Undirectional over strip planking?

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?

 

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.

 

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

 

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 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.

 

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.

 

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.

 

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.

 

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

 

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.

 

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.

 

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.

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.

 

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.

 

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).

 

...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.

 

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