filling in large voids?

I have a '67 24' fibergass cuddy cabin that I'm in the process of restoring (the whole boat, not just the cabin). There's a section of the keel, right at the entry below the water line, that has hit a cross-beam of the trailer enough times over the last 40 years to break through the fiberglass. It's been patched at least three times in the last 20 years.
After grinding the old material away for yet another fix, I stuck a nail up into the hole to see what was behind it - and it was nothing. I figured the "keel stringer" must have rotted away in this area. I went into the boat, chopped out the front 2 feet of the keel stringer (a heavily glassed in 2x4) and found it to fine - wet, but very little rot. It must have been preserved by bilge water . Whatever was used as fill ( up to 1" thick at some points) under the 2x4, however, had pretty much disintigrated. It seems to have been tufts of balled up glass and some kind of filler which had degraded to the point where I could pull it out with my fingers. Basically whatever crap the builder had.
I'm going to splice a new 2x4 back into it. I guess my question is: What do I use as bedding/fill for the new keel stringer? I have epoxy and various fillers; but this seems very expensive bedding. Would bondo or something similar work?

 

Maybe I'll just use a 4x4 instead and shape it so it approaches the bottom of the hull more closely - that way I wouldn't have to use so much epoxy.

 

Bondo is a no-no, add enough material (fabric and goo) to make the laminate at least as thick as it once was, thicker wouldn't hurt. Since the remaining portions of your hull, likely have the stringers and other structural elements bonded directly to the hull shell, worrying about a hard spot in this location is moot. What you're possibly seeing is mat that didn't get wetted out properly when the keel batten was installed. Cut you new keel batten scab to fit tighter against the hull, bond and tab well and you're just left with fairing and paint.

Don't be tempted to use a big hunk of wood to fill the void, use a wooden laminate sandwich if you need the bulk. Epoxy doesn't like to hold onto big pieces of wood well. If you need thick pieces, make them up from 1 by stock, glued together, so the internal stresses within the wood and moisture gain can't bite you in the butt.

 

Thanks for the advise. I will do exactly that. It'll be more interesting that way as well .

Speaking of the keel batten and hard spots-

There are three, roughly evenly spaced, cross members made of 2x12 that run between the stringers. They follow the contour of the hull down to the keel batten; but flatten off about 3/4" above it and never touch it. They are fully glassed to the hull and stringers with woven roving; but not the keel. Is this to avoid hard spots, or just a factory short cut to avoid custom cutting each cross member to fit tight between the flooring and keel (they are an integral part of the floor structure) ? I'll be cutting new ones regardless, so I'm not sure if I should bring it down to the keel or not. Also, because the keel batten is raised above the contour of the hull, there will still be plenty of room for bilge drainage either way.