Do not pay attention to the number on the tension. It would be easier to see than to explain how to adjust the tension. Here is a link I started a long time ago - I didn't check to see if they are all still there:
http://www.quiltingboard.com/vintage...s-t170748.html anyway it is a bunch of links. It could be that you have the threading wrong around the tension. The thread comes across the top of the machine, goes through the tension, through the tension spring, over the bar, then up through the take up lever. If you do the spring after the bar and then go from the spring straight up to the take up lever it might do weird stuff.
The Sunbrella should be light enough to be sewn by a Singer 15 if the project isn't too huge. If it is a large piece you may have to 'help' the fabric through the machine. When I made a 30 X 50 foot awning, I used some ramps and I folded the material a lot. I was using a Consew walking foot sewing machine. I wonder if you got the tail end of the thread caught in there? You can go up to about a size 18 needle - any bigger and your machine can't handle it very well. The needle won't fit the hole/bobbin/shuttle quite right and an over sized needle on that machine could damage the bobbin case or the shuttle or the throat plate and could make burrs. Trying to pull too much heavy fabric through could bend the needle shaft or the needle. The other thing I can think of that could make stitches like that could be a burr on the hook or somewhere. You have to feel around and then polish out the burr.