I getting ready to do my first center fin box. I’ve been using the ofishl snap in boxes with no real problems except for my last post about the crappy resin job and post cure bubble. Anyway, I just finished building a jug for the 10.5" box. I made it so that it fits real tight. I can just barely squeeze in one layer of 6oz cloth underneath/sides the box. I read with interest the post about re-enforcing single fin boxes and it seems to make sense. The ofishl box has straight sides, no concave. It is also capped and it appears that you can glue in the box before the lam, glass over the lip on the lam, and then grind off the top cap after the hot coat. My question is whether my jig should be a little larger to fit more cloth/resin in or it is better to keep it really tight? Less resin will keep the heat to a minimum, but should I worry about squeezing the cloth dry as I stuff it? We all like a tight fit. But is there such a thing as too tight?
Your jig sounds good. It is supposed to be tight. The ofishl center fin boxes are made to be installed before you glass just like the sides. Make sure the front and back of the fin below the cap are slightly below foam (about two business cards worth of space), then glue it in without cloth. Then cut a small patch to be put around the box by about an inch just as you would the side fin boxes. Then you cut a slit in your glass for the bottom, so the patch is under your regular glass.
If you need pics of each step for a better visual, let me know. I’ll be gluing in some boxes later this weekend.
Like Schroeder said, its supposed to be tight. You won’t ‘squeeze out’ the resin from the glass putting the box in, because you do it like this: Mask off the foam or lam around the hole, so any resin that squeezes out doesn’t make a mess on your board. Move the board forward on your stands until the box area is level. Pour the hole about 1/3 full with resin. Wrap a layer of dry glass around your box. Push it into the hole. The resin will squeeze & wick up into the glass and even squish out around the box a bit. Tape/brace until set. Grind it flush & pull off the tape and you’re ready for lamming or capping. That’s a good install…
Forgive me, but I love this stuff.
Toughness
You could say that there are at least two issues here; bonding and toughness. The bonding issue is pretty clear. There’s doesn’t seem much controversy regarding the need to have the most appropriate bond possible between two materials, whatever the required bond characteristics. However, the toughness issue is often under appreciated. It relates to crack propagation, and that’s where the composite matrix of resin/fiberglass comes in. A good composite matrix stops small cracks from becoming big cracks (among other things), and the last thing you want in a single fin board fin-box installation is a big crack.
Resin/chopped-fiber mixes are adequate for some installations, like FCS installations, leash plugs, etc. What tends to characterize the use of resin/chopped-fiber applications is the small amount of filler used –e.g. the relatively thin annulus of filler around the FCS plug, etc., you could say, most of the time it’s used as sort of a reinforced glue. However, it’s not the best material to use structurally.
The thickness of the material between fin-box and foam, as well as the expected level stress on a single fin board fin-box warrants a maximum composite application to prevent small cracks from becoming big cracks –i.e. crack propagation, hence the layer or layers of cloth between box and foam in the installation.
Using a resin/chopped-fiber mix in this kind of an application would be sort of a crapshoot. Maybe the chopped-fiber resin matrix will have enough of a composite nature to stop a crack, or maybe it won’t. Usually when then region (volume) is small enough, the chances are good that it will –i.e. where the crack’s directional options are limited. But as soon as you get a little volume, the crack will find a way and propagate. (See reference below for an excellent explanation as to why cracks propagate.)
A Word on Terminology
Toughness may or may not mean strength, at least with respect to the common usage of the term. Here the actual strength of the composite comes from the materials used to make it –i.e. glass and resin. Toughness tends to refer to preventing failure, in particular preventing crack propagation. It usually comes as surprise to most to find that resin/fiberglass structures are riddled with small to micro cracks, even though they look completely new (or are new.) Ahhh, but the neat thing about composites is that they are tough, or they can live with cracks, without failing.
Oh, and one more thing. The fin boxes themselves are not the most structurally sound objects. I suspect the makers actually expect you to use the fiberglass wrap reinforcement that some of the posts above have described.
Kevin
References:
The New Science of Strong Materials or Why You Don’t Fall Through the Floor, J. E. Gordon, ISBN 0140209204
Howzit KC, I use the same mix for boxes as for FCS installs and find it works great. Only put a layer of glass in the bottom of the hole,not around the sides of the box. One trick is to punch a hole on both sides of the stringer at the front and the rear which seems to prevent any cracking from stress. Since doing my box installs this way I haven’t seen any cracking and the box is set firmly in the board. I suggest you make the mixture a day before using it so all the bubbles go away and gives you maximum strength. Aloha,Kokua
Sounds like an interesting twist (no pun.)
The whole point being that maximum ‘toughness’ is achieved with a layer of glass. But there are other benefits, less shrinkage as it cures therefore a better bond (doesn’t pull away from the foam as much), it’s also smidge stronger if you’re a little heavy on the pigment, and a feeling a safety if you’re a modest overkill freak (like me.)
As for making a prep ahead, sure but not because of bubbles, it’s just convenient.
Bubbles use to drive me nuts, in the fin installations, in the deck, anywhere they might appear, but for me the occasional small bubble was more of a cosmetic problem (unless things started to look like a carbonated soft drink.)
Bubbles, which have a comparatively large radius (with respect to the diameters at the ends of cracks), are actually crack stoppers. For example, one way to stop a crack from migrating in a sheet of metal is to drill a small hole at its tip. In fact there are any number of plastic objects out there that have curious round ends to a slit or cut, see picture. So I would tell myself this, and do my best to forget about the occasional bubble – it sort of worked.
Crack tips concentrate stress, and the smaller the tip the more concentrated the stress. In fact any sharp corner concentrates stress. And being into modest overkill, I would tend to suggest that when you dig out a fin-box hole you might want to leave the corners rounded (which comes free of charge, if you router.) It will at least delay if not reduce the chances of the appearance of those corner cracks that occasionally appear on some single fin fin-box installations.
But it sounds like you’ve got things down to where they work for you. No controversy here. In fact, interesting technique, thanks.
Kevin
