Stoneburner, yes, but what does that box bear against? Is it tied to the stringer? if not, it is just bearing against foam.
Sharkcountry,
“In most of my boards the bottom of the fin box is close to the deck, maybe less than 1/2 inch. I placed 4” or 5" long balsa strips perpendicular to the box near the front and back to stabilize the box, then route through it when I insert the box."
Exactly. Did that many times particularly in 1 pound foam. But I am convinced using fiberglass strips does better with fewer steps.
all the best
Huck, I believe that tying the cloth to the bottom skin is key. It the pic you show, it seems that the cloth stuffed in the hole does not overlap onto the bottom thereby tying the cloth in the hole to the bottom skin. Just stuffing cloth into the hole and stuffing the box in afterwards is not the same.
to each their own, por supuesto, but the next layer of glass as shown in the second pic overlaps the boxes and including the glass around the perimeter.
“Greg Tate wrote:
Stoneburner, yes, but what does that box bear against? Is it tied to the stringer? if not, it is just bearing against foam.”
The top of the cradle is 2" is wide and 10.5" long – 21 sq. inches of balsa surface bonded to 2, 4-oz layers of FG deck skin.
The bottom of the balsa cradle (10.5-sq. inch surface) plus FU box (minus slot = 5.25-sq. inch surface) is 1.5" wide and 10.5" long – 15.75 sq. inches of balsa+FU box surface area bonded to 2, 4-oz layers of FG bottom skin.
Total surface area, top + bottom = 36.75 sq. inches.
Deck cradle surface area would be slightly more than 21.5 sq. inches because the top of the cradle is angled from front to back rather than parallel to the bottom.
The cradle+box is bonded to (covered by) both deck and bottom FG skins.
As a short wide I-beam, the logic is, the top and bottom skins would handle/disperse a significant portion of the stress load.
As such, the balsa cradle and skins should bear the principal load.
EDIT: A stringer 1/8" thick x 90" long has 11.25 sq. inches of top surface area and 11.25 sq. inches of bottom surface area – approx. 22.5 sq. inches total – bonded to the deck and bottom FG skins.
Hi Greg, I’ve had proboxes blow out on the sides in EPS and XPS. I used to wrap the box and have a layer of glass inserted that stuck up over the hole hoping that it would attach to the sides, just like the photo Huck posted. I found that in several of the boards the router cut a super smooth hole and I don’t think the resin mix had enough to grab on to. The failed boxes came out pretty clean and I was able to see that the hole was smooth. That’s why I started poking holes along the sides and making a groove along the bottom edge, to give the resin more attachment points. On the latest stringerless singles I’ve cut an FCS type hole under the front of the box for the resin mix to attach to the deck lam. The back of the box also has a hole to the deck, but it’s because I put the leash cord through the box. So the box has resin mixed with glass fibers that connects to the deck lam.
I haven’t had a long center box fail, but now I probably will (murphy’s law). I also just started using the FCS fusion boxes for the sides. In the past, I’ve had glass on side fins fail too. Usually on my front side rail. I must exert a lot of pressure on that side.
Stoneburner, the top and bottom bonding is golden as you well know. Love it.
Sharkcountry, I hear you. I’m no genius. Again top and bottom bonding will help enormously but nothing is fool proof. On those Proboxes, as with Huck’s install pic, you have to be careful when sanding the finished board not to cut into the glass that connects the the boxes. Hard to do when you install post lam. Of course I’m preaching to the choir.
All the best