Fiberglass fabric and tape

The main function of the fiberglass cloth (fabric is used interchangably with cloth in this article) is to provide a strong exterior sheath for the hull. Epoxy/fiberglass sheathing provides a very strong structure, indeed, and blend perfectly well with plywood. Fiberglass cloth comes in standard widths (like 50 or 60 inches) and 10 or 20 yard or longer rolls. It is up to you to decide on the proper length. The cloth is wowen fabric with 90° threads, quite similar to ordinary cloth. You can cut it with scissors or safety razorblade to obtain any size you want. Depending on the thickness, the fabric may have various grades like 4 oz., 6 oz., 9 oz. and 12 oz. (I think that these are the weights per square yard). In Turkey they sell it in square meter units and the weights are 135 gr/sq. meter and 200 gr/sq. meter, corresponding respectively to 4 oz. and 6 oz. grades, I believe. Of course, the heavier the cloth, the stronger your sheath will be. The negative side is that more epoxy is needed to saturate the cloth and fill the weave. This contributes to the strength, though. Multilayer cloth application is also possible, though not necessary for boat sizes in the realm of most amateur builders.

There are also non-woven (or knitted) cloths and mats. These are stronger, but also result in a rough surface, hence do not usually find much application in stitch and glue boatbuilding. One exception is the seams, where tapes made from knitted fabric are recommended, especially in larger boats. The threads in knitted fabric (which is frequently called "biaxial fabric") are ±45° and this is said to contribute to the extra strength. It is also usually of heavier grade than wowen fabric. To obtain a relatively smooth surface, it is recommended to apply a layer of woven tape above the biaxial tape. Otherwise, you'll need to sand these hard to access corners for days and nights. Fiberglass cloth and tape application is described in the article on epoxy, therefore I am not going to repeat it here but stress only what -I believe- is important to the first-time builder.

The hull seams of a stitch and glue boat deserve the most attention. Therefore, after filleting the seams, you have to apply tape over these. On a boat of the size of the D4, a single layer of tape inside and another at the outside of the hull is enough. I used two layers inside the hull, and one layer outside (the outside will benefit from the fabric sheathe as well). For larger boats, more layers are recommended and especially if the boat is longer than say, about 5 meters, it is best to use a knitted tape as the first layer inside. Epoxy manufacturers sell wowen or knitted tapes of various grades and widths (like 9 oz. and 2" wide, 3" wide, 4" wide, etc). An important point in taping is the following: When you are going to apply multiple layers of tapes, the narrowest tape has to go first, followed by tapes of progressively larger widths. This is said to distribute the load more evenly. Of course, you can cut your tapes from the fabric instead of buying ready-made tapes. While cutting tapes from wowen fabric, do not cut in parallel to one of the weave threads. This results in loose fibers from the edges of the tape. Cut with an angle of about 20-25° to one of the threads. The advantages of home-cut tape are the following: It can be wetted more easily, you are not limited by standard widths and can cut to any size you want, and it turns out to be cheaper. The major disadvantage is that even if you cut with the angle mentioned, some fibers eventuallly come loose and mess up your work, especially if you play with the tape too much. For the inside seams, I used 5 cm home-cut tape for the first layer, and 7.5 cm ready-made tape for the second layer. Ready-made tape does not have the problem of loosening fibers, therefore it is much easier to work with. Because of this very same fact however, it has thicker edges which can be felt, and do not look good in the finished boat.

As for the outside sheathing, make sure to overlap the cloth (by at least 10 cm) when your cloth piece is not enough to cover the hull alone (it will rarely be enough). Don't be afraid to use multiple pieces provided you observe the overlap rule. It is a good idea to overlap the cloth at the stitched seams even if your cloth has sufficient dimension to cover that section in one continuous run. In this case cut the cloth and overlap at the seams by at least 10 cm in each direction (total overlap becomes 20 cm). This is easier if you apply the length of the cloth in the fore and aft direction. Since the width of the cloth will not be enough to cover the width of even the smallest of hulls, you'll have to make overlaps along the seams.

Fiberglass cloth, when epoxied, cannot make tight turns and needs a minimum turn radius. Therefore, the corners which will be coated with fabric have to be rounded somewhat. The inside of the hull does not require fiberglass cloth sheathing and sealing the plywood with epoxy coating is usually enough. It does not do any harm though, but keep in mind that the inside of the hull has many structures and tight corners which make it difficult to sheathe. In bigger boats, it is recommended to sheathe relatively flat areas where more resistance is required. An example is the foredeck where the anchor may be dropped.

Sanding is what a stitch and glue boat requires most, and you'll be sanding over fiberglass cloth and tapes all the time. Avoid inadvertently sanding into the fabric. If this occurs, stop immediately, repair the damage with unthickened epoxy, and sand later.

One final word about fiberglass: I have read somewhere that some fiberglass fabrics used with polyester resin may not be compatible with epoxy. Keep this in mind if you buy your fiberglass cloth from a source other than your epoxy supplier. If you are unsure, and cannot obtain a reliable answer, test first!