Sink box with Q-cell and resin only.
Cap the box with one layer.
WAY strong. Much better than sinking the box with glass and not capping.
Sinking the box with glass, and capping, is overkill, if you are going to cap there is no benefit to glass in the sink, it just adds weight.
Blakestah, maybe you don’t surf over shallow rocks as much as I do 
As I said above, both failures are very common so there’s a reason for both types of reinforcement…
Of course, I’m also talking single fin longboards where the fin & its impact are large. Small boxes require a lot less reinforcement because the damage caused by impact on a small fin is generally less severe…
Interesting.
I lied about the hearsay. On my first longboard I didn’t put the extra layers in the hole. On a nice day, maybe shoulder high, I blew the box out on a bottom turn. I had used a resin/fiber mix a friend was using for his FCS repairs and installations, etc. I just figured it would be enough. It wasn’t.
Also, slapping an extra sheet on top around the fin, though unlikely to make much of a difference in terms of hydrodynamics, it would have been a ‘hump’ that would have driven me nuts. And I’m not that convinced that given the amount of overlap with the box (and filled region), it would be more than a band-aid.
But it seems there’s plenty of opinion otherwise. Like I said, opinions differ.
Kevin
By the way, you might get a kick out of the following books. They read like novels and can probably be picked up used for a few bucks each on the Net.
The New Science of Strong Materials or Why You Don’t Fall Through the Floor, J.E.Gordon ISBN 0140209204
Structures or Why Things Don’t Fall Down, J.E.Gordon ISBN 0140219617
The chapters on crack propagation and material failure in general, in composites and otherwise, are a treat.
(Edit 10/10/06. Added the references.KC)
Ben/Dave,
When you cap off a finbox with glass, how do you protect the slot from resin getting in and then open the slot back up after the resin is gelled/cured? masking tape/X-acto knife, or is the slot cut out of the glass cap beforehand? I imagine the glass cap will also need a ‘hotcoat’ as well. Is all this done before or after hotcoat of the main lamination?
Thanks,
JSS
1/2" tape, leave a little sticking up at one end like a handle…
Cut off/out with exacto…
MErcy, considering this is my first board… take my advice with some caution. i have already done one hotcaot on my board. So im sanding down around the finbox, then going to lam a patch over the box (which will be taped off very well). I then plan on cutting the tape out after the epoxy has begun to kick. then a little cheater coat to fill the weave(if necessary). then i will be ready to “gloss coat” my whole board. hopefully this time it will be free of dry/pinholes, and actually look glossy.
in the pics you can see a few air bubbles around the box, i plan on dripping some resin in them before i put cloth over the edges of the box???
Anyone have suggestions on the capping of the box??? should i just cut a slit out of the cloth beforehand??
i plan on making a few fins for this board… anyone have a similiar ride??? i would be interested in some favorable fin styles for lined up point breaks, i will be taking the board to baja pretty soon. that fin on there now is a yater 7.5 rudder. came off a buddies longboard, worked well on my funshape, but i wanted to get a more flexible one for this 6’6" egger. i havent found any good templates on here, any links???
here some of the recent progress. you can see how bad the 1st HC went. but i kinda like the added weight, this thing is gonns be a cruiser!
Im getting excited about taking it out. How soon is too soon after my final HC??? Im in FL, so temps are 80s-70s most time. ive heard about 3-4 days should be long enough. using RR w/ Fast H, ADD F
Blakestah, maybe you don't surf over shallow rocks as much as I do ;) As I said above, both failures are very common so there's a reason for both types of reinforcement...Of course, I’m also talking single fin longboards where the fin & its impact are large. Small boxes require a lot less reinforcement because the damage caused by impact on a small fin is generally less severe…
This is worth a little digression into the forces on the fin and finbox, and how the installation deals with them.
In normal usage, the main force on the fin is perpendicular to the fin, and near the leading edge. This force is transferred to the glass near the fin leading edge, and is distributed forward and rearward on the box. It also transfers to the stringer as the box is mounted on the stringer. If the box is not capped, the force will hit the glass from the side. Typically, since the box is installed with resin, there is enough contact between the resin and glass to transfer the force. The force transfer depends entirely on the surface area of contact, and this is poorly controlled. The FCS repair manual even recommends removing foam at the glass margin to ensure good contact. But this is still quite weak. And prone to water leaks. (FCS is not weak, but the bond from finbox to hull glass IS weak without a patch).
So, cap the box. Grind the lip on the box down an appropriate amount (1/4 mm or so), lay glass patches, feather into the existing glass, and then hotcoat. Then the force has a large area in which to transfer the load to the surrounding glass, and you’ve dealt with the leaky problem.
BTW, boxes that are wider get much much stronger, because the width of the box acts as to give leverage to the glass. This is the main strength difference in various boxes that all install in the hull glass only (Futures, Lokbox, FCS if not drilled through) - the width of the box near the leading edge of the fin.
OK, another prominent failure mode is “I ran into a rock”. This pushes the box rearward and towards the deck. The box SHOULD be mounted on the stringer, and with or without glass in the hole SHOULD adhere to the stringer well, and transfer this force to the stringer. There is no where else for this force to go. If you sink the box with glass, the glass will transfer the load to the hull glass (barely), to the foam (which will yield readily), and to the stringer. In other words, the force is going to the same place anyway, with no net gain in strength. Glass weighs more than Q-cell, so you’ve added a significant weight to the installation, with no gain in strength.
Of course the hole should have been routed cleanly and tightly so there is a large good area of contact with the stringer to begin with.
I’ve installed with no glass in the hole and no glass patches and had problems. I’ve installed the same with glass patches, no problem. I see no benefit from using glass in the hole AND using glass patches. Just using glass in the hole, without patches, is barely better than not using the glass at all, and is still prone to leakage. Installing in a tightly routed hole with Q-cell, then capping, is by far the best combination of weight and strength.
Some fin companies drill through to the deck. This is stronger. But a digression from the topic at hand.
To keep resin out of the box mask it off. Or, as a few drops of resin will barely stick, you can get them off later, but an ounce of prevention…worth at least 50 grams of cure.
As to having a hump there, it is not a problem. Simply grind the 1/4-1/2 mm off the box before you start. Drop the glass patch, cover with wax paper, and finish it well. A good glasser or ding repair guy can make this patch completely invisible.
hth
Ben/Dave,
When you cap off a finbox with glass, how do you protect the slot from resin getting in and then open the slot back up after the resin is gelled/cured? masking tape/X-acto knife, or is the slot cut out of the glass cap beforehand? I imagine the glass cap will also need a ‘hotcoat’ as well. Is all this done before or after hotcoat of the main lamination?
Thanks,
JSS
I just used masking tape, the thick stuff from the auto paint shop. Had some drip in the box but it didn’t interfere with the fin entry or moving as it was at the end of the box.
Dave,
Do you still feel a glass cap patch is necessary if the finbox is installed in a balsa block that spans all the way from deck to bottom? This is what I have done, and did not use glass at all, either in the box hole or as a cap patch. I wonder if I will find out later it was not the way to go…
Campbell,
Nice work on a finbox hole with hand tools!
JSS
Dave,Do you still feel a glass cap patch is necessary if the finbox is installed in a balsa block that spans all the way from deck to bottom? This is what I have done, and did not use glass at all, either in the box hole or as a cap patch. I wonder if I will find out later it was not the way to go…
Hope you have a good bond between the glass and balsa…
Otherwise leaks could still be a problem…
The easiest workaround is the FCS one - remove the foam
for 1 mm under the glass around the hole, and let the filler bond there. But capping is lighter and stronger, although it takes a little more effort.
I actually used a large sheet of glass 4 oz so there would be no bump. I took it to the rail and used it to help shape the bead. As for the overlap to the rest of the bottom some fine sanding and two shoots of “hotcoat” later it’s invisible. I would hope your balsa will work but balsa is a soft wood so I don’t see much strength over foam eh… I think if I wanted wooden support I would go marine ply or the likes, otherwise hardfoam. I just sank the box in the hole with glass bubbles and chopped fiber, also scarred the foam quite deeply beforehand “under” the glass to get good bond. Doing it bit by bit to avoid exotherm… hey man, if you’re in doubt cap the box, its safe and easy.
Capped the box today w/ one layer 6oz. I wish i would have left more covering the edge of the box. Cut out a little more then i wanted, but thats because i taped too much off. Oh Well, it should still keep it watertight. Also filled in some low spots and pinholes. The bottom needs a good sand, then shes ready for final coat. I really wanna surf this thing… How long does it need to fully cure??? i would like to take it out this upcoming weekend, but dont want any troubles.
Okay (Dave), not to jack the thread, but I want to do a singlefin EPS shortboard without a stringer (3" thick, 6, 2 x 6)–how would the box be healthiest?
I hadn’t thought about caps. I had thought about cutting a slot to the deck skin all the way around (or something) the box like FCS does in a circular fashion for the resin to form a sort of column to the plugs…
but the cap makes a lot more sense, thinking about the loads at the bottom surface/fin interface…the cap layer of glass goes over the box? I keep thinking both…
Hot coat, cut box hole, cut groove through to deck around perimeter of hole(?), lam patch layer around slot (?), epoxy in slot, box goes in, lam cap layer…
LOL
This is all a question
personally, without a stringer i would put some higher density foam around the box. Unless you plan on using a high density eps. Without a stringer, you are gonna have some strenth issues around the front and back of the box. I would maybe do a extra deck and bottom patch, on the last third of the board. Seems like the flex of a stringerless EPS (depending on glassing schedule) would create a problem around a rugged/stiff finbox. but im a first time builder, so this is all theoretical BS. anyone else???
Looking good campbell!!
I haven’t played with RR yet but our local epoxies seem to be good to go as soon as you can no longer indent them with your fingernail. This has been 30-40 hours in my experience but that is obviously dependant of temp etc my last baords were during the summer in temps 25 to 30 c…
Okay (Dave), not to jack the thread, but I want to do a singlefin EPS shortboard without a stringer (3" thick, 6, 2 x 6)–how would the box be healthiest?
I haven’t made a stringerless (yet) but I think the frenchie crew (PierreK??) may have had some experience as I believe his bro is making eps blanks for this purpose. I know Lob has made one, check the thread “first try EPS” and his webpage: http://www.barrelsurfboards.com/atua.php
Okay (Dave), not to jack the thread, but I want to do a singlefin EPS shortboard without a stringer (3" thick, 6, 2 x 6)--how would the box be healthiest?
Because of the narrowness of the box, and the density of foam it sits in, if you use a simple strategy of sink it in Q-cell and cap it, the box may still be wobbly, or become wobbly over time. The deep end of the box would wobble, only being supported by foam.
There’s multiple cures. The simplest is drilling to the deck, but not through the deck, at the 4 corners. Then, in those 4 holes, place wooden dowels with resin, or drop resin and chopped glass. Then, do the rest of the install normally. Something like a 3/8" hole or 1/4" even. This may not be aesthetically pleasing enough for some people (ie: it doesn’t look super pretty).
You could also sandwich the sides of the box in stringers. Contrary to popular opinion, placing the stringer cross-wise and not length-wise will do the most to strengthen the box. Just a 2 inch long stringer sandwiching the box at the front and rear, and the stringer oriented perpendicular to the box, running deck to hull. This is stronger/lighter than a lengthwise stringer because the maximum width of contact with the glass is the principal determinant of strength (remember the forces on the box are mostly side-to-side).
You could also install a stringer only for the length of the box before you route the hole.
You could also cut out a large block of foam and replace it with a higher density foam and sink the box with glass instead of Q-cell.
I prefer the first strategy, and since I’m too lazy to go get wooden dowels (which would be lighter and strong enough) I use chopped glass. But all will work, it is just a matter of preference and end-result weight. If you drill holes to the deck and fill with resin or resin and q-cell or resin and chopped glass, you are violating an FCS patent. If you use a wooden dowel, you are not violating their patent, and it is lighter. Violating their patent is of course a non-concern unless you are selling a competitive product.
Thanks, Dave. Good stuff.
And yeah, no, violating an FCS patent by allowing resin to run through to the deck skin for fin mount stabilization on a personal board isn’t really anything I’m concerned about!
Howzit janklow, Some people swear by capping boxes but the resin doesn’t adhere to the box all that good. If you cap a box and then cut around the tape you can see the glass delamming from the box surface as you cut. A well installed box is as strong as one capped. I’ve even seen capped boxes crack in front of the box from stress.Aloha,Kokua
Kokua–I’m hoping you’re around and wouldn’t mind telling me what method you’re talking about when you said “if the box were well-installed…”
in this stringerless EPS I’m finally happy enough with to finish off.
Thanks,
Greg
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