Core for daggerboards

I posted this in another part of the forum and got no replies. Maybe here will be better. What I really need to know is how and what type of core goes into a daggerboard made by mating two halves. So many foils are made this way, there must be a common practice for what goes inside them.

I am building new daggerboards for my Macgregor 36 catamaran to replace my erstwhile and apparently poorly designed boards that broke at the hull exit (of course). Lots of mistakes on the earlier ones: Essentially, for a 450 mm chord, I made a couple of fiberglass I beams about 125 mm apart, glued on A600 corecell, shaped the foil and vacuum bagged the skins. Neither I beam was at the maximum width, and you can probably guess what happened. Examination of the stubbs shows evidence of the board crushing and then breaking off.

For the new boards I want to skip the endless fairing, and am building a female mold in the manner recently described in a pdf by Kurt Hughes. I can use the same mold to vacuum bag 4 skins, and then join them to make two boards.

So what do I do for the core? I want to vary the skin thickness to have the maximum carbon at the hull exit point, and laying in a rigid core seems impossible. I am open to suggestions. I have thought about just building up a solid section of fiberglass about 100-150 mm wide on each skin, and routing it flat when I trim the leading and trailing edges. That might be heavy, but a) I know it would be virtually incompressible, and b) would automatically fit the uneven inner surface of the skins.

In my research, I read of people talking about pourable epoxy foam for this, but all I can seem to find is urethane foam, which I understand is unsuitable. I came across two other ideas that seem attractive for weight and cost issues. One was to make up a bog of epoxy and microballoons, which really is a sort of epoxy foam. This could fill the cavities around my solid glass sheer web, or perhaps be the entire core. Anybody have any experience with this?

An even lighter, and cheaper, core was suggested in a boat design forum: using styrofoam beads. Normally I would not consider styrofoam, but the guy claims a good pedigree for the idea: "the epoxy /Styrofoam mix is an idea i picked up from Kiwi multihull designer Malcolm Tennant as it was something used to fill rudders and daggerboards on his cat designs. The materials are easy to get,the styrofoam beads are what is used in a bean bag chair, or should be available in a craft or fabric store for next to nothing, with the epoxy you want to use the slowest hardener. Ive used this method for rudders on a Searunner 31 tri, a macgregor 36 cat and monohulls as well as several bulb keel struts.
Steve."

Has anyone else ever heard of this? I can get a large supply of 1-1.5 mm beads pretty cheaply, such as here: http://www.milkbottlefill.com/. Seems there is a whole milk bottle collecting community on the web. Who knew!

Sorry for the length. All thoughts appreciated.

 

Either construct things such that a core is not needed, and indeed use something light and cheap, or use a constructive core.
The constructive cores break down in 2 variants: sheet foam (A600 should be enough, but at the exit point, where I would use high density balsa or a very thick laminate.)
The other indeed is pourable epoxy foam. Sicomin in Europe has it, and I have a customer using it for daggerboard production. No idea of availability in the USA however.

 

I've build mostly hollow foils, but I broke both hollow and cored foils.
On the point where foils come out of the hull or casing, there will be a lot of compression. Once you'll get a failure in your core on that point, the foil will go. I recommend a hard point in the foil there (at maximum thickness), and the rest either hollow or filled with a foaming epoxy. The last has some constuctive value too.