Divinycell question re: sandwiches

Bert, Mr. J., Greg, everyone else helping me understand the process & concepts of sandwich construction, vacuum bagging, and so forth - the inevitable has occured. I now understand just enough to be dangerous but have more questions to ask. Pandora, what have you done?

Divinycell is available where I am (North Cal). But the longest lengths are 8’. So, what would you do to make a 10’ longboard? (What about a 12’? I saved the 12’ rocker/thickness schedule from Paul, who wouldn’t? Thanks, Paul!) I’ve had fine luck splicing together 8’ pieces of EPS to make a 10’ board, but since the construction will get so much of its strength from the high-density foam, I have concerns about running a butt joint across the board 2’ from either the nose or the tail. Add a stringer and don’t worry about it? Rip the sheet in 6" strips and stagger the joints nose,tail,nose,tail? Use 3 stringers and cut all my foam in 6" strips?

And the rails. To adequately shape pinched egg rails on a 3.5" thick log, I’m sure I need more than 1 layer of 1/2" divinycell on the rails. Do you glue those up together or one at a time? In the bag or with tape?

And how to vacuum a 10’ board? I’m thinking clear plastic over sawhorses, 3 or 4 10’ 2x6’es, then the hot-wired offcut from my EPS foam as a rocker bed, another layer of plastic so it doesn’t stick, then my breather cloth, EPS, epoxy-saturated glass cloth, Divinycell, more breather, and flip the first sheet of clear plastic back over the whole mess and tape it shut, closing up the works except the sawhorses. Is that right? It won’t fit on a plywood table, unless I make a bigger one than either my workbench or assembly table.

What if a use an existing board as a rocker guide when I vac together the EPS & D-cell? Inside the bag with 'em, I assume, but will the peel ply/breather really keep extra epoxy from sticking a finished board to my EPS? Should I wrap the finished board in stretch film? Wax paper?

Double 6 oz on the EPS bottom and double 6 on the d-cell top? I’m 220 lb and ride all over the board, noseride, turn aggressively, and put myself in closeout barrels for the view. I’m hoping that this type of construction will be lighter than my strongest boards and more durable than my lightest boards. Another form of compromise, but one which covers much more middle ground.

Sorry I ask so many questions. Please don’t feel you have to answer everything, I’m ok at connecting dots if you point me at links, photos, etc. I just remember from school that if one person asks a question, many others were often thinking it…

Thanks so much.

Ben

EXCELLENT! I was thinking the same thing! I won’t be doing a composite very soon but… I was looking at the EPS at HOme Depot… sure is cheep but I like longer boards than 8’. Butt joint, scarf, alternate 6" strips, what to do guys?

In advance… THANKS GUYS!

Les

In boat building (quite a few years ago) we used to join 1/2" plywood with a scarf that was about 5" wide. The wide scarf was so that the two peices were joined well. Not sure you need this with sandwich construction though. Maybe just a butt joint because if you use epoxy it is gunna be pretty strong (stronger than the foam itself?). You could put a bit of angle to the joint to give it more area (stronger) if you like. Of course, best would be to have a solid peice.

We always just butt joint for longboards. EPS is available in almost every major city from a manufacurer in any length you’d need for surfboards. And in thickness you need as well. Home depot is not a very good place to buy this. Look in the yellow pages under “insulation”.

As for the bagging questions, we always used our hot wire cut offs as rocker beds with a piece of plastic over, then the blank with the D-Cell inside the bag with the breather inside as well (I like small bubble wrap for breather). You generally need to apply some weight to make sure the blank stays true to the rocker bed (small bricks).

It sounds as if you just going to use D-cell on just the deck. We did this and most of them broke. All the load gets shifted to the bottom and… snap! Use core materials on both sides unless your engineering some other entity to carry some load.

Scarf joints are the way to go, especailly if there is going to be even the slightest amount of flex. The fact that epoxy is stronger than the joint iteslf is the root of the problem.

Butt joints with epoxy make hard points on the joint line that can end up causing the laminate to fail around the joint in the core.

www.boardlady.com has pictures and a better explanation - examples of poorly-designed production windsurf boards.

With plywood 8:1 is recommended for scarf joints, I’m not positive but probably the same would be in order for a foam core. In 1/8" foam that just means that the angled part of the joint takes up 1" in the length of each of your two foam pieces.

Funny that I know all this stuff about vacuum bagging and some of the core materials but I don’t really know squat about surfboards other than what I’ve learned here so far. There is a Yahoo group at http://groups.yahoo.com/group/boardbuilding/ where pretty much everyone using the group is using vacuum bagging techniques for building kite, skim, landboards, and the ocassional sailboard. If you check the old messages you will probably find lots of usefull information. - Hey that’s where I found out about this site!

Greg & Trent - thanks! I was planning on EPS underneath & 1/2" d-cell on top with a stringer & maybe balsa rails (maybe d-cell) to stiffen it up. Was also thinking of vertical d-cell as the stringer. Greg, do you think that would still snap?

I know from Bert’s thread, he uses 1/8" d-cell top & bottom. Obviously, that’s better, but the stuff is $$$$! But then again, a broken board (that will probably break again, if its a poor design) is even more $$$.

Glad to hear I’m on the right track about the bagging, anyway. Thanks so much.

Ben