We tested three methods for threading yarn telltales; direct threading using a #18 sailmakers needle (0.080-inch), pulling through with upholstery thread using a #16 needle (0.061-inch), and pulling through using a large household sewing needle (0.034-inch). We used acrylic/wool blend yarn, 65-weight upholstery thread, and medium stiffness 6-ounce polyester sailcloth.
To sew directly with the yarn requires threading it through the eye of a large sailmakers needle. This calls for square trimming, moistening the yarn, and concentration. We tried threading the yarn through a #16 needle, but even when we succeeded the thread was damaged in the process.
Its much easier to pull the yarn through using smaller thread. To do this, you double the thread through the needle, pass the needle through the sail, and then pull the needle off the thread. Then pass the yarn through your new thread loop, pull the thread and yarn back through, and then adjust the length of the yarn as needed.
In all cases, the yarn is secured with an overhand knot close to the fabric on both sides, leaving an 8-inch tail. In all cases, the yarn broke before the knot pulled through the fabric. Using the household needle, the yarn was often damaged by hard pulling through a too-small hole.
Lightweight household thread will break. Light whipping twine can be used, but it adds volume when pulling the yarn back through, eliminating a key benefit of the method. Size 60-70 polyester upholstery thread seems a good compromise between strength and size.
Bottom line: Although the smaller needles leave smaller holes, there is no point in damaging the yarn by using a tiny needle. The #18 needle leaves a large hole in the cloth, potentially weakening it, although this is probably acceptable for leach telltales pushed through multiple layers of material. For lighter sails, pulling the yarn using a loop of upholstery thread through a #16 needle-hole is the best compromise.