Fin Box Reinforcing

Question: do you reinforce your fin boxes before laminating or is it enough to just laminate the board? I just installed fin boxes on an old short board and will cover the whole thing with one layer of 4 oz cloth. I used a healthy amount of construction adhesive to set the boxes but I’m debating on whether to reinforce the fin boxes as recommended by Future Fins. Do fin boxes ever really pop out and does reinforcing really help?

I always make up a mixture of cab-o-sil and resin. Pour it into the fin hole and lay 2 layers of 6 oz. glass into the hole making sure that there is enough glass to come up the sides of the box and exten over the top. Then install an old fin into the box, and tape around the fin to make sure no resin get into the box. Install the box into the cut out hole. Move it around a little to make sure any air bubbles have been removed. Line it up using the fin to make sure it is sq. and setting just where you want it to be. Allow the resin to set up tell trimmable, and using a sharp knife trim off the glass/resin. As far as adding anymore reinforcement. I only do this if the board is real big 11 foot or so, and the guy plans on using a real big fin. If this is the case I will route out a slot down each side of the fin box that extens a inch or so past each end of the box, and install a piece of wood in the same manner that I installed the box. cab-o-sil/resin/glass. Never had one of these come out. I have done repairs on broken out fin boxes.

Bagman: Future Fins recommends laminating football shaped fiberglass reinforcing over the boxes before you laminate the board. It sounds like you skip this step typically. I don’t see how these football shaped reinforcing would help keep the boxes in. Thanks for the info.

probably because you’re building up the thickness of the glass in the fin area which in turn stiffens up the finbox area?

I always add the extra layers of fiberglass for sanding rather than added strength. Sanding around the plugs/boxes can be tricky, so I like to have a little bit extra to work with to prevent sand throughs.

It doesn’t seem to make sense to try to strengthen the fin box with extra layers of glass, since the box goes in after the board is glassed, and you cut through the glass to set the box. The strength comes from whatever material is surrounding the box itself: in the meat of the board. If the box was set in place before glassing, and some of the glass extended over the top of the box, then it would seem to lend a little resistance to tipping. I usually order my blanks with two stringers 1 1/8" apart so that the finbox is locked in on both sides by wood. Doug.

Howzit Bagman, I find that milled fibers work better than cabosil for a filler. I also stopped wrapping the box with fiberglass and only put a strip of 6oz in the bottom of the routed out area. Since i’ve been doing it this way I’ve noticed that the boards don’t get those unwanted cracks at the front of the box.Aloha, Kokua

Hey Kohua, I like the milled fibers idea. Your saying that if you don’t wrap the box that helps stop the cracking at the front of the box? Hummm. Is it the milled fibers or the lack of glass wrap? I can’t see how that would work. I will try it on my next repair on a fin box. I don’t do new boards, I just do repairs. Your way would sure make it easier. TK Bagman

Looks like 2 different types of fin boxes are being talked about here. The big fins unlimited style longboard boxes get routered in after glassing,and a nice butterfly patch in the tail during laminating is the norm.This more or less is for sanding purposes,being that you have to grind that sucker down with a 50 grit nasty-pad,and a little buffer between foam and glass is nice.I abandoned sinking these boxes with cloth a few years ago;a semi-runny mixture of cabo-sil,milled fibers,or micro-balloons works just fine.(I combine all 3). For a comp-finish board(no gloss coat),I usually laminate 2 layers of 4 oz over the sanded down box.No need to tape the slot either, the resin doesn’t drip thru if you use 2 layers.A roto-zip trimmer or carefully sanding along the slot will re-open it for use, and now it’s a little stronger @ mainly it’s more waterproof. I also rough up the boxes on the sides with 60 grit to help that plastic-resin bond be as good as it can be(I hate those micro-scopic separations that sometimes happen after grinding.)Now, to the Futures…I’ve only been installing them for about a year,but again,the football ovals that lay over the boxes(underneath your main bottom layer) are more or less to help with grinding down the excess box that sticks up above the bottom of the board. Plus that leaves at least a couple layers of cloth over the flange part of these boxes(for strength).About the only way I can think of to re-inforce futures is to do the obvious things such as 1.Wipe them off with acetone before sinking,2.add some milled fibers to your sinking-batch of resin,3.put more than one extra patch over each box.(The company actually says to put a minimum of 12oz. of cloth over each box.Using 4oz. cloth,that would be 2 ovals and your bottom layer.Using a bunch of 6 oz. layers could lead to obviously darker zones around the fins as well as being a pain to fight the air bubbles that try to stay around the raised part.

Glad you caught the two styles of boxes (I was a bit conecrned that someone would try to install Futures after glassing). The boxes that are installed after glassing have an inerently weak transition from the box material to the surface glass. Over time and with power surfing (or doorway boo-boos) the transition begins to crack. The same thing happens to leash plugs. A bit of extra glass in the surface around the box can keep this from becoming a delam - but if I really want a board to last I often lay glass over the box after insertion and cut it to the fin hole - leaving a layer overlapping the box flange - or plug flange. In general I prefer Futures since their design eliminates this weakness (they are inserted under the glass and already have the overlap by design). All my big wave boards sport extra glass over plugs also - I dont need one of these to pull out.

does Future have a website for the fin boxes??? or anywhere with good info?? appreciate it…

Let me ask a different question (I am already learning a lot in this thread). If YOU were designing a fin box for installation in a board, how would you spec it out? Last time we talked about materials - the importance of glass-reinforced plastics and Tom’s preference for polyurethane. But how should the side of the box be shaped? small holes in the side to support resin? Installed before or after glassing? Assume you are designing the box, and you get to make the call. My best guess is to put 0.050 deep grooves in the side of the box, rough it with 50 grit sandpaper, epoxy is in place, use a football patch, and then glass over it all. I am assuming I’m not interested in the Red-X style mount, which has clear advantages, but requires a bit more from the glasser at installation.

Well I’m glad you guys are finally figuring out that the “capping” method is the way to go! That’s what it’s called when you glass over a box. The seperation you see around leash cups and fin boxes is a combination of shrinkage(the resin is pulling away from the plastic) and heat from a worn out grinding disc. If you use a fresh disc, you will minimize this by grinding it “cold”. Most fin boxes have a melting point of around 140 or 150 degrees. If your disc surfaces it before it has a chance to heat up and soften the box, it won’t seperate. Also attempting to surface the box within a couple hours of gell time causes sepo’s too. If you cap a fin box and cut the hole back out, you’ve sealed the bond between board and box, eliminating the chance of a separation at that point. You also keep water out that eventially will weaken the area around the box and cause failure of the box. Water doesn’t compress, so if you have water working it’s way in around the box, eventually it will start to force the box up and out. Any fin box that is installed after the hotcoat,and not “capped” is vulnerable to this separation as flex moves through the tail from lip impacts and standing right over these boxes and pumping the board. The patches, whether they be football patches or butterflies, placed onto the foam under the lam layer,reduce flex locally around the box, minimizing the chance of cracks ever starting. One layer of glass (especially 4oz) around a bahne, or future, or lokbox etc… will allow movent of the box within the board,and cracks to form. When multiple layers are used, the strength(locally) increases substantially. The best installation is when the layers are squeegeed nice and flat, and the “cap” layer/s are applied flat and clean to avoid them being stripped off in the sanding process.

Howzit Bagman, Another thing is I punch a hole on each side of the stringer at the front and the back of the rout out about half way down with the small end of a chop stick. I’m sure this also helps. Aloha, Kokua