.

GaryHLucas · #7 (**permalink**) 09-20-2008

I am assuming that these are through bolts, not screws. If so, then there is another issue. A 1/4-20 fastener can supply a clamping load of 3,000 or more EACH! The coring material probably can support 500 pounds per square inch at best. So the bolts will crush the core and everything will loosen up. By removing the core and replacing it with a donut of epoxy about 1-1/2" in diameter, with a compression strength of maybe 5000 pounds per square inch you provide proper support so crushing doesn't occur. This is at least as important as keeping water out.

Interesting little story along this line. I was at the Atlantic City, NJ sailboat show about 10 years ago and I was sitting in a 32 foot J-Boat that was very nice. I made a comment to the guy showing it, about the chainplates with HUGE 3/4" bolts through the bulkheads with Nylok Nuts. The guy said the bolts weren't large for strength, they were large to spread the load an prevent them tearing through the fiberglass bulkhead, and they were only tightened until snug so they don't crush it. I thought "Wow, nice engineering" and it was then that I looked at his name tag. It was Rod Johnstone, Oh!

SVDistantStar · #8 (**permalink**) 09-20-2008

How about useing unthickend epoxy just to fill in holes? I just removed 2 winches that were for the traveler from my cabin top and have a bunch of little holes now. I was just going to put some tape on the bottom of the holes and fill them in with epoxy.

mazzy · #9 (**permalink**) 09-20-2008

I filled holes in the coachroof with MarineTek epoxy. Dried white and I left a little dimple to hold a bit of gel coat touchup.

I'm thinking,along Gary's comments, to make the holes a bit oversized, but scoop out a good inch diameter of core and, using a syringe, cram that with epoxy. That'll give me a high compression donut to support the loads.
Mike

sailingdog · #10 (**permalink**) 09-21-2008

BTW, while you're doing this, you should countersink the tops of each fastener hole. That will give the sealant a place to form an "o-ring" type seal and make them much more resistant to leaking.