Undirectional over strip planking?

...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....

 

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.

 

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

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

"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.

 

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.

 

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

 

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.

 

Do you know much about sandwich construction?

I think you will find that this vessel here

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

 

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.