FIn box failure

General Discussion
1

I had a fin box failure today.  On my first wave I turned and pushed my board into the water and my fin hit the reef.  The board felt strange and when I turned my board over half the box was sticking out.  The bottom of the box separated from the sides.  With the heel of my hand I pounded it back in and surfed another hour.  I’m not sure if it was all the box or if I had an install issue.  I made the board a few months ago and have ridden it quite a bit with no problems.  I did lightly sand the fin box before installation but I didn’t clean it with acetone.  I wonder if the sides didn’t stick well and the bottom did.  I used 4 ounce cloth and the board is poly.  Has anyone seen this before?  

For the repair I plan on routering out the bottom of the box, sanding the sides a little (so I have space to put cloth in again), cleaning with acetone, and installing a new box.  Anyone have any other suggestions on how to get the bottom of the box out?  I’m open to all suggestions.

Thanks


2

Interesting failure mode. Is it a genuine FU Bahne box or another brand?

Had it been glued in better…the board may had been more damaged by the reef.

Router out the bottom and starting over sounds the easiest to me but there are more experienced voices here.

I have yet to tear one out like that.

 

 

3

It is a fins unlimited box.  I’ve used them before and never had a problem with the quality.  Strange I didn’t hit the reef all that hard.  It didn’t damage the fin.  You’re probably right, I should be thankful more damage wasn’t done.  I’m glad I didn’t lose the fin.

4

I was surfing a new-to-me spot in Baja once and didn’t realize how shallow the reef was.  Lost the box and my favorite 10.75" Velzy Classic.   It looked just like that.  No damage to the glass, but the box and fin just pulled straight out.  

I didn’t glass that board, but I learned a lesson from it. Epoxy+veil+risers  

5

Aloha J3, 

Take the path described by  Jrandy, above.       Sand the new box completely, and wipe with acetone, before you install.

6

Did it snap around the channel that the plate slides in ?

7

Wow, the bottom is still in there? I’d probably try to drill a few screws (shallow) into the box and lift it out with pliers. Looks like a box failure to me.

Did you do only 1 layer of 4oz? I would wonder if that had anything else to do with it.  Epoxy or Poly resin?

8

A very good reason for a tail patch.  I put mine in with milled fiber and a tail patch that actually covers the box.  I don’t hit the reef,   If I did it would break the whole tail off before the box would give way.

9

I’m gonna agree with jrandy and Bill- that the box broke is a good thing, if the box had stayed intact it could have done a helluva lot of damage to the tail, deck and so on. I’m also thinking that you hit the reef harder than you thought. It happens. I’ve seen similar before.

This would also kinda color my approach to installing a new one: do you really want it to be so strong that a horrible crunch results? The way you did it the first time worked pretty well. Instead of a buckled tail and a fin box jammed in crooked that has to be dug out and a lot of filler to do and so forth, you have a fairly simple fix. I’d call that a win. 

It probably goes without saying, if you used a jig to rout out the original fin box slot, use it for taking out the busted off fin box bottom. If not, well, make one. Don’t even think about doing it freehand. Secure it well. And that should also get into the sides of the opening plenty good enough to accept some cloth along the sides without going overboard. After all, it’s the original size of the slot. If you have to make a jig, I’d go no more than a fat 1/8 inch over the size of the fin box. 

hope that’s of use. 

doc…

10

When the tab is on the front of the fin, one can use a break away screw plate and prevent damage to the board/box.

https://shapersupply.com/products/breakaway-screw-plate

 

I’ve often considered break away roll pins too, but now just use screw plates front and rear, and no roll pin.

 

I am in favor of rock solid fin box retention, as I don’t want any flex between my foot and the fin box/ base of fin, or have water intrusion between box and resin or  between resin and the foam/wood or have a premature failure of a box.

 

Never seens a box fail in that manner before.  Not sure if they use adhesive on that bottom portion or heat weld it, but it appears not only was that portion not upto snuff, but there was little adhesion between the sides of the box and the resin surrounding it.

 

One can go through the motions, and remove the shine of the box with dull sandpaper, but new sharp sandpaper makes for much better mechanical tooth and much higher adhesion.  Solvent wipe/remove the mold release agents on box before sanding, so that the sandpaper does not get contaminated with them, and push  release agents into the valleys, and prevent a good bond.

  Wiping with acetone after sanding rounds those sharp mountaintops and fills the valleys, inhibiting mechanical tooth.

 

One can go to extremes of mechanical tooth, using a new sharp razor on an opposing angle in an X pattern so that the cured resin fills the cavities and locks the box in place even if the resin/box bond fails.

 

I also believe that many use way too much pigment when setting fin boxes, which can weaken the resin and its bond to the finbox.  On single fin boxes I’d either not use pigment at all on my own boards, or on someone elses who wanted pigment, do it in two stages, fill to 1/8 inch below the hull with lam resin, then use pigment in sanding resin to fill to the tape dam afterwards.  I’d usually cap them too.  Lots of extra work and perhaps unneeded though, since so many view boards as disposable. 

I don’t work with PE resin or make boards for others anymore so not an issue.

11

Thanks for everyone’s responses.  In answer to some of the questions.  The bottom of the box broke off, not the channel.  The resin was poly also.  The bottom is still in really good, I don’ think I would be able to pull it out but thanks for an alternate plan.  I was also wondering why the bottom adhered so well and the sides didn’t.  That’s why I was wondering if it was due to not cleaning with acetone. I’ll use some new sand paper and not use pigment in the resin this time.  Thanks again.  I’ll post a pic when I’m done.

12

The Fins Unlimited aka “Bahne Box” is a two part molded box. 

When windsurfing became all the rage we blew these apart every time we were learning how to get big air at Jalama. We went thru many trial and error methods of reinforcing the boxes using “woodies”, capping over the boxes with glass, drilling holes at each end and bolting them to keep the corners from cracking.  On and on… one team rider I had was blowing them out of a small leading edge 7’8" I made him that I routed out the whole area and set the box in a hunk of Doug Fir.

We finally figured out (by accident) the importance of the “skin to skin connection”.  These were the early days of windsrfing, so I was buying Delrin rod from a plant in Camarillo and making my own footstrap inserts.  I would install them once the boards were glassed and hotcoated but previous to the fin boxes being routed. When I routed the fin boxes we would route part of the previously installed Delrin rod.  The boxes now had a seat on the rod.  I was selling a lOT of sailboards in the Bay area thru a dealer in San Rafael, and they told me “your boards are THE ONLY SAILBOARDS WE OFFER THAT THE FIN BOXES DON’T BLOW OUT”.

Upon hearing that, I thought about our production process and realized what we were doing right.  It was a fast moving, fiercely competitive business, so I kept my mouth shut and enjoyed the benefit.  Eventually people caught on that the Fins Unltd. boxes had design issues. Fins Unlimited produce a “red box” but the only difference was that it was double plated on the bottom.

Long story short, the REAL SOLUTION came about when CHINNOOK produced a one part/piece fin box and in order to mold such a box and release it from a mold they had to use a CNC machine to drop into the channel to rout the track. They also included a post that was part of the unit that could anchor to the deck basically the same thing that I had discovered with the Delrin rod skin to skin connection.

13

Same story with fcs 1 plugs, connected to a decently deck laminate, as it was design for, it’s a realy effective fins system.

14

Right on all counts. And the real story.   I’ve never had one blow out that I know of.  I’ve fixed a few, but none that were a failure to the box. Mostly cracked boxes from impact.  The Chinook box is a very good box and I have installed them, but minus the posts.   I really believe the Futures “Strong Box” is the best on the market.  For surfboards anyway.

15

More and more bad plastic plugs copy on market. Cheap but shit.

Deck connection is a plus for strengh when needed, avoid plug go in foam when fin hurt soil. Plugs for foil must be connected to deck i repair many where all plate move in foam…