Biax with or without mat?

[Ryan]

Has anyone ever used biaxial fabric without that attached mat to make repairs in places that you don't need/want the added thickness buildup? I have a hull repair to make in a spot that the hull is only a 1/4 or less thick. If I use the standard 17 oz biax w/mat (epoxy resin will be used, regardless), I'll be lucky to be left with two good layers once I am done sanding. On the other hand, I could lay down a few more layers if I stick with mat-less biax. Is there strength to be gained with the mat?

[Tim]

I have used the mat-less biax a number of times, and it works very well. The mat is there more to help the stitched material hold its shape than anything; you'll find that the mat-less biax will easily distort and stretch out of shape.

Beyond this one caution, you will do fine with the mat-less biax, and will be better off for what you are proposing.

[Harry James]

Is the mat in the biaxial suitable for use with epoxy? I had always heard that the binders in mat wouldn't disolve in epoxy. Is there a mat made to use with epoxy?

[Tim]

Is the mat in the biaxial suitable for use with epoxy? I had always heard that the binders in mat wouldn't dissolve in epoxy. Is there a mat made to use with epoxy?

The mat attached to the back of materials like common biax and other similar materials is lightly stitched in place, and does not contain binders. These materials are ideal for use with epoxy.

Plain chopped-strand mat, which relies on binder to hold it into usable sheets, can sometimes be insoluble in epoxy, but this is not a hard and fast rule. Many binders are soluble. Even the so-called insoluble binders will still allow the material to be wet out with epoxy, but with less than stellar results. Fortunately, there's little reason to consider using CSM for repairs or construction.

[Ceasar Choppy]

The biggest problem I have heard with using mat (and this goes for woven roving too) with epoxy is that it is not always easy to thoroughly wet-out.

But I have used it successfully by using plenty of epoxy and wetting out both sides well before applying it to the repair surface.

[cliffg]

I have installed new marine plywood coring material to most of the deck on my Kittiwake 23. I now have to recover it. I had assumed that I'd just use fiberglass cloth, something liek 7 oz. Recently I read somewhere else that fiberglass cloth shouldn't be use, but mat should instead. I'd apprecaite any thoughts or suggestions.

[Jason K]

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[Tim]

You will find that many, many people seem to think that the term "mat" is some sort of generic term encompassing all sorts of fiberglass. IT IS NOT. This is a real pet peeve of mine because it leads to a lot of unfortunate misuse of fiberglass products, and using the incorrect terms for anything is always a poor idea as it leads to confusion.

Mat is a specific product, also called CSM (Chopped strand mat), in which hair-thin strands of fiberglass are held together in a random matrix with some sort of binder material--glue, for all intents and purposes. The binder is designed to be soluble with the resins used--usually the styrene in polyester resin. Mat has very little strength on its own, and should never be used in a structural capacity. Mat used to be commonly used in alternating layers with the old heavy woven roving 90? material that many old boats, including your Kittiwake, were built with. The mat helped the wide-open weave of the roving bond successfully together. But this technique is inefficient and produces heavy, resin-rich laminates that are really not that strong for their weight. The newer materials are a better choice these days.

"Mat" never means "cloth", or "biax", or "roving", or anything else. Mat is mat.

Mat is not, in my opinion, anything close to what you want to use on your deck. And Jason #218 is right in that 7 oz cloth is pretty lightweight, and you'd need a large number of layers to build up any reasonable thickness. Your outer skin on your original deck is probably 3/16" or 1/4", so to replicate that your best bet is a heavier material like biaxial cloth, 1808, or other similar structural fiberglass product.

A readily available material is 1708 biaxial cloth in 17 or 22 oz. weights, including a lightly-stitched layer of mat (yes, real mat) on the back. A similar biaxial material is also sold by West System in a 15 oz. weight, though this has no mat on the back and can be easily stretched badly out of shape. Biax of either type is relatively inexpensive and easy enough to work with, and is a good all-around material for most structural fiberglassing work. Jamestown Distributors is a good source for this material.

[kendall]

Just finished a new seat pan for my bike, several layers of fiberglass, and a couple layers of mat to stiffen it up some, after doing so I came to the conclusion that mat was invented by masochists for their own personal use.

Hopefully I can use cloth for everything in the fuure.

ken.

[Ceasar Choppy]

This is a real pet peeve of mine because it leads to a lot of unfortunate misuse of fiberglass products, and using the incorrect terms for anything is always a poor idea as it leads to confusion.

Kinda like how a lot of folks use brass and bronze interchangably.


Thanks for the edumacation there Tim. I'd also add that while mat can sometimes be hard to wet-out, woven roving sould be avoided because it is even more difficult to wet out-- and like Tim suggests, can be too resin-rich. Alternatively, if it is not wetted out enough, when used in a lay-up, it can starve a lay-up of even more resin as it continues to soak up the resin around it.

[cliffg]

Tim,

Thanks for taking time to give such a thorough explanation. I've done a little bit of fibergalss work but not very knowledgable. I will follow your advice. I'm hoping to get a good bit of the work done this weekend, unless of course my wife comes up with something critical at home!!!!!

Thanks so much.

[Tim]

...woven roving sould be avoided...

As far as I'm concerned, there is no longer any reason why woven roving should ever be used. It's an outdated material that has been replaced with vastly superior products that accept resin more consistently, are stronger for their weight, and produce much higher fiber/resin ratios.

[FloatingMoneyPit]

So, I wonder if Tim or anyone else could propose an update to the old "mat-mat-roving-mat-roving-mat..." laminate schedule for a hypothetical all-epoxy structural piece (e.g. single-skin hull/deck, battery box, hard dodger), using modern fabrics. I've used 9 oz cloth and roving for a few small projects, and as suggested above, the number of layers required and resin ratio are not ideal.
All biax? Cloth-biax/mat-biax/mat-cloth? Anyone happen to know the cured thickness of say 22 oz biax vs. 9oz cloth?

[Ryan]

When I laminated new decks on my boat, I used four layers of 17 oz biax (biaxial fabric with 3/4 oz mat stitched to the back of the fabric) and one layer of 10 oz cloth on top. The cloth final layer was a suggestion from Tim and others here, mainly to help smooth out the rough texture of the biaxial fabric and soak up any excess resin. It worked wonderfully.

Ryan

[Tim]

Anyone happen to know the cured thickness of say 22 oz biax vs. 9oz cloth?

Not exactly, but suffice it to say that the biax is substantially thicker when cured.

It's not all about thickness, either. One layer of 22 oz. biax, by virtue of its construction, is vastly stronger than several layers of cloth, whether or not they come out remotely to the same cured thickness. Thickness of laminate also translates to impact resistance, which is why certain areas of a boat need to be thicker than others (hulls, for instance).

Those of us with old fiberglass boats often like to brag about the hull thickness. The thickness does provide a sturdy, flex-free laminate, but as far as the ultimate strength goes, a modern hull built with less thickness and less weight would be stronger in most applications and under most testing methods. Thinner laminates will flex more; flexing is directly related to the overall thickness. This is why cored structures work, in part: they effectively simulate a very thick structure, but without the weight. (There are other engineering reasons why cored structures work the way they do, such as the "I-Beam" concept, but these are not necessary to go into here.)

Material like the fine fiberglass cloth (6 oz, 9 oz, 10 oz, what have you) is really a sheathing fabric, not a structural material. With enough layers, you will eventually build up thickness, and a fabric/resin ratio similar to that found in an old woven roving structure--that is to say very resin-rich. But despite the thickness, the structure will not be as strong as it could be.

Laminate schedule for any piece depends greatly on its need for strength, first, and also on the need for stiffness in the completed piece. For example, one continuous layer of something like 22 oz. biax, laid up over a mold shaped into a battery box, would be strong enough to hold the batteries, but might contain too much flex to be effective. You could deal with this by adding additional material (heavy and ultimately expensive), or by adding structural stiffeners to reduce the span of the solid laminate, or by adding core material and an additional skin.

For solid hulls, 1/4" of thickness, using modern materials, is surprisingly strong, but flexes substantially. Significant stiffeners are required to reduce the span to a reasonable level--bulkheads, stringers, and that sort of thing. Or, one could add core material and significantly stiffen this structure.

I say this only to illustrate the point; I am not advocating 1/4" single-skin hulls. But the strength comes from using the right materials, such as biax, or even more exotic materials like carbon or kevlar or unidirectional fiberglass in certain areas, and from acheiving excellent bonding and controlling the glass/resin ratio, and not necessarily from the thickness.

For a battery box, it would be fun to build one from single skins of fiberglass on either side of a Corecell or other foam core; it would be lightweight and very strong, particularly if vacuum bagged. Same for a hard dodger: instead of a boxy, heavy, awkward plywood structure covered in a layer or two of sheathing fabric, as with those that are usually seen, how about a more rounded form that actually blended with the lines of the boat, made from structural foam and vacuum bagged with a couple layers of strong material on each side. Now that would be slick.

As to the laminate schedule required using modern materials and epoxy resin for a single-skin hull: that's far too much into engineering for me to begin to guess, and I never would. It would partly depend on whether you wanted to build a hull that was strong enough without being vastly overbuilt, or if you just wanted to build up enough layers to feel safe.

We've seen hull engineering stretched to the bare minimum limits in many of today's race boats, especially the America's Cup class, which has provided us with several spectacular hull structural failures. Obviously boats like those that broke apart were clearly not strong enough despite the rigorous engineering behind their construction, and they were built with no saftey factor at all, but from this sort of engineering and failure the entire industry learns and grows.