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"BASICS of Boatbuilding CLOTHS” 

                                  .... what they look like, and what they are used for.

This article is for those who are just starting out in boatbuilding and who might be overwhelmed with all the cloth options seen & talked about.   Once you have this clear, you should be better able to make the right choices for whatever reinforcing you are needing.

(Although I will start with FIBERGLASS  (FG), other cloths like the gold-colored ARAMID (Kevlar) or the black CARBON FIBER (CF) are basically similar in that they all vary in weave, utility and available weights).   

FIBERGLASS comes in different weaves, each designed for different uses.  The basic cloth is what we call a Bi-Directional one .. with roughly equal strands both up & down and across the cloth, each interwoven.     This is often sold as ‘Boat Cloth’ as it’s most commonly used for sheathing plywood or even a planked wood hull.   As fiberglass has greater elasticity than wood, it can accept the normal expansion of wood without being overstressed.

When purchasing cloth, you will need to specify not only the type of WEAVE, but also the unit WEIGHT.   (There are also different strengths, but we’ll cover that later in the article).   Weight of the cloth will relate to its designated duty and represents the weight of a unit of cloth, typically 1 square yard, though different countries and companies sometimes use different standards .. such as 1 sq. metre, or even a yard or metre of a specific width – sometimes even up to 50” wide … so be careful when comparing the claimed weight of different brands, that the related surface area is the same.

Boat Cloth is available in very light weaves (for models etc), but typically the lightest we use for small boat sheathing is 4 oz, with 6oz generally being the minimum for underwater surfaces.

Adding such a cloth is a good way to guarantee and control an even thickness of epoxy over the wood surface.  It also adds some strength, toughness and rigidity, but always at the expense of added weight …. so for a light performance boat, one needs to think carefully about where it’s to be applied.    Personally, I recommend PRE-sheathing the interior of any panels that need a fabric so that the cloth passes under any framing, as all cloths have a problem to lay tight into sharp corners.

Any interior glassing AFTERwards is full of problems.     You can be sure that any void will ultimately find water in it and as it can no longer dry out, rot will start BEHIND the glass in such a case.   For this reason, a bad fiberglass job (passing over structure and framing), is FAR worse that using no fiberglass at all in my opinion.

All inside (concave) corners first need fillings of epoxy putty (made to ‘a consistency of creamy peanut butter’ by adding one of various types of filler powder to catalyzed epoxy resin) and formed smoothly with a plastic squeegee, shaped with a suitable radius that may vary from 5mm up to even 30mm for a high strength joint.

All exterior (convex) corners need to be radiused off with at least a 5mm radius, so that the cloth can be pulled snug to lay totally in contact with the corner surfaces until cured.

A heavier boat may need a sheathing boat cloth of 12 oz or more, but this is more typically for a larger cruiser (over 20ft) or workboat.

Because the weave is fairly even and flat, this Bi-Directional boat cloth is also sometimes used as the final layer over composite sandwich hulls or other surfaces requiring lightweight protection.

It's worth adding a note here on Elasticity.    We often see it written that "it's worthless to add Fiberglass over wood as it's far more elastic than wood so offers no advantage but just adds weight".    Here's my personal take on this.    Yes, it does add weight and therefore its use needs to be measured against the advantages of adding it for each specific area of a boat.   But FG adds abrasion resistance and also helps to control the thickness of the resin layer.  But here is where I may disagree with some.   My tests show that it can also add some stiffness and here I think is why.   The figures for Elasticity are based on how much stretch the material can endure at the point of failure.    This is certainly significantly higher for fiberglass than for wood, but there is STILL some resistance to flexing offered by the FG at the lower level, where the bare plywood would normally fail.    Yes, the full resistance of the fiberglass will seldom be reached when the plywood fails, but on the curve of stiffness vs load, the fiberglass will still add 'some percentage' to the panel stiffness.     There is also a gain in stiffness due to the increase in thickness from the sheathing.  Stiffness goes up by the Cube of the thickness, so only a 3% gain in thickness theoretically gives 9.3% increase in stiffness (assuming the same material).    Stresses are also higher at the outer fibers of any panel so that's where failure will first occur.  Having an elastic boundary layer there is not such a bad thing, even 'if' the wood were to fail first.     And finally, fiberglass with cured resin, is no longer as flexible as the cloth alone.  It's relatively brittle in fact,  as when encapsulated in resin, its seemingly high Modulus of Elasticity is typically chopped by about 65%, according to test figures I've seen using vinylester (epoxy similar), from a federal university in Rio.     So overall, its not quite as simple as the opening statement implies.    

Make your own tests if needed.  Just sheath some plywood and cut 250 x 50 strips in different directions and compare this with unsheathed plywood.    You just have to decide if the improved surface resistance and moderate gain in stiffness is worth the extra weight.    In some locations, I think it is.

Another weave that is being used more and more is called UNI or UNI-Directional, where the majority of fibers lay in ONE direction.    Of course, something is needed to hold these fibers together so some cloths have a few thin cross threads woven in, while others have the cross thread bonded or stitched to one side of the UNI strands.    There is an advantage to the latter for minimal stretch and greater strength, as in this case, the UNI strands lay totally flat and do not need to weave up and down over the cross strands.    So for things like ‘building a highly loaded beam or mast’, the use of CF-UNI strands that are stitched and do not use woven cross threads, is advantageous and recommended.

The reason UNI directional cloth is used more and more is that engineers now do more calculations and have found it’s best to orient fibers as directly in line with predictable stress as possible and when pre-woven cloths are used, they sacrifice that ideal far too often.    To make this even more effective, strands of a fiber (FG or CF) are often bundled together in what we call ‘tows, so that we can add a lot of strength in small areas.    You just need to make sure that ALL the fine strands are epoxy coated though and that there’s not some dry area out of sight in the center of the tow.      

One place that UNIdirectional cloth is used, is as a binder around and across a boat built with wood strips.  While the wood strips give good longitudinal strength to a boat hull, they have little strength across the boat as there are seldom any transverse frames in a stripped boat.  To compensate for this, UNI cloth is laid from gunwale to gunwale perpendicular to the wood grain, both inside and out, effectively serving as transverse framing, yet leaving the interior free of such structure.  

In a system I call TSF for Tension Strip Framing,  I often use UNI tapes (instead of more traditional internal wood framing) to limit deflection and add internal resistance against impact.   This not only saves internal space and reduces the number of troublesome hard spots, but it’s faster, more economical and easier to both build-in and maintain.  See sketch.

CF-UNI is particularly effective for creating very strong connections between parts, with tows of CF being laid in all the stress directions, before being covered by a bi-directional woven cloth to make a smooth covering surface.

Another cloth that’s also used more and more, is called BI-AXIAL.   In this case, the fibers are laid in two directions at a significant angle to each other .. an angle that can be anywhere from say 60 to 90 degrees, with half that angle being from the main line of the cloth roll.   So a 90 deg bi-axial, has strands at 45 deg one side of center and 45 deg to the other.   Such cloths are available either interwoven (and therefore not very flat) OR are laid one over the other, with light cross stitching to hold them in place (see photo).    Typically, both UNI and BI-AXIAL cloths are heavier than Bi-DIRECTIONAL (boat) cloths …. starting at around 6oz and going up to even 30oz per unit weight.  9 and 12oz are very popular as they give good strength but are still thin enough to allow epoxy to penetrate to all fibers.   Once wetted out, the challenge is to then pull as much resin OUT of the cloth as is possible or otherwise there can be more resin than cloth and anything over 50/50 is not desired or recommended, whereas a ratio closer to 70/30 (cloth-to-resin) is more desirable IF achievable.    (This is where vacuum-bagging of a wet laminate for compression by the atmosphere can help, but it’s not practical for all applications).

Another weave used for high strength where weight is less critical, is WOVEN-ROVING.

Rovings are like the TOWS described above .... bundles of strands/fibers all together.    So Woven-Roving are interwoven Tows.    Being large coarse bundles, these cloths tend to readily slip apart but also take up different shapes quite easily.    They are often used inside bow areas, on hi-load chain-plate bulkheads or to reinforce transoms for heavy motors and also used as the inside skin for motor and work boats.    You may even find a relatively lightweight one as the inner skin of a general purpose fiberglass canoe … but it will not be a lightweight one, unless the fiber is of Kevlar.     (Kevlar is a noticeably lighter fiber than Fiberglass and being stronger in tension, is often woven with thinner tows).

Another bi-directional cloth that you will see advertised is ‘TWILL’ .. often with an added note ‘2x2’.   This means that instead of each strand going under and over 1 strand, it goes under and over 2 … and in both directions, hence the '2x2' (see photo).      This creates a less stable cloth that will deform more easily, making it more ideally suited to covering shapes that are 3 dimensional, with a hemisphere being a good example.    It’s very seldom needed or used in boat building, unless you have a specific item to sheath that has an odd shape.

The other cloth I should mention is the classic MAT or CSM as it was called for Chopped Strand Mat.  This cloth is made of short fibers (about 40-50mm long), blown down in totally random arrangement and held together with a bonding agent.  This was once the basic material for all early fiberglass boatbuilding back in the 1950’s as it added bulk thickness fairly easily and also offered ‘adequate’ strength in all directions without needing to do much calculating.  While it still has the occasional use due to its ability to mold around any shape, boat building has since become more sophisticated, with composite cores and hi-tech weaves that allow the material to be more effective in dealing with the calculated loads, thereby reducing weight and achieving far more effective glass-to-resin ratios.    With MAT, it’s a struggle to keep the resin below 60% of the layup, whereas with the new weaves and application methods, the resin part can be brought down to as low as 30-40%.  This offers more strength and endurance, with lower weight.

Something else worth mentioning here is that the binder for typical random MAT is one suitable for using with polyester resins, as that is the common combination.   If you DO use MAT with epoxy, you’ll need to be patient and really work the resin in, as the binder just does not breakdown so easily without the styrene in polyester.  It can be made to work, but takes more time.   There are a few suppliers who DO make MAT for either resin, so look into this if you need it.    But as MAT cloth is a very effective binder between two hard surfaces, one can peel off a thin layer of dry MAT and this is then easier to impregnate with epoxy - see more below on this.

Although we seldom use mat anymore in modern boats, it does have some properties that are worth remembering.  The random fiber arrangement bonds very well to a variety of surfaces and it does not peel off or delaminate from plywood like a sheet of regular cloth will.   Many years ago I designed a 15ft double chine canoe for my 12 year old son to make as his first boat.  (I still have the offset table ;)

It was to be really inexpensive with a total budget of $100.   We bought a couple of sheets of 3mm birch plywood (door veneer) for the skin and used ‘stitch & glue’ to hold the panels together until joints were made.    We used CSM cut into 2.5” wide strips for the longitudinal joints inside and out, wetted out with polyester resin to cover the grey polyester car-body filler that we used to cove the joints and cover the wires.

The bottom was stiffened by bonding in ¾” polyurethane foam panels with contact cement and then glassing over on the inside with the whole boat then painted inside and out.   I called the system PLYFOAM and also used it for a 12’ sailboat that my daughter made, also at 12.    Well, the sailboat fell apart in 3 seasons (after too many capsize soakings ;) but the canoe was a miracle … lasting 29 years before the thin plywood became too rotten to accept any more fiberglass repairs.   But what was interesting is that after all those years, the CSM was still strong and effective and the wood veneer it was bonded to, was still intact (see photo).   Because of that experience, I always keep a little CSM handy and to improve a bond, I peel off a very thin layer of the random mat .., perhaps only ¼ of the full mat thickness, and drop that on say a metal surface that I am bonding too, to help the bond at the initial interface.   Although CSM is undeniably heavier than cloth and not mechanically as strong, its ability to bond and seldom delaminate is an interesting aspect worth remembering and using to advantage.

Finally, CSM is useful as the first layer when filling a Vee'd joint .. as when making a 'hybrid' scarph to join two pieces of plywood with a butt join using glass tapes.    While such a joint can be made with a shallow Vee on both sides, I find it's easier to achieve on the boat itself if the inboard side has a thin butt strap (grain parallel to the plywood its joining and with beveled edges for the butt strap).   Then the Vee is well ground out to finish the joint on the exterior, using several layers of different width tapes.   This is where starting with say one or two strips of random mat (CSM) can really help fill the Vee and make the initial bond, with bi-axial and bi-directional tapes over to complete the joint.   (This is a system I recommend for the plywood side panels on my W22, and in this case, the interior butt strap - thinner than the main plywood - is also faired and glassed over with boat cloth to remove any hard transition).    

Although this covers the most common weaves, there are also combinations of say ‘UNI with BI-AXIAL’ that are available stitched together (called Tri-Axial), as it’s rare to see UNI applied without a layer of BI-AXIAL.   If you wonder why, it’s because UNI offers virtually NO shear resistance, as one parallel fiber can easily slip on its neighbor. But BI-AXIAL is the master-fiber to resist shear (that typically runs at 45 degrees to the direction of tensile or compression forces), as the Bi-Axial weaves are typically at that same 45 degree angle.  Generally though, we only need about 20-30% of the fibers to be Bi-Axial, to match with 70-80% of the fibers in a UNI direction, although certain connections may demand a more equal distribution.

Occasionally, one sees some fiberglass cloths that are ‘outside the norm’ and perhaps selling at discount.  These are probably ‘left-overs’ from some special order for a particular weave for a custom job.     Whether they are worth buying, depends on what the weave is and what your needs are.  One particular weave I found a few years back was a 6oz cloth but with a very open weave almost like a netting with open squares of about 4mm (see photo).   Now this would be useless in any areas that needed finishing or where water could collect, as the heavy strands around each open square would not allow good drainage.  But as a cloth to strengthen the underside of plywood decking, it was great.   The open weave allowed it to bond well with epoxy and the mesh was very strong with fibers concentrated into heavy strands about 4mm apart.     I used it under the cockpit floor and also under all the decks of my boat and it’s proven to be most effective, both to lower deflection as well as offer good surface protection.   

So if you see something ‘On SALE’, try to get a small sample to see exactly what you are getting and if it fits your need, go for it.   I also once bought a very lightweight 4oz UNI cloth, but that was not successful.   It was so tightly woven that epoxy would not easily penetrate, so the cloth tended to float on top instead of bonding to the plywood.   So test out a sample.

I had a similar experience with some 5 oz bi-directional Kevlar, but found ways to make that work and that one was certainly ’cost-effective’.

You will also find and need some cloth tapes. These are pre-woven into certain widths (typically 1” to 6”) and of different weights and weaves.  The reference weight is still based on a sq yard (or metre), so 8-9oz is a good weight for a bi-directional woven tape used to cover the epoxy filled coving.    Two 3” wide tapes can be laid with half the width (1.5”) overlapped over the center of the coving to create a double thickness at the critical throat of the corner.    Tapes are generally available for all weaves of cloth and are convenient to use, though more expensive than cutting your own from a wide cloth.   They also have a somewhat raised woven edge that will need sanding off or fairing in, but at least the strands stay more in place compared to cut cloth.

In the case of CARBON FIBER, there are both structural and cosmetic finishing cloths so don’t get them mixed up.    Buying a roll of CF Tow is always useful as this can be laid anywhere to reinforce a highly stressed area and then over-sheathed with a FG cloth, so that none of the important CF is ground away while fairing.     While it’s clear that CF is used for highly stressed areas, it’s important that enough material be applied.  This is because it stretches less than the other fabrics, so will carry almost all the load before the other materials start to really work.   

The use of KEVLAR is for abrasion resistance and as a lightweight fabric if under tension.  As it is very weak in compression, it’s only effective on the tension side of any beam or surface and useless for say a mast where loads alternate..   When exposed, KEVLAR will fluff up and be almost impossible to hold down for fairing, so frequently it's used with layer of glass over it to combat that difficulty.     Kevlar is relatively light and tends to float on any pre-resin'd surface (as does Dynel), so special application techniques are needed to get/keep the air out and avoid excess resin.   One way is to wet out the cloth on a separate table and then roll it up wet on a dowel or tube.  Then unroll this on to your clean, dry surface and you will have 'just' enough resin to make the bond without enough to float the cloth.   If a few spots are dry, its easy to stipple in a few grams of extra resin and roll it out.   Kevlar is also notoriously hard to cut, but again, there are tricks.  One is to add masking tape to the cutting line to stiffen it and then it can typically be cut by any heavy duty steel scissors.

Fiberglass fabric comes in two basic strength qualities.   The standard one is called E-glass and the other is S-glass which is significantly stronger but far more expensive.  The E was originally for ‘Electrical Grade’ when used for insulators, but you might want to remember it as ‘Economical’.  The ‘S’ grade can be thought of as Super-glass, and there are now even S2 and S3 versions.

By the way, 6oz cloth is approximately equal to 200grms, so 300grms equates to 9oz and 400grms to 12 oz cloth etc.

You may find this YouTube adds some visuals to this discussion …


Fibreglass does have a higher elasticity than the other fabrics and this leads to both positive and negative aspects.   Certainly no good for a stiff mast, but if you are making up a fairly thick laminate that will be under stretch, you have a better chance of all the fibers taking load without breaking with fiberglass.   The Hinges and Latches on my W17 are a case where I believe FG is more suited than CF because of that aspect.  With CF, there is less margin and all fibers must be more evenly stretched out to share the load without internal strand overload.    Once some CF fibers fail, their neighbors immediately take more load and full fracture can follow fairly quickly, often surprising those not prepared.    Some CF structures and beams (incl some masts) are not always adequately designed and while wall thickness may be adequate for the tension and compression, it may still be insufficient for local buckling.  (As a guide for beams or masts, I’d be wary of any wall thickness under 3% of the beam/mast diameter, without some internal stiffening).  

While the above introduction is by no means ‘complete’, it should ease you through the first steps of familiarity with the various weaves and materials.    Keep exploring and testing.                                                                                                                                                   

COMPARISON of Properties                                                                                                          

 Although this is only a general info article, it’s of interest to compare basic fiber qualities.   It may surprise some to see that S2-glass is significantly stronger than both carbon and Kevlar – but is both heavier (Density) and less rigid (Youngs Modulus) than Carbon or Kevlar 49.   And note that Kevlar is only 56% the weight of E-Glass.    What is not shown here is the weak compressive strength of Kevlar that prevents it from being used far more broadly.  

(See 'Building the W17 Main Hull Part 1 'for more on using KEVLAR , via the W17 Main Page) 

Although S2-Glass is more than double the strength of E-Glass and 10% stiffer, it’s also (at this time of writing) many times more expensive & also 73% heavier than Kevlar.   But its high strength associated with high stretch without failure, can make it ‘a good fit’ for a few specific applications.

For passing interest, the above table also includes Alumina-Saffil, a fiber used to reinforce aluminum and magnesium castings to create Fiber Reinforced Metals (FRM).   For some 20 years now, such fiber has been used in some alloy engine parts (such as pistons, engine blocks and cylinder heads etc) … making them as strong as a cast iron product but much lighter.   More info can be found here:

http://www.saffil.com/index/fibre_home/metal_matrix.aspx                                                                                                                                                                   Mike 2018

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