I just finished shaping my first board (poly blank). I am ready to glass it (polyester resin), can any one please give me instructions on how to install a fins unlimited 10.5" center box. I found bits and pieces in the archives but nothing from start to finish.
After lam coat or after hot coat?
cabosil, milled glass, or plain resin?
wrap the box in a sheet of glass?
When do I put the box patch on…
just afew questions any help would be great .
Will post pics. soon
thanks
dave
I’ve done both after lam coat and after hot. I find after hot was better. I used glass, resin & cabosil, makes it real permanent. If you glass over the box you could use just resin & cabosil.
A lot of shop boards are just resin and Q-Cell that way if you hit a rock/ bottom the box can pop out. I find this can more likely lead to cracks i the front of the box that can leak and cause other issues.
Look at this link:
Well the box is 10.5 inches long, 1 inch wide and about 1 inch deep. You’re going to use it as the center fin of a multi-fin setup, or a single fin, so you’re gonna have to cut the stringer.
You can cut the hard way: lay the box on the blank and trace the outline
You can cut the easier way: make a jig for your router and machine it. Make sure the jig doesn’t shift from the vibration of the router! The size of the router base plate, or the router bit and ball bearing if used, determine the jig size. I couldn’t tell you, not knowing what tool you’re using, what the size would be, but it’s pretty easy to make a jig from some 1/8 or 3/16 plastic sheeting, or make it from 1/4-inch thick mahogany ply, well sanded to remove any burrs, and coated with some leftover resin so it’s nice and smooth. I use spring clamps to hold the jig to the board.
Some cut before glassing, some after. I have for a long time clamped the jig to a laminated board, but lately I’m using a Dremel more often and it’s simpler to clamp to a shaped blank. Up to you which way you go. If on lamination, be sure to put some tape on the lam so (if your baseplate is black plastic like most) the lam doesn’t end up with ugly black sh*t from scratching the baseplate.
As for extra glass patches… if you put the router to a shaped blank, then install boxes, you’ll have to put some masking tape over the box prior to the lam, and lay a extra patch with your lam. In my opinion (looking at box failures, glass-on fin failures and break-outs) almost all the reinforcement needs to go around the front of the box.
If you rout a laminated board, I’d put extra glass patches in/under the lam prior to routing, or put small patches on after the cutting and box install. With UV cure poly resin, it’s soooo quick and easy to slap-on extra layer of 6 oz or two of 4 oz where it will be needed. No need to put extra glass on or across the entire tail, though we have seen it done for "effect"l. I’ve not seen a fin or box breakout extend more than about 2 inches around the front of the box. I have seen the entire tail break off, at the front of the box. That’s harsh, but very seldom happens. Hey, don’t surf over any big rocks, okay? My brother did it to an old wing swallow of mine at Diamond Head; I did it to my all-time narrow diamond tail coming out of a nice little shack at Log Cabins. Either way, bummer, no mo surf that day.
Tip: when you rout the hole for the fin box, any fin box or plug, try to keep it very close tolerance. Here’s why: one, extra resin doesn’t make the install any stronger. The strength of a box or plug install,that is to say the weak point, is almost entirely in the bond between the resin around the box or plug, and the foam. More resin won’t increase the bond. Adding glass fibers, chopped strand, Qcell, whatever might make the resin stronger, but won’t increase the bond. Easy to see and understand, but there are a lot of idiots who insist that chopped strands make it stronger. Not. Anyway, another tip if routing EPS: epoxy heats up a lot when it goes off in a large mass, and reports of melted foam are not uncommon. Try it. Learn the hard way, I won’t care. But at least (assuming the original poster reads this) you will have been warned.
I’ll be back. Post additional comment or questions. Send checks to my retirement fund. Nah-nah-nah, this is kokua, manuahi, hope it helps.
What is better cabosil or milled glass. (does it matter) how much should I add?
Thank you for your help
We use a router jig that cuts the hole really tight. It’s held down with spring clamps. And we do that after the board is hot coated.
I like to use 5 minute epoxy to set fin boxes. It’s nice and thick and gets hard really fast. It’s great for EPS blanks cause it gets hard so fast the foam doesn’t melt. My brother used 1 minute epoxy this weekend. We routed and set 10 boxes in about 15 minutes.
Hey. What 5 minute epoxy are you using? I am afraid to use my System 3 for fin installs (I’m not home, but I think that is the right name, 50/50 mix). It gets so hot and it seems rubbery when set. I just use it in small batches < 1 oz. I know that isn’t the way it is supposed to work, and I’m using a scale for measurement.
Howzit fdpiper, Ilike using milled fibers for box installs but that’s just me. Aloha,Kokua
Honolulu, good tips.
I agree about the chopped fibers and such. Just doesn’t add anything. It’s all in the flange.
On my finbox installs (Futures, not FU, which is anchored in the stringer, generally), I have started routing a little 1/4 inch round hole down to the deck at the fore and aft in the slot to help anchor the box, FCS style. This has been on my bamboo compsands where you can’t see the hole on the deck. This is antecdotal (ha), but it adds no weight to speak of and it makes me feel better.
We use system 3, and all the little syringe type epoxies available at any hardware store.
I never weigh the mix just eyeball it. For the 1 minute epoxy, my brother uses the mixer applicator nozzle. Problem with that is you end up wasting a lot of epoxy cause it hardens in the nozzle.
The 5 minute gets hot, but it stops curing after 5 minutes. I’ve never had foam melt with 5 minute epoxy. I have had it melt almost every time I’ve used regular epoxies for fin boxes. I think it’s because the curing reaction lasts a lot longer.
Howzit Greg, Good thinking there, I punch holes into the foam on each side of the stringer at both ends of the routed hole when installing boxes. Seems to help prevent cracking the glass from flex.Aloha,Kokua
What is better cabosil or milled glass. (does it matter) how much should I add?
Thank you for your help
Hi fdpiper, I would not use cabosil or milled glass. Both have no structual support in a board, will cave into the foam in time. I perfer straight glass like the pic shown below. This gives you a barrier between the fin box and the foam wall for lateral structual support when load from large fins are applied. Sorry didn’t have a pick of a center box, but we do the same with a ProBox install.
Mahalo,Larry
Thanks, how much glass one 6oz or two?
Hey Kokua,
I am just about to do a couple of these boxes, but am not sure I understand where the holes in the foam you are describing do. Can you post a pic or sketch?
Thanks,
Howzit tigermeat, I was reffering to Bahne like fin boxes so if it’s center fin boxes here it is. The holes are at the front and rear of your routed box hole about half way down on each side of the stringer. Hope that explains it for you. It came to me years ago that this might prevent cracks due to flex. It seems to have worked since all the boards I did this to and checked later this to didn’t crack from flex. I then add milled fibers like a FCS mix. Some feel that milled fibers don’t add strength but it does by interlocking in the resin like it was designed to do.Aloha,Kokua
Yup, that covers it. Sounds like you are spreading out the stress concentration at the transition from box to stringer. I’ll give that a try.
Thanks,