Straight-stitch sail seams?

[Paul356]

Is it possible to straight-stitch sail seams, or do they need to be zig-zag? Our youth boat-building group inherited a "canvas machine" sewing machine that we had expected we could use to build a sail kit, only to learn that it only does straight stitches, not zig-zags. Do we need to keep looking? I'm aware we can try to press a home sewing machine with zig-zag feature into service if necessary, and have done it on my own projects, but we were hoping this donated machine would set us up for good. Thanks.

[Todd Bradshaw]

It can be done. On the old cotton sails it was quite common. On Dacron sails it is not ideal, but it will generally do the job. Seams should be tape-basted on synthetic cloth and the seam tape is actually doing as much or more to hold the sail together as the stitching is.

[Paul356]

Thanks for replying. I'm not quite sure what "tape-basted" means. Is that double-sided sticky tape?

[Todd Bradshaw]

Yes, Most have a thin Mylar carrier with sticky stuff on both sides. It comes rolled with a removable paper backing. The most useful size for sailmaking is 1/2" wide and typical panel seams for most sails are around 5/8" wide with the tape inside the seam. Dacron is so slippery that you need something to keep the panels aligned as you sew them together and the tape does that and also adds tremendous strength to the seams.

I'll normally tape-baste just about every single seam on a sail. You can buy rolls of sail basting tape from places like Sailrite. The one I have used for the past ten years or so is called "Venture Tape". A lot of industrial thread is treated with an oily lubricant since industrial machines run so fast. It helps to keep the polyester thread from melting due to friction during the sewing process. You may find that your sails develop tiny "grease spots" around the stitch holes after a while. This is a reaction between the thread lube and the adhesive in the seam tape. They will usually fade away in time. The Venture Tape tends to do this less than others that I have used.

Before we had basting tape we used a thin line of silicone sealer to glue panels together before sewing. It was somewhat messy and took time to dry, so the basting tapes were a real improvement. In any case, something has to be done to keep the fabric pieces aligned, especially since moving big pieces of fabric through the machine gets pretty awkward at times. Lofts that build cheap production sails often skip the tape. They "baste" their seams using a gizmo like a soldering iron with a tiny needle tip. They go along the seam and ever so often they melt a pinhole through both pieces of Dacron, sticking them together. With care, it holds long enough to get it through the sewing machine, but in the end, it adds no strength to the seam so you aren't likely to find it done on high quality sails.

There are also some specialized versions of seam tape. On Mylar sails, we use a tape with a very light woven Dacron layer in the middle. Mylar has lousy tear strength around needle holes and the woven carrier in the tape helps to keep the holes from expanding or tearing. There is a tape called "3M Super Tape" which has no carrier at all. It's just backing and sticky goo. It is commonly used on seams of spinnakers and other light cloth sails. Since it has no backing, the tape will stretch as the fabric stretches, making for sails with a smoother shape. It forms a very strong bond, but it is rather expensive to use and not needed for most sails. A few even use a tissue paper carrier, since the flimsy paper will deform as needed to generate smooth spinnakers. There are also some tapes with a rather gooey, rubber-based adhesive which are intended for use on materials like synthetic canvas boat cover fabrics, rather than sails.

It's pretty much guaranteed that during any sailmaking process you will find yourself in some pretty awkward moments as you try to get the cloth through the machine without wrinkling it up and making a mess of the sail. Good seam basting tape is a tremendous aid at those times.

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[The Q]

Todd, you're a lot fitter than I am, I'd need a long time to get back off the floor after sewing in that position..

[Todd Bradshaw]

Originally I had about 32' of infeed and outfeed tables and the machine set at typical table height. I was living in a rented house making computer plotted sails when we first got here, in a big living room with shag carpet and I needed something the sails could slide on while sewing. Unless I can be right over the needle I can't sew a straight line to save my life, let alone two lines side by side, and bending over the table to see and sew was an absolutely horrible position for my back. Once we bought a house I set the machine into the floor and adopted this rather curious, but reasonably effective, sewing position that was at least easier on my back.

I started making sails in 1980 while also working on hot air balloons and retired a couple of years ago. Since a typical sail involved two or three days of crawling around on a hardwood floor, my knees got the worst part of it all. I was lucky in that I made almost exclusively one of a kind sails and they were never boring old plain white sails, so it was always exciting and motivating to see how they would turn out - but my knees REALLY don't miss crawling around on that floor at all. I made a big Top Gun canvas cover for my grill a couple months ago and couldn't believe how much my knees hurt from just a couple of hours working on the floor.

[Paul356]

Thanks, Todd!

[pvg]

The instructions that came with my Sailrite kit said for most seams to use 2 rows of zig-zag or 3 straight stitch; this for dacron sails...

[Todd Bradshaw]

If the seams are tape basted, two rows of stitching is likely plenty. Whether adding a third line of perforations actually strengthens or weakens the area is a valid question. Basting tape has what they call "dwell time". What this means is that its bond strengthens dramatically over the days after application. On dinghy-weight sailcloth it will get to the point within a few days where pulling a taped seam apart will permanently distort and damage the cloth.

[pvg]

If the seams are tape basted, two rows of stitching is likely plenty. Whether adding a third line of perforations actually strengthens or weakens the area is a valid question. Basting tape has what they call "dwell time". What this means is that its bond strengthens dramatically over the days after application. On dinghy-weight sailcloth it will get to the point within a few days where pulling a taped seam apart will permanently distort and damage the cloth.

Yes, I forgot to mention Sailrite also recommends the use of and includes in the kit the basting tape Todd mentioned...