Sewing sails is the part I dislike most. It seems like every time the sewing machine is out to make it hard for me. To make the task easier, be sure the sewing machine is working correctly and is well oiled. Next, only use Ball Point needles, size 9 for delicate knits. For sewing thread, use 100% Polyester, it performs well and does a good job.
Before sewing any sails, make test pieces that matches what you are doing. This is the only way to make machine adjustments. Sewing sails can cause puckers in the seam of the panel if certain steps are not done first.
The first main step is to regulate the correct thread tension. To get a good idea of this, read your sewing machine's owners manual. On your test pieces, if the thread tension is not properly balanced, your work will not perform well (do the best you can with what you have to work with). Sometimes the sewing machine sews well on the seams (the most important) but not as well on the re-enforcement corners. What ever the case, spend time sewing on your test pieces, it can make the difference. On foot pressure on the material; keep it light but steady. It too can cause puckers to be sewn in.
When sewing, don't pull the material, let the machine do the job. Pulling the material through causes problems, so don't. Again, take your time, think it out first, then make a lot of test runs on your test pieces. This part takes time to get it right. The first, second or third time sewing will teach you and cause you to find better ways to sew.
I sew straight stitches on my panel seams, right through where the panels are joined. Because of this, the needle gets gummed up. To stop this, clean the needle after each sewing panel and use a light, clean oil. Next month, more on putting sails together.