Hi all- I’m in the middle of my first attempt to replace a center fin box on a longboard (9’10"x23" Anderson log, 10"Y true ames fin, I paid $100, still looking forward to riding it. It has a printed Scott Anderson signature laminated in the glass near the finbox, but says “Dane Peterson #85” hand written on the deck, so I’m not sure if it was shaped by SA for DP, or jsut some kind of DP limited edition shaped by SA, or if DP shaped a few boards at SA’s shop- but that’s a separate issue, which won’t make me surf any more often, or any better, either way.)
Anyway, I used a router to remove the old box, which appears to have been held
in place with resin and qcell or some other kind of filler. The box
was cracked and the edge of it had lifted out of the glass, but the rest of the bond was secure, so this was a chore. I’ve discovered what looks like some kind of cloth (looked white, but similar to 4 oz glass?) under the fin box- hopefully it will show up in the picture.
I’ve only worked with glass ons before so I’m inexperienced with boxes, but I haven’t heard of putting cloth under the box. Has anyone heard of this, and/ or think its a good idea? I guess it makes sense because based on my experience fixing dings, qcell can be a little brittle. But, a system is only as strong as its weakest link, so I’m not sure if the glass was doing anything helpful. The box was certainly secure, (evidenced by the fact that the box was still secure after it cracked- also, the fin has a good size nick in it which I suspect is related). As I was removing the box, the bond failed either at the interface between the foam and filler, or at the interface between the filler and box, so maybe the glass didn’t do anything- like strengthening a single link in a chain. On the other hand, if the qcell was less likely to fracture, maybe it could rely on a greater surface area/bond between the box and foam which would make the whole thing stronger?
Does glass unbder a finbox help?
Maybe what I thought was cloth was something else?
thanks for all the tips. here’s the final result: I went with glass under/around the box, black resin/filler and two sheets of glass over the box. No additional wood. the resin cleaned up pretty well, but the edges of the old glass weren’t perfect so its still a little rough looking. I’ll let you know when it fails again and/or gets eaten by seals.
Sorry buddy. Hope you can clean it up with the sander. I've done FCS plugs, leash-plugs, fin boxes etc with colored pigmented ground or milled fiber. Because you are working with straight lines at the fin box(10 1/2")the tape- off will give you a clean straight edge with minimal clean-up. I've done them in white, black, red and blue. lowel
I am going to tell you all (some of you already know), the strongest center fin box install in the world is a "Chinook" box with glass in the slot and wooden "wedgies" or a "Chinook" box sunk between two center stringers at 1" or 1 1/8" with a glass tail patch over it all. WOW! I'm trippin'. Don't know how that happend.
Some of you already know; The strongest center fin box is a Chinook box with glass under it and wooden wedgies along side or a Chinook box between two 1" or 11/8 " stringers with a tail patch.
You are correct about 'the weakest link.' This often is the surrounding foam. I've seen the Chinook boxes (with serrated edges to "increase bonding surface") blow out the same as normal FU boxes. The block of box/resin crunches in to the foam just the same.
If you think about the forces involved when your fin strikes a rock and the structural aspects of a single stringer, you'll likely agree that the other weak link (arguably the weakest) is the violated stringer immediately in front of the box... a likely spot for stress cracks to develop or even for the tail to snap off completely.
If weight isn't a huge issue, your best bet on future boards is to order a double stringer spaced so a box can fit between the stringers. Another option, and one I would recommend for you on this repair, is to route two slots - one on each side of box. Extend the slots a few inches past the box - maybe a bit more on the forward side, and resin in a couple of pieces of wood. If you line it up right, the resin will make contact with the box and the wood and provide a far stronger installation. It will be more resistent to sideways forces and to inline fin strike forces.
On a recent board I made I used a double stringered blank - I cut slots cut horizontally inside the routed slot so there is a resin bridge from the box directly to the wood about 3/8" under the surface glass. I lined the slot with some cloth - just because that's the way I've always done it -not because I want to start WWIII over the risk/reward ratio or anything, I also capped the whole thing with glass so the seam is fully sealed.
It's not a technique you'll see on a production line but I can show you plenty of boards where the fin box installation has failed. By "failed" I mean leaks, cracks or broken tails. It's an area of the board that gets a lot of force working against it. With fins sticking out, it's also vulnerable to striking rocks, sand, other boards and or bodies.
IMNSHO, glass under the box provides no useful function. I’d not replace it, just a humbug to install and trim. Better to inset the box below the surface and fair in one or two layers over the edges of the box. Given the small area of overlap even that won’t provide much but it’s the best one can do.
This is the standard procedure on most surftech, hawaiian island and other pop-out style (and even some standard style) epoxy boards. Typically it’s done when the board is made of an extremely light foam (think 1lb EPS which usually requires a veneer on the deck to keep it from denting and dinging). The reason you line the box with glass is for extra strength since the foam itself isn’t really strong enough to hold the box in place with just the resin/q-cell mix. The torquing and flexing of the fin over time would create a cavity around the box and weaken it without the glass present. Every SUP I’ve ever repaired has had this tecnique.
If your board is EPS you should absolutely be sure to seat glass in the box with your filler mixture to ensure the strongest repair possible. If the board is standard-density PU foam it’s not really necessary, but does still add some more strength to the overall repair which is never really a bad thing. Looks from your photo like it’s a PU board but I can’t really tell if that’s the case or if your router just cut the foam so smooth I can’t see the beads.
Most importantly when replacing your box though, is to be sure you prep the box itself with something like 60-80 grit paper. This will ensure a good mechanical bond of the resin to the box itself. Every box I’ve ever pulled that followed this step has resin/filler stuck all over it and is a bitch to get off - any box I’ve pulled that didn’t have this step comes out with almost no resin attached to it, meaning it is a weaker install and more prone to failure. Don’t just depend on the ridges and channels on the box to hold it in, always sand!
When you re-seat the box, if you choose to use the glass-under method, be sure to cut a piece that will nicely line all 5 sides of the hole you routed out, fill the hole with your resin/filler mixture and make sure the glass is fully saturated before you insert the box and add the rest of your filler.
…the other day came to the wshop an egg that I made couple of years ago with a 10.5 FU and a 7.5 fin. The board went through the rocks and hit the fin, the break away fin plate acted as a fusible so the rear edge smashed the back of the box slot but nothing more occurred with the box; then I checked the plate but did not broke actually, the bronze thread part pulled out from the plastic flat part and stayed with the screw, so I unscrew and put it again on the plastic part with a hammer and ready to surf.
That box did not have fiberglass into the box only 2 layers on top.
Thousands of windsurf boards, poly glass and clark foam, and hundreds of big wave boards of the same material used double footed, 5 side wrapped, then woodies beyond the main stringers and in front and behind.
If you want it strong, you have to make it strong.
Ever see a picture of a big wave rider from under water during his bottom turn? The fin flexes like you'd never ever imagine.
Thanks for all the insights. To clarify, the board is pu/pe, heavy glass. As I mentioned, it has a giant fin (for now) so I’m thinking I should get that box in there as solid as possible. THere are some good options here, so I’m not sure what I’ll go with, but I appreciate the advice and I’ll let you know if I ever end up having to re-do my work.
On a separate note, has anyone ever used pigments in filler? My local surfshop only carries black boxes, and I’ve got some opaque black pigment sitting around somewhere… It will probably end up messy and be more work than its worth, but it got me thinking…
I seated a black leash plug into black carbon fiber with some black resin pigment on my last board - just tinted the pigment super black and set it off extra hot to offset the pigment retardation - you don’t need to do the extra catalyst if you don’t mind waiting an extra few hours-days for it to cure, but I’m impatient and always go for the quick solution. It came out pretty good using q-cell and resin pigment. Super stealth-looking leash plug
