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Biaxial fiberglass notes

Page history last edited by PBworks 14 years, 5 months ago


Woven cloth weaves bundles of fibers over and under one another. This is the same weave you typically see if in a table cloth, or wicker basket. Biaxial fiberglass is laid down straight, with no tight weave. On the other hand, a woven cloth relies on fibers that weave over and under each other, creating crimps that tend to be fracture points in the fibers, and in the case that they do not fracture, they will stretch far more as the loaded fibers seek straighten out. It's also import to note that in all hull laminates where biaxial is allowed, Winner uses a 100% biaxial cloth.


The straight, flat paths of fiber bundles in biaxial leads to greater strength and stiffness. By putting down straight, flat fibers, directly in line with loading forces, the fibers will resist stretching and breaking immediately and with 100% of their strength. Biaxial also tends to have a higher fiber count than woven. This has three advantages. First, more fibers means greater strength. Second, higher fiber concentration means less matrix (resin) and therefore far less brittle laminate. Third, less matrix (resin) means less weight. So in addition to the fundamental physical advantages of biaxial, we get the secondary advantages greater fiber content, greater impact resistance and lower weight.


By contrast, the crimping of the fiber bundles in woven cloth leads to stress differentials in the fiber bundles and a lower overall strength. When fibers are bent (as they are in woven cloth) they tend to fracture, weakening the composite.


The crimp in a woven fabric also has a tendency to straighten itself out under load, reducing the stiffness or modulus. Another disadvantage of woven cloth is bumps, or high point that naturally come from the weave. When the laminate is laid up in the mold and the resin is rolled out or squeegeed, these high point are damaged and fibers are broken. Further more, when the surface is finished, it has to be sanded, which again damages these high points and fiber bundles. Biaxial tends to have fewer high points and therefore is damaged less in building process.


Our cloth is what's know as a double bias +45°/-45°. What that means is that the fibers are rotated on a 45° angle from vertical and horizontal. This has several effects:


First, it lines up the fibers with the stresses greatest stresses on the boat. An boat is constantly be twisted by two great levers, the mast and the sailor, at opposite ends of the boat. The harder the wind blows, the harder the sailor hikes in the opposite direction. The result is a huge twisting force on the boat. Every millimeter of hull twist that can be prevented is energy that's transmitted directly into forward motion instead of hull flex. The orientation of the biaxial puts the fibers directly in line to resist that twisting. In contrast, a woven +90°/-90° , which most builders use, has NO fibers in line with these twisting forces and presents it's weakest axis to the most important forces.


Second, a double bias +45°/-45° is oriented so that all the fiber bundles cross the floor/side joint in an Optimist. A woven +90°/-90° is oriented so that half the fibers are running the length of the hull and do not cross the hull/floor joint at all. Further more, the +45°/-45° orientation of Winner's biaxial creates a greater effective radius because the fiber bundles cross the hull/floor joint at 45° angles. This means that the loads are dispersed over a greater length of fiber which leads to a 41% joint over any given radius. More than twice the fibers crossing the joint, plus 41% greater strength per fiber bundle equals a 282% stronger joint. Materials do make a difference!

In larger boats a knitted fiberglass tape is used. Many plans & designers will refer to it as a biaxial fiberglass tape, bias tape, or use nomenclature such as DB120 or DB1708. These are heavier fiberglass fabrics that have all the fiber on one plane orientated in the same direction and on the other plain turned and orientated in another direction. For taping the seams of stitch and glue boats double bias, meaning that the fibers are orientated at +/- 45 degrees, is what is used. Knitted fiberglass fabrics however have not been designed for use as a cosmetic fabric and often in areas where the seam will be seen it is top coated with a lightweight

fiberglass tape or cloth.


Strength of woven fiberglass and biax composites


From: http://www.tackleshack.com/sailing/opti/mk12.htm

There is more than one kind of biaxial cloth. In fact there are many kinds of

specialty unwoven glass fabrics, but "biax" as it is normally referred to is

the most common.


The +45/-45 degree biaxial cloth is ideal for making your own glass tapes for

joints. When you cut long strips of this cloth both layers of fibers are

oriented 45 degrees from the lengthwise cutting direction, thus both layers

of fibers cross the joints -- and the more fibers that cross the joints the

stronger they will be.


We also use 0/90 degree biax for strengthening and reinforcing longitudinal

structural members such as stringers. This cloth has 80% or 90% of its

fibers running in the lengthwise direction specifically for this purpose --

it is much stronger in one direction than the other.


In general, all biax is stronger (for the same weight) as woven e-glass

because the fibers are not bent up and down like they are in woven cloths, so

they are already straight. This means they do not have to bend when

stretched before their ultimate tensile strength is reached, so they can

resist bigger impacts.


However, biax also costs significantly more than plain weave e-glass cloth,

and in most home-built boats it may not be worth the added cost. Then again,

if you have the money, why not use it?

You can get a smooth finish over biax, so there's no need to cover it just to

get a smooth finish.


I don't know how much strength is required on the joints of Enigma, so I

cannot recommend any particular glassing schedule. However, two layers of

biax will be stronger than one layer of biax and one layer of woven cloth of

the same weight.


Biax costs a lot more though, so if the added strength is not needed, why use


> In regards to fiberglass, I have been happy with this combination- 4

> oz glass on the topsides, 6 oz on the sides and several layers of 10

> oz on the bottom. I like 10 oz glass instead of the woven roving for

> the bottom because I think it is easier to finish. On Little Cruiser I

> used 5 layers of 10 oz on the bottom, but if I were to do it again I

> would have done only 3-4 layers. For swamp thing I used 2 layers of

> 10 oz on the bottom, and Matt used the same amount on Enigma. I think

> for Paradox you will probably need a minimum of 15 yards of 10 oz, 8

> yards of 6 oz and 8 yards of 4 oz. Of course I would buy extra so you

> have enough to do the rudder/blade and mast.



If I pull my finger out, I should be just about able to glass my boat

before the cold weather sets in.

Looking at cloth I'm really confused about the properties of different

weaves. I've alreay got 450g/m biax for the bottom, but when it comes

to the topsides I'm unsure what to use.

The only 4oz (135g/m) cloth I can find is plain weave, but it isn't

wide enough. To get the width I'd need to go to 200g/M which I reckon

equates to about 6oz/y but this is only available as twill or crows

foot. I can get 25 metres of 260gm (7.5 oz) woven roving for less than

the price of 10Metres of 4oz plain weave, which is more than enough for

the whole boat. Do you think 7.5oz WR overall is too heavy?

Jack, in a previous post you mentioned you used 12oz Biax on the

bottom of your boat. Did you use just a single layer. I was going to

use 600gm biax (which is about 20oz I reckon), and was wondering if I

needed 2 or more layers. Matt specifies 3 layesr of 24oz WR if my

memory serves me right.

I cant remember exactly how much stronger the 12oz biax is to WR, I

think it was 12 times, the reason being that a triangle is stronger than

a box, with one layer of 12 oz the boat is ridged, stiff. I thought that

the WR would soak up $$$$$ worth of resin and I could get the same

properties with one layer of biax. less weight.


I built a flat bottom 19 foot power boat, plans specified 12oz biax 1

layer on bottom and 2 layers along the chines, boat is much much heavier

than a paradox and will do 28 mph, I have beached her, and scraped a few

rocks, no damage just some scratches, also very rigid hull.


If you do elect to use biax be sure to use no mat backing type, as the

mat backing is for the smelly resin, not epoxy, and it actually melts

the mat, epoxy will not, it will not become part of the matrix.


At the time Paradox was built and used by matt, I dont think biax was

availble, most parts for paradox are common hardware store items,

simple... I just felt it fitting to make it even more simple, I think

the cost is prob less, its gonna take alot of epoxy to fill 3 layers of WR.


Paradox isnt a very big boat, and not a heavy boat, nor fast, beaching

on a large rock, the boat will just move wherever it has to go rather

than having the momentum to hole the bottom, or create heavy damage.

course being hit by something that has speed and mass and wieght is

gonna be a problem, but then bottom isnt the problem, sides, and top

are, unless its a sub :)


In any boat, the Captain is the most important part of the boat, take

care of her and she will take care of you!!

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