Vac bagging balsa skins, tape the inside?

General Discussion
21

I guess uninterrupted balsa skins would be the strongest. I’d love to find some sticks 10’ long.

But maybe what you’re getting at is that the joints should be placed in very specific locations to contribute to flex characteristics. In fact, since they don’t seem to be a failure contributor, you could line up all your joints at a spot where (for example) the tail rocker transition begins and end up with a flex tail. And, of course, that flex tail would behave differently if the joint lines of deck & bottom were directly above & below one another or if they were offset forward or backward by, say, the thickness of the board at that point…You could do the same at the beginning of a step deck longboard and truly identify & control the spot where noseriding decreases rocker.

This is one of ideas which won’t seem natural to me, comning into this as a carpenter rather than a surfboard maker. Perhaps Bert or CMP - who I gather were surfboard makers first who learned about wood and not woodworkers who learned about shaping - can espouse on the seemingly risky idea of allowing wood joints to line up & flex together in a wood sandwich skin surfboard?

22

At the moment, Its hard to tell what’s the limiting factor in the overall design.

Thats probably more a function of each specific design and layup.

…Perhaps the rails or core will fail before the skins.

But if the skin could be made stronger, then thats just one more place where you may tweak the design to suits your needs.

-not that i really have any particular needs at the moment :slight_smile:

If the butt joints tend to fail in tension before the actual wood fibers, i would suggest something like a scarf joint.

I would feeel the same way if the joints affected the modulus somehow.

I guess a simple test is probably the best way to circumvent all this rambling.

-bill

23

A little while ago I made a scarf joint to join two pieces of balsa together with titebond woodworking glue (I was playing around with trying to preform the rails). Anyway, I was totally shocked how easily the joint broke.

24

exactly… i was thinking all along why dont you use plasterers tape on the inside, it would wet out fine and hold well enough for working with,

25

Looks like a good call. 2" would probably be too wide though – with 4" panels it’d leave you with only half the balsa uncovered. I found some 35mm tape (~ 1 3/8") which would probably work, but it’d be nice to get it down to 1" if not less.

26

Scratch that. I should have taken a closer look at the tape before posting. There’s hardly anything to it, maybe a strand every few mms or so.

27

Thanks Wahoo/Feral Dave

great idea… I’ll go price the stuff at the local hardware store this weekend hopefully you don’t have to wet it to activate it …

Lunch time brain fart (gotta be this hawaiian laulau and pig plate lunch I’m scarfing down)…

As far this tensel strength longitudinal torque horizontal camber skin stress and whatever…

kind of seems like we’re constantly over analyzing the whole thing versus going out and trying it firsthand.

In the span of all these discussions Benny’s made his stringerless fish and I’m sure if he was industrious enough he probably could’ve built a couple more projects and reported back here with the results…

FlavianSan has done just that causing some concern about going too long with the stringerless option. I think around 6 feet maybe 6.5 feet you can go stringerless without the balsa rails and still have plenty of strength going with 6oz in side and outside of a 1/8" balsa lam…

7 feet and over and I think you’ll definitely need the balsa rails and some type of horizontal stringer arrangement to control the amount of flex stress passed on the external shells.

For my 7’4" and 8’ 1lb EPS projects I’m thinking that on top of doing the bamboo layer between the two 2" EPS sheets, to also do the a sandwich on the deck and bottom of 2oz+1mm bamboo+6oz+balsa+6oz.

In this case your sandwich would be glass then balsa then glass then bamboo then foam then bamboo then foam then glass then bamboo then glass then balsa then glass.

The bamboo will support some flexing after glassing across the long axis but not much across the short axis.

The balsa will provide the external insulation and surface tension like Divinycel but with some flex

The foam core will provide the needed floatation and shock absorbtion

And the glass and as little epoxy as possible holds everything togethor.

Without the balsa rails you probably could do the top or bottom shells of glass+bamboo+glass+balsa all at the same time.

Of course you still have to shape da bugga which will always be my personal down fall.

But the skins should be relatively easy to do if you keep the internal bamboo and glass layers well with in the rail line for sanding purposes. I profess not to know how much additional weight the epoxy laden 2oz with the 1mm bamboo will add but if you had to you could do all the internal layers with 2oz instead using the 6oz under the balsa to reduce weight a bit.

Going down to the store to get some more bamboo this weekend (CMP said it’s on sale), then cut it and my 2oz to size for my 6’3" “post modern fish” or as I like to call it, “the one on the left of the picture…”

I was planning a thinned out (2.25") but widened (21") version of my 6’3" Bushman with a slightly concave deck to test some of bert’s EPS board design theories might as well slap on the new sandwich skin design at the same time…

Will report back when it’s done…

28

The fiberglass mud tape isn’t sticky enough to hold balsa pieces together. It sticks just ok to dry sheetrock, and is really held on by the mud. Its only there to give the mud something to stick to in between the sheets. It peels off wood as if there’s no stickum on it at all.

Any titebond joints will pop right apart if there was any moisture in the wood or the air when they were made. If you have dry wood & air and clamp properly, the joint should actually be stronger than the wood around it. Its extremely water sensitive. If I’m going to use titebond, that wood better sit someplace flat & dry for at least a couple weeks first. A scarf joint - or hell, a tongue & groove, a dovetail, why not go crazy - is only there to introduce more glued surface to a joint. In a 1/8" thick piece of the softest wood in the world, a 45* scarf might give you 1/32" more glued surface? Really, don’t bother. I don’t glue the balsa pieces to one another at all, just tape & go. An extra 1/32" of glue coverage of yellow glue isn’t going to make the slightest difference when the thing has 100% epoxy coverage over the entire top and the entire bottom of the wood.

Balsa is good for this because a) its light b) its good at taking up space between the 2 layers of epoxy & glass c)it doesn’t introduce air which can later expand & cause delams (if that didn’t matter, I’d personally use cardboard for the weight & strength considerations) and only, lastly, d) its relatively strong for its weight & price. Perhaps even more importantly, its nice to look at.

But the crucial thing seems to be having a space between your two layers of glass so that the box beam/sandwich skin/parallelogram / whatever you want to call it - has the thickness necessary to make it stronger than just laminating 2 layers of 6oz together. At least, that’s what I’ve inferred from Bert’s posts. So you could use anything, provided it meets the requirements of: light, solid (no void space), available, and aesthetically acceptable.

edit: oh yeah, whatever you use, I guess it should stick to epoxy resin too. It’d be a shame to build a sandwich skin out of expanded low density polyethylene (the foamed stretch wrap that TV’s are packed in) or something and then peel your glass right off…

29

I’m not sure if this is relevant because I’ve never done a surfboard this way. I may one day.

Anyway, for wings we constructed the balsa panels using superglue. We also perforated the balsa to allow the excess resin to transfer to the surface, so no pooling and encouraging a better pressure bond.

And sorry, yes we did use peel-ply, breather cloth and all that stuff, but hey, if you want quality you got to spend that little bit more.

30

I love this forum!

I agree with Benny on the drywall tape. I find the stuff really doesn’t want to stick to much.

Oneula, I think you’re on to something with that bamboo. When I talked to Greg Lohre he seemed really positive on using bamboo for the bottom of the boards (except he was using some type of bamboo vener). He also really liked the hemp cloth, but as a teacher people might freak out.

I agree with Benny, Meecrafty and Oneula. Its possible to overthink the whole process. In some movie I saw somebody surfing bookcases. Especially on your early boards, how long do you really want them to live. On any board I make I don’t really think it matters if it fails down the line. Its not like anybody is going to come complaining to me about it. I doubt I’ll ever replace the pros. That being said, I do believe in developing best practices that you can build on.

31

If you want quality you have to spend a little bit more??? Peel ply on a wood lam surfboard wood totally be wasted if you use the least amount of epoxy necessary for the adhesion. Peel ply is beneficial on non wood lam, straight up fiberglass vacuum bag jobs for a nicer finish and excess resin absorbtion.

I am making surfboards not space shuttle parts. The reason why the wood pieces are staggard is for looks more than function. In 1991 I scored some 16" wide by 50" balsa sheets. They made the board look weird because of the seam across the whole board at one spot. It was perfect for mixed wood applications.

To change flex characteristics you use stiffer wood materials in different lengths and different locations to acheive this. Or strategically lay up double layers of glass under the wood lam to gain stiffness or shift a flex point forwards or backwards. I have experimented with my personal boards to know what seems to work. I have never stress loaded one of my boards to breaking point or physically measured the flex characteristics. Mission control(my wallet) has never allowed this.

20 years ago my motivation was to make an unusual, cool looking inexpensive wood lam surfboard. It just so happens it was stronger, as light as polyester boards and certainly cooler looking. I was able to laminate wood on foam for 8 years with crude taping techniques. Now the motivation is to do it more efficiently and get boards lighter.

I would try whatever you think might work. I’ve learned the hard way mostly. If you mention something to try that I saw fail in my trials I will say so. That’s what makes this process so cool. The final product and all the different ways we got to that point. I have not inside seam taped for 20 years. The last 3 months of inside taping has revolutionized my ability to save time and money and mostly get a superior product than I made in Dec 2004. I don’t plan on ever going back to those days.

32
Quote:

…

Balsa is good for this because a) its light b) its good at taking up space between the 2 layers of epoxy & glass c)it doesn’t introduce air which can later expand & cause delams (if that didn’t matter, I’d personally use cardboard for the weight & strength considerations) and only, lastly, d) its relatively strong for its weight & price. Perhaps even more importantly, its nice to look at.

But the crucial thing seems to be having a space between your two layers of glass so that the box beam/sandwich skin/parallelogram / whatever you want to call it - has the thickness necessary to make it stronger than just laminating 2 layers of 6oz together. At least, that’s what I’ve inferred from Bert’s posts. So you could use anything, provided it meets the requirements of: light, solid (no void space), available, and aesthetically acceptable…

fair enough.

I’m not even sure i disagree.

But if all that is true, why not use divinicell for the outer sandwich?

Or even end-grain balsa for that matter?

I bet Bert prefers balsa planks because of better flexure properies, not because its prettier.

Its certainly not easier.

-bill

33

Oneula

A couple of things, I am fully aware of keeping costs and times down, gorilla glue is cheap, as you use so little of it, it goes off so fast that I can assemble a side in 15 to 20 mins and do other stuff while waiting for glue to dry. I only had a few spots of resin bleed through, so sanding time is at a minimum. I am no expert here, but when vacuuming other cores in composite structures, no skin tie is necessary, the only reason for perforated core is to stop air being trapped under the core, the area we are using , this shouldn’t occur. You will only get resin pooling if you use way too much resin! Weigh your cloth then you know how much resin you need, if the cloth is wet out evenly, no pooling!

Cheers

Mark

34

35

Zfennell

There is a relationship between sandwich skins and thickness

First just so there is no confusion

skin is the glass on each side of the sandwich

core is the substance between those two skins

The strongest sandwich is the one with the most thickness ie thicker core separating the two skins

If you have a thicker core its compressive strength is far more important than its tensile strength

Which is why boats are made using pvc cores and end grain balsa

No one that I now of uses balsa length ways to make boats!

Now if you make a thiner sandwich ie less disance between the two skins

You have lost some of the strength benifits of a sandwich, to offset this you need a skin that can handel more tensile load rather than comperssive loads

Balsa lenght ways as a core? looks nice

Compare the tensile strengths of the two skins to the tensile strength of the balsa,make your own judment on that one.

While you are at it get a ordinary sheet of cardboard about 3mm thick skin it each side with the same glass you use on a board.

Yes it will be heavier but compare the tensile strength! Looks like crap though.

Its all swings and roundabouts thicker vs thinner fexi vs not so flexi

looks nice vs looks crap

enjoy your own way

Mike

36
Quote:
fair enough.

I’m not even sure i disagree.

But if all that is true, why not use divinicell for the outer sandwich?

Or even end-grain balsa for that matter?

I bet Bert prefers balsa planks because of better flexure properies, not because its prettier.

Its certainly not easier.

-bill

D-cell works fine for the sandwich core - that’s what Bert showed in the vast ‘vacuum forming & sandwich construction’ thread. I think end-grain balsa would slurp resin out of your lams at an alarming rate. End grain, after all, exposes the wood’s moisture-transportation system…

Anybody ever used Gatorlite? Sintra? Dibond? …

Ben

37

“I think end-grain balsa would slurp resin out of your lams at an alarming rate. End grain, after all, exposes the wood’s moisture”

Hey Benny, if you want to really experience slurp…try d-cell…then when thats done, you gotta deal with pin holes. I’ve heard of those other products…they can be found in industrial plastic sheet suppliers…heavy stuff.

38

Benny

Lots of people seem to have problems with d cell and k cell,Ive used both many times with out problems. It makes me think that most people are squeeging way to much.

With epoxy you take time and let the resin soak in to the cloth by it self do not force it.

Also try pre weting the glass before putting it on to the d cell

Pin holes can be caused by many things from contamination to over squeeging which will get the resin foaming and result in small air bubbles

I can only suggest slow down. when you mix the resin do it slowly with a wide flat stick not a round one,slow down and try to minimmize introducing air at all stages of your resin application.

Mike

39

Hey Glenn, thanks for the heads up…the stuff is available at HD in the drywall supplies section…I bought a small 50ft roll for less then $3…it sticks good and feels good under tension…has multiple app potential…thanks!

40

thought i’d add my combo platter method…

i have tired taping the outside only… not much fun.

i have tired the veneer tape on the inside… works good as long as your using a wet out table, the stuff is just paper and dissolves and looses it’s bond very quickly… slow to put on. i let it dry a good day before using. not so good if your trying to wrap the rails fully like cmp, also you can still get some bleed through…

so i came up with the combo method of the veneer tape and 3/16 masking tape down the seams… works well, but i am going to continue to try other methods…

the fiberglass tape won’t stop the soak through so i think i’ll pass on that one, also it will give me nightmares about my drywall days.

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