Undirectional over strip planking?

[NoEyeDeer]

I'm going to be building a rowboat out of strip planked cedar with the ubiquitous glass sheathing for cross grain strength. It occurred to me that using unidirectional for the sheathing might not be such a silly idea, as really the transverse strands are what is needed and the longitudinal ones are more or less wasted. Since the standard sheathing seems to be 6oz(200gsm) cloth a 3oz(100gsm) uni would, if I can get such a thing, give better transverse strength at half the weight.

Has anyone ever done this?
Is it practical in such a light layup?
Does it require anything over the uni to get a decent surface?

[Landlubber]

Yes, you can use glass sheathing if you wish, I would use 45/45, as it will suit the application nicely, and use epoxy resin. Do a cloth on the inside to assist balancing the layup, and I assume that you will varnish the inside, paint the outside. The epoxy resin layup will need UV protection anyhow.

[gonzo]

45/45 is the best. Unidirectional would not give as much puncture resistance. The fibers would split along the lonigitudinal direction.

[NoEyeDeer]

Landlubber, I already know I can (and will) use glass and of course I will use an epoxy resin. I wasn't asking if using glass sheathing was possible. I was specifically asking if using unidirectional instead of cloth was practical for such a light layup.

The problem with trying to use a double bias (45/45) is getting one light enough. Since I was aiming to save weight compared to the 6oz cloth most people use I obviously don't want the layup to be more than that. The lightest 45/45 I can source locally is 8oz. No good. :)

However, I can get this (http://boatcraft.com.au/Shop/index.php?main_page=product_info&cPath=40_42&products_id=692) which would be stronger even if it doesn't save any weight. Might be worth thinking about.

Gonzo, good point about the splitting on impact. I was thinking I might need a fine cloth over the uni anyway to give something to sand for a decent surface finish. This would probably be the case no matter what sort of stitched fabric I went for. Uni, 45/45 and triaxial all seem to be a bit rough for finish.

[gonzo]

I don't know who makes 45/45 in 6 or 8 oz. You can cut fabric on a bias, but it will have some seams at 45. I don't know if you can live with that.

[michael pierzga]

For what its worth a friend of mine used 6 oz 45 45 biax on a project and he told me it was difficult to work with...left a coarse finsh that required much epoxy fairing work. Evidently the 6 oz biax used the same fiber bundles as heavier cloth but with less bundle density. Perhaps what Gonzo says with normal 6 or 8 oz cloth oriented on a bias might be good. S glass is availble in light weaves.

[NoEyeDeer]

There's not much point in using standard cloth on a 45. The crimped fibres aren't as strong as a proper double bias anyway and the seams would be nasty. Better to do something else I think. That 189 gram triaxial would probably be a better bet.

[michael pierzga]

Too bad you cant get the 0600 biax locally. Did you look hard ? Perhaps give Raka a call and they can put it on the plane and ship to you ?

http://www.raka.com/Stitched_woven_biaxial_triaxial.html

[PAR]

To directly answer the questions of NoEyeDeer, yes, it's practical to use uni-dia, you'll have a stronger, thinner laminate, but you have to question the scantlings a tad to make any advantage of the extra work.

Typically, a row boat or other small craft, as you've surmised has most of it's longitudinal stiffness in the strips. The glue lines working in pure tension and the long walls of the wooden fiber cells make up the cross grain strength. A cross grain application of uni-dia will stiffen up the cross grain, but you have to ask yourself if it's necessary with the strip scantlings employed. In other words, if you want an advantage and not just more cross grain strength then necessary (extra weight) then a thinner strip is the usual choice.

Yes, biax is a pain, but the first thing I do with biax is take a grinder to the stitches and knock them off, which renders the surface a much smoother thing.

On a typical jon boat, you're only talking about a few pounds difference from a light (6 ounce) biax and a couple of layers of uni-dia. So, unless you shave the hull shell down a little to decrease strip weight (thickness) then the savings is a relatively moot issue. My point being a good two coats of primer and two coats of paint will nix any weight savings you might muster from a uni-dia layup, without a strip scantlings review.

[michael pierzga]

Ive never tried it, but it Must require careful attention to make two layers of diagonally laid uni conform and stack at the transition zones in the bow and the stern. Thats one of the charms of biax...the corners and chines only need one carefull going over, with sweat dripping off your nose as you jab, poke and squeegee.

[NoEyeDeer]

I was questioning the use of one layer of uni running directly across the boat. In other words, using it purely for cross grain strength with perhaps a fine surfacing tissue over it to get a decent finish.

Since the standard sheathing seems to be 6oz(200gsm) cloth a 3oz(100gsm) uni would, if I can get such a thing, give better transverse strength at half the weight.

I wouldn't bother using two angled layers of uni as a pre-made 45/45 or triaxial would be much more sensible. Basically, my original plan was to effectively throw away the longitudinal glass that is not doing anything useful and end up with around half the weight of glass doing the job I want it to do and nothing else.

[michael pierzga]

Sounds logical. Plenty of for and aft strength in the wood strips. If you use a rowing seat and outriggers hardly any load is transfered to the skin. Perhaps some laminate deatailing in the ends and bottom.

[NoEyeDeer]

I'll have a harder look and ask some more questions about what I can get locally. At the moment that 189gsm triax is looking the best. It's quite cheap too. Although it wouldn't save weight, a stronger boat for the same weight is never a bad thing (if it's cheap).

[Landlubber]

...NoEyeDeer...."seeing" as your boat is only a little row boat, why not just sheath it in Dynel and epoxy...easy to use and last basically forever.

[gonzo]

Dynel uses a lot of resin and has a very rough texture. I can't see any advantage.

[Landlubber]

...Dynel advantages..it is more abrasive resistant than glass (about 2x), it provides a FLEXIBLE waterproof coating (its flexural rate is that of timber), it is nominally thin (drapes well for compound shapes), it weighs about half that of glass.
...if the boat does not need structural strengthening, then it is a viable ALTERNATIVE....

[NoEyeDeer]

...NoEyeDeer...."seeing" as your boat is only a little row boat, why not just sheath it in Dynel and epoxy...easy to use and last basically forever.
Because it would be a complete waste of time, effort and money. Dynel does not have the same strengthening properties as glass. I want any sheathing to be structurally useful. That's the only reason it is ever used with strip planked boats.

ETA: I you re-read my OP you'll see that strength to weight ratio is what this thread is all about. :)

[Landlubber]

...sorry mate, just thought that alternatives may interest you, sorry to touch onto ya toes.....

[NoEyeDeer]

No prob. :)

Dynel is good stuff if you just want abrasion resistance but that's not my main interest here. I wont be dragging this boat around and I'll be patching anything that gets damaged before it becomes a problem. What I'm thinking of is getting the boat to be adequately stiff and strong while keeping the weight as low as possible. I'll have to get this thing on top of my truck to cart it around so excess weight doesn't sound appealing.

[PAR]

Actually there are at least a dozen different variations of strip planking, some require a structural sheathing, others only for abrasion, while others till none at all. As a rule laminate is heavier then wood in small craft, which is why you see stripper canoes and kayaks with a 4 ounce skin, which imparts very little physical strength the the wooden strips.

As I mentioned previously, if you reduce the strip thickness, making up the hull shell stiffness with laminate is possible, though you'll be fairly hard pressed to compete with the usual wood species choices, in terms of weight. This said, with the advent of a moderately high end laminate, to replace a substantial portion of the strip thickness, it is possible, likely costly, but possible.

[Gilbert]

Why not use a very light glass cloth oriented at about 45 degrees to the wood strips? Then all the glass fibers will be working across the grain of the strips.

[PAR]

I want any sheathing to be structurally useful. That's the only reason it is ever used with strip planked boats.

Actually, this isn't correct. In most cases the sheathing is abrasion protection alone, in small craft.

Dynel or better yet Xynole are highly flexible, and dramatically improve abrasion resistance. Unfortunately also dramatically increase weight once it's wetted, and faired. For example; an 4 ounce 'glass cloth, wetted well and filled is about 10 ounces per yard. A 4 ounce Dynel or Xynole sheathing well wetted and filled is about 24 - 30 ounces for the same yard. So you need some "substance" of boat under you to justify the weight.

Fabric used in conjunction with a wooden "core" does help in a load bearing way, but this is entirely different then an exterior sheathing. In these cases you actually have to engineer the loads for the 'glass portions, with the core material's physical attributes taken into consideration as well. This would be a true composite.

Back to the original comment about structural sheathing. If you weren't willing to engineer a sandwich composite, you could opt for a one off single skin, which is essentially a thick sheathing over something. Unfortunately, these tend to be quite heavy as the 'glass portions take the load and the "something" is usually just along for the ride, offering little to the structure. In these cases, which are very common among the one off 'glass builds, you could use pretty much anything you want to lay the fabrics over until they cure, so diagonally stacked rhubarb pies would work and once the goo cured, they could be eaten if desired, with no harm to the laminate.

If you want to impart strength to the wooden structure then biax or uni-dia are the answers. These non-crimped fabrics will impart all of their strength without excessive distortion, elongation and fiber breakage, which are common flaws with cloths. In other words biax and uni's will make stronger, lighter skins that do more work, for less bulk, which I think is the key to this discussion - the desire to use a wooden something or other and skin it with a material that will also bear some load, preferably in the direction(s) desired.

[NoEyeDeer]

Well it depends on what you mean by "small craft". In the case of frameless strip planked canoes, kayaks and rowboats the sheathing is primarily structural.

If you're talking about a more traditional build over closely spaced frames then yes, any sheathing would be primarily for abrasion.

[PAR]

This is not correct NoEyeDeer. A 2 to 10 ounce cloth sheath on a canoe or kayak has very little bearing on the load paths of the structure, just abrasion resistance. Strip planking as a method uses the glue lines as the cross grain strength for the most part, assuming reasonable strip heights. Strip thicknesses and their longitudinal orientation provide the fore and aft stiffness.

This said, if a uni-dia or knitted fabric is used, instead of cloth, then yes, you'll get some stiffness and strength improvements, based on the weight of the fabric and resin combination. You see, with 'glass, you need bulk and thin sheets of cloth don't offer much, it's just too thin and the fibers very "wavy". uni-dia, biax and tri-ax have straight fiber orientations and offer the best bang for their bulk, so a thin skin can preform some real work.

Unfortunately, the vast majority of builders don't use uni-dia or knitted fabrics, but prefer woven cloths. In short, if you leave the 4 ounce sheath off, a brightly finished canoe, it'll be just as strong, several ounces lighter and of course more easily damaged by casual use.

Frameless strip planking has been around for about 200 years. It was often skinned with a few molded veneers, because they lacked truly waterproof glues until WW II. It was edge nailed and glued with heavy shellac. Resorcinol was also employed. Many of these builds used bulkheads and interior furniture as the athwart bracing, with no frames, which employ the same engineering concepts used today. Naturally, these none skinned builds may have incorporated slightly thicker strips to absorb surface damage and refairing in the life of the boat, which is another engineering "device" to improve longevity. For example some steel hull scantlings may have as much as 50% of the hull plating, scheduled for life of the vessel corrosion waste. Yep, that's right, 1/4" plate, where 1/8" will do, but across 40 years, they need the extra 1/8" because they expect it to rust away!

This isn't to say they aren't any composite canoes or kayaks, but as a general rule a thin cloth sheath is none structural. I'll repeat, with 'glass cloth you need bulk for strength and there's no strength with a 6 ounce cloth skin.

[dougfrolich]

"I'll repeat, with 'glass cloth you need bulk for strength and there's no strength with a 6 ounce cloth skin."

You need bulk for stiffness, not nesc strength. 6oz Cloth certainly adds strength but very little stiffness. Especially adds strength as a X oz. uni oriented accross the grain of the wood, and especially if that is the short span of the panel.

[NoEyeDeer]

PAR, I suggest you try an experiment. Build two identical frameless, strip planked canoes. Sheath one inside and outside with 6oz cloth and leave the other one unsheathed. Paddle both of them around a bit. Bounce them off a few rocks and beaches. Decide for yourself which one has the most transverse strength. :)

[PAR]

Yea, your right, I haven't a clue . . . maybe some personal laminate testing will help . . .

[Steve W]

On strip planked canoes,kayaks and other small craft what sheathing with light cloth,particularly dynel adds is a"toughness" that just is not there without it and is well worth the added weight which is not as much as theory would suggest.My own kayak for example and my sons sistership we built from plans out of woodenboat and are nearly 18ft long(they are not strip planked but stitch and glue) but we reduced the ply thickness from 4mm to 3mm and sheathed them with dynel and they ended up at around 42lbs even with doubled up bulkheads (so they come appart into 3 pieces) and they have proven to be very tough indeed,( not necessarily stiffer) much more so than the ones we did in 4mm and just taped seams as per the plans. Of course on larger strip planked boats we have built we use uni.
Steve.

[sabahcat]

"I'll repeat, with 'glass cloth you need bulk for strength and there's no strength with a 6 ounce cloth skin."

You need bulk for stiffness, not nesc strength. 6oz Cloth certainly adds strength but very little stiffness. Especially adds strength as a X oz. uni oriented accross the grain of the wood, and especially if that is the short span of the panel.

Do you know much about sandwich construction?

I think you will find that this vessel here
http://www.multicats.dk/images/splitenz1.jpg
http://www.multicats.dk/?Sailing_Cats:Split_Enz

is from memory built from 8mm WRC planks and around 8oz/200gsm uni inside and out

Quite stiff

[NoEyeDeer]

PAR: Personal laminate testing is a good idea. I should do some. The thing is I have a pretty good feel for the properties of thin bits of cedar. Quite apart from anything else I've clad whole houses in the stuff. Its tensile strength across the grain and its splitting strength are not that impressive, despite its other good qualities. I personally would not trust an unsheathed, frameless boat built from 6mm strips. I think it would be far too prone to splitting in normal use. 6oz glass cloth, although it's not the ideal choice, does have reasonable tensile strength.

I haven't built a small stripper myself yet, but everything I've read about them (written by people who have built lots of them and used them extensively) emphasizes the necessity of having glass on both sides for transverse strength. This makes sense to me. If the glass was solely for abrasion resistance nobody would bother sheathing the insides of the hulls. It would be easier and more sensible to put all the glass on the outside. Putting the glass on both sides is exactly what you would do if trying to use the glass for strength.

If you can show me experimental results which demonstrate that unsheathed 6mm cedar is a strong across the grain as as 6mm cedar with a layer of glass cloth each side then you'll probably manage to convince me, but in the absence of said experimental results I'm not likely to be convinced.

[PAR]

I'm not going to debate folks that can't understand the obvious differences between uni-dias, knitted fabrics and traditional cloths. If you're going to debate apples and oranges, then have at it, but be informed, you're taking out your butts if you do as the physical differences under load, particularly in very light structures when stress concentrations on the skin are comparatively high.

. . . what sheathing with light cloth,particularly dynel adds is a"toughness" that just is not there without it . . .

It's just this form of ridiculousness that I'm talking about. If you want to mix and match your discussion variables, like this, I can't be bothered as it's gibberish and nonsensical mush. Mixing polyester and acrylic fabrics with 'glass cloth and uni-dis and knitted fabrics in the discussion is just absurd, and out of hand silliness, frankly. So, if you'd like to discuss the physical properties of these materials, that's one thing, but if you arbitrarily intermix them, then don't expect your comments and opinions to be valid or taken seriously.

[NoEyeDeer]

I'm perfectly well aware of the differences between the various types of cloth. That was why I was specifically asking about uni to start with. However that's pretty much been dealt with on the first page and the thread is doing what threads often do and wandering a bit. I'm still not convinced that in the "usual" construction for small boats the "traditional" glass cloth used does not contribute to transverse strength. I might be wrong, but I'd like to see some evidence.

ETA: I should note that I'm talking about panel strength, not the strength of the hull as a whole.

[dougfrolich]

I agree totally Sabahcat.
Using WRC for the 0degree strength and the glass Uni for other off axis strength (90 and or +-45 ect... as nesc.) and then providing the stiffness by bulking up or adding thickness with a low density core separating the skins---superb!
Actually working on a project at the moment doing just that.

[PAR]

Okay then, maybe we've all veered off course on this thread a little, but the last post shows just the problem. Sandwich construction techniques and light skinned strip construction, two completely different animals and shouldn't be used in the same sentence, so to speak.

We're in agreement that cross grain strength and stiffness can be improved with perpendicular uni-dia or knitted fabrics, naturally with some issues. As I mentioned on the opening page, there are many different strip plank types, so being specific is important. This is simply because there are huge differences between say Load method strip planking, which uses nothing but high elongation fabrics like Vectra and Xynole over a strip core and a Baltek strip method, which requires knitted fabrics and/or uni-dias to be successful. These are just examples and extreme to each other.

If we divide the many possible methods into a few categories then things get simpler. The first category is plane old "wood/epoxy strip plank" this is essentially a carvel hull with narrow edge glued planks, from an engineering point of view. It can be 'glassed, but the 'glass isn't really contributing to the strength of the hull shell as much as it's abrasion and moisture vapor resistance. In this case the high elongation fabrics will be more effective (in both regards) if you can tolerate the weight penalty. Most small boats fall into this category, amazingly enough.

Next would be the "wooden core, light sheathing" methods. These are more composite then traditional structure and typically have no internal framing, just fabrics. This is the biggest column of different methods and should be divided in two. External only and both internal and external sheathings. The externals only are usually closer to traditional, though sheathing thickness can play a role. The both sides method is where things start to get mixed.

For example, there are formulas for reducing the strip planked core thickness for both sheathing weight or, and the usually more preferred method in larger craft, diagonal veneers. This is because it's faster and lighter to use veneers then laminate, but in small craft, not typically employed.

Lastly are the "heavy sheathing strip plank" methods. These are the true cored composite building methods. The core in many cases can be your grand mother's dried up Thanksgiving mashed potatoes, just so long as the peel strength is there and the potatoes stay in place during the cure. Okay maybe a bit extreme, but not as out of line as you might think, given enough laminate thickness.

I have a lot of experience with the Lord strip plank method and it's amazingly light (the lightest of all the strip plank methods), yet uses literally junk core materials. Keels and engine beds are usually plywood and it doesn't have to be especially pretty stuff either, just well coated and particularly well entombed in fabrics. With these types of structures, core compressive strength, peel strength of the resin/fabric matrix and laminate skin density are the keys to success.

When discussing this topic with a rather eclectic, recently past designer. He described it uniquely, which knowing him was his fashion. He said the best way to understand these differences, is to look at the core to laminate percentages by weight. I replied this would be an unfair bias, but he disagreed and his point was if you're using 60% laminate and 40% wood core then you've got a plastic composite of the type seen in most production builds. While on the other hand a traditional glued seam strip plank is 90% wood and 10% plastic. We spent the better part of an hour discussing and categorizing the different common strip planking methods into 30/70's, 60/40's, etc. It was our last conversation and I miss the old, but genial bastard.

[uncookedlentil]

epoxy is heavy, significant gains in strength to weight in glass layups can be made with the use of peelply and it has the added advantage of smoothing out lapped or cut, glass edging.:)

[PAR]

How do you justify a comment like this Uncooked? What is peel ply bringing (or in the case subtracting) to the table that isn't derived with a good hand layup?

[Landlubber]

....leave out the word "significant" and it helps.....

[PAR]

John you're killing me . . . shall I squint too . . .

[ancient kayaker]

Jesus was a carpenter. All of you who insist on layering glass and epoxy over Nature's already perfect composite will go to hell.

Merry Christmas!

[uncookedlentil]

Jesus was a carpenter. All of you who insist on layering glass and epoxy over Nature's already perfect composite will go to hell.

Merry Christmas!

at least I'll be warm and dry for a change! Happy La Nina and Merry Solstice

[Landlubber]

"shall I squint too".....na mate, sometimes it is simply better to close your eyes.......


...somethings are just too painful....

[ancient kayaker]

Jesus was a carpenter. All of you who insist on layering glass and epoxy over Nature's already perfect composite will go to hell.

Merry Christmas!

- sorry about that! One of the inmates at The Wooden Boat Forum escaped but he's back in custody again, so you're all safe.

[uncookedlentil]

How do you justify a comment like this Uncooked? What is peel ply bringing (or in the case subtracting) to the table that isn't derived with a good hand layup?

well par, peel ply buries and totally flattens the glass in one wet out, whereas a proper weave burial without peelply takes three, 3, to get the weave print out of the picture. greater time and weight savings are to be gained when lapping of the glass or working off non selvedged edges.

Sorry to get everyone so excited over the word ''significant'', my experience comes from years of building competitive boats where skinning five pounds was discussed in tones usually reserved for audiences with the pope.

[michael pierzga]

It is nice to use peel ply with hand layup. . Compacts the fibre, drives excess resin out and leaves a much smoother surface Biax fabric print and as a result requires much less fairing and filling. I dont know how much weight you would save, but its Hard to find any reason why not to use peel ply on lightweight, exterior and high visibility surfaces.
I recently watched a French gang of laminators replace the outside skin of a 20 meter cat. Overhead work with peel plied Biax and the results were remarkable, Very little filing and fairing required. Very skilled laminators.

[variverrunner]

Nick Schade of Guillemot Kayaks posted these test results in Jan 09.

The test studies various strip thicknesses and reinforcement types.

http://www.guillemot-kayaks.com/Building/Testing/index.html

[PAR]

Nick also willing admits the tests are inconclusive for several reasons, though test sample grain orientation differences, is a big one. The very limited number of samples and several other obvious procedural necessities, for making reasonable and reliable results, are also noted. It would be nice to see these tests taken to a higher level of reliability.