After about a year nearly all of my fin boxes have leaked into the EPS. I use the 10" chinook strong box ones. first the capping starts to lift then the h2o finds it’s way under the glass and down past the pvc sandwich around the PVC block the box is set into and into the foam. I sand the boxes with 80 grit and dip in acetone before glassing.
I think as the salty water gets under the capping it dies out and the remaining salt expands the lifted area further and further each time it gets wet and dries out between use.
Cool diagram, but it doesn’t really show where the leak is, does it? I might be stoopid and missed it though. It looks like your boxes is well covered in glass and reinforced with PVC. Does the cap cover the box with just the slot cut out? Did you make sure the box was dry after you cleaned it with acetone? Acetone has a nasty tendency to soften epoxy(and melt EPS). If it’s not dry it think the bond to the epoxy is compromised. I think your right about the salthy crystals expanding.
Question: Does water soak through PVC or is it closed cell?
Also, what density PVC, is it Airex water proof or Divinicell semi, what’s the depth of the PVC, what length fins used, how big are the surfers, did you check tip of the fins.
I guess the glass was well saturated and catalysted, the two layers between the PVC and the plastic box.
Ok heres a modified diagram showing the leak. The H80 divinycell block 50mm wide thru to the deck is set in after the 3mm h80 div sandwich has been vacuumed on. That post on the box goes thru to the deck, I put the boxes in before glassing the board, after the board has been glassed I route out the opening, bevel the glass near the opening so the fin doesn’t push against the glass. But the glass starts to lift at the front of the opening. The boards are single fin 9’ plus and the fins used are 10" plus and yes they do have gouges and scratches on the leading edge and tip. yep the glass was well saturated and mixed. I think the real problem is the bond between the glass/resin and the box.
if im installing a fin box of that size i do what leedd said and run 9mm ply about 3" longer than the box at each end ,next to the box giving it more lateral support…
i also sand and gently round off the the inside edge of the hole in the board before the box is set in …plus make sure theres heaps of fibre in my resin mix …at the end when you cut your hole out of the finished box round it off as well so you got nothing to catch on putting the fin in and out. one last thing i always use 40 grit for roughing boxes and sometimes it still happens where the glass lifts…
its painful when its the box thats let water in the board…
since ive been doing alot more fin systems lately …similar things have happened …
set fins in eps/epoxy boards are definatly more durable…
Hmm I only sand them with 80 grit, I only use qcell and resin plus 4oz to set the box I didn’t think adding fibres to the mix wouldn’t add any strength, the woodies are a good idea. Next time I will put the box in after the board has been laminated but instead of grinding the 4oz that wraps around the box I’ll try and bend it over while still wet onto the laminated glass. Then I just have to wait for a years use and see if that leaks.
I would drop the cap if I were you as it doesn’t seem to do you any good. I wonder if the routing of the slot heats the glass/box and compromises the bond? I route out the slot first, cut the glass so it folds nicely into the slot and then lam. Alltough it can be a bit tedious I feel this reinforce the box considerably.
What I don’t understand is how your reinforced box leaks and the normal uncapped box installed in PU foam works as the normal ones bonds only to foam and stringer. I guess the torque on the box is not all that much during normal turns.
Haavard I don’t understand either I tried to make it as strong as possible I think EPS wants to suck h2o easily but with PU it’s fairly closed cell and a higher density offering more resistance to sideways force.
I have repaired a few sailboards from the cobra factory in Thailand and they have an amazingly lot of glass and resin around their boxes I think as they are made in a mould they set the box on capping glass on the mould and put heaps of material over it, then it seems like they build the board around it.
