The Seamy Side of Covers

Why is my cover leaking? That is the first question an end-customer asks when they find water on the floor of their boat or on their restaurant’s patio tables. Whether it is a boat cover, an awning, or anything else that is protecting possessions from the elements, the cover is being used to keep those elements out. And expectations are that water won’t leak through. Most materials being used today are highly water-resistant and don’t leak, even those without a coating. So, when Marlen Textiles gets a call about a cover or top that is leaking, the first place we look is at the seams, especially if the covers have been sewn. More often than not, that is the source of the leak. So, if that’s the usual culprit, why use sewn seams in the first place? Many fabricators utilize sewn seams in their offerings, and they do so for a variety of reasons, including cost, convenience, strength and versatility.

The basic premise of sewing involves using a needle to punch a hole in material, allowing the thread to be pushed through. After the needle goes through the fabric, the hole does not magically seal up. In fact, it has just created a path through which water can begin to leak. Even so, people have been sewing industrial textiles for as long as there has been needle, thread and fabric, and the vast majority of sewn seams have no leaking problems. Yet, there are the occasional issues and some of this may be due to not fully understanding the dynamics of what is going on.

The Science behind it all: What you need to know about sewn seams during fabrication so you can avoid leaking at that spot. But first, a quick history lesson!

Once cotton duck and cotton thread become popular for use in industrial applications (around the 1920’s), it quickly became the industry standard. So, for many, many years, cotton thread was being used to sew cotton duck fabric and leaking seldom occurred at these seams. This is due to the fact that cotton is a natural fiber and one of its main characteristics is that it absorbs water (significantly more than nylon, polyester, acrylic or really any synthetic material). When the cotton absorbs the water, the fibers (both thread and yarn) actually swell and become larger. For instance, when you take cotton blue jeans out of the washing machine, they are heavier than normal because of all the water being held in the cotton fibers, and they are “bigger” than when they are dry (as they have “grown” with the addition of water). Likewise, when cotton thread and a cotton duck cover get wet, the thread and yarn in the duck expands and basically closes off the holes that were created when sewing, closing off access for the water droplets to use to cause a leak. Decades ago, as more UV and mildew resistant, stronger man-made thread replaced cotton thread, the effect of the “cotton” thread swelling to close the needle holes has been significantly reduced. This also coincided with the advancement of synthetic fabrics (polyester, nylon, acrylic, etc.) that gave superior performance and longevity, and as a result, cotton duck is not typically used in most marine and awning applications. So, both the thread and fabric became significantly less water absorbing and neither swelled. This led fabricators to take extra precautions to prevent water from leaking at the seams.

Now, some specific information about our fabrics and how they interact with sewing seams. Marlen Textiles’ coated products (Top Gun, Top Gun 9, Top Gun 1S, Top Gun FR, Top Gun FR Lite, Top Notch 1S, Top Notch FR, Odyssey, Odyssey FR, & Softouch) are all produced on polyester substrates. These all have coatings that are highly water repellent (which is a feature the industry demands) due to the coating containing various waxes and components that help bead water. We design the coating this way to prevent the water from “wetting out” the fabric. By causing the water to bead up, the water then turns to droplets and rolls off the cover. The water beads up due to the surface tension of the coating being lower than the surface tension of the water itself. Therefore, the water molecules will be more attracted to other water molecules (rather than the surface of the material) and they will group together to form increasingly larger drops, until gravity does its magic, and they roll off the fabric. Another way to look at it is that the contact angle between the water and our coated products are high. This indicates non-wettability of the fabric by the liquid. In its simplest form, the higher the contact angle, the more resistant the material is to wetting out.

(Picture of three different contact angles. “Bad Wetting” means fabric won’t wet out. “Good wetting” means fabric likely to wet out)

All of the technology we use to help the fabric from wetting out works to our disadvantage when it is in a sewn seam in relation to not allowing the water the penetrate through the needle hole. This is because not only does the coated fabric not swell when exposed to water to help seal the seam or the needle holes, but the water repellents in the coating allow the water to slip through even faster. The higher the spray rating the faster the water will roll off (and through any holes). 

That explains our coated products. But what about Top Notch & Top Notch 9, which do not have a coating? These products do have a treatment that helps bead the water. This treatment provides an even lower surface tension than the coated products (remember the lower the surface tension, the more likely the water will bead up and roll off). This is why the water repellency of these products is so high. But the biggest distinction is that the film that is created is more thoroughly wrapped around each individual yarn (versus the coating that is over the entire surface of the coated products). We create it this way to help maintain a more breathable fabric. Hence, when a needle is used to punch through, it will have less impact on the non-coated products because it isn’t really punching a hole in a “solid” material. 

In summary, here are some important points to remember when sewing seams:

  • It is very important not to locate a sewn seam where water is likely to gather or pool.
  • The size of the thread should match the size of the needle for the most efficient sewing performance and reduced leakage through the needle holes.
  • If possible, use double-sided tape. This not only helps keep the fabric in place when sewing, it helps fill the holes up with the “sticky” material of the tape as the needle penetrates. 
  • Apply a seam sealer when finished sewing the seam. Some fabricators rub dirt along the inside portion of the seam to “clog” up the holes.
  • Don’t make the cover too tight where the seam is located. If it is too tight, it will stretch the seam and make the needle holes even bigger.
  • Due to their construction, some types of seams provide more water resistance than others (i.e. full flat felled seams are more water resistant than overlapping seams).
  • Thread’s resistance to water will depend on the thread’s surface, porosity, and whether the water source is dynamic or static.
  • Some specific needle brands claim to help seal the hole back up on its upstroke. Check with specific manufacturers.

We hope you now have a better understanding of how water acts at sewn seams, and we hope this new knowledge will allow you to proactively prevent leakage. Happy sewing!