We are building a boat bimini (a fabric shelter that shades the cockpit) out of 8 oz. Sunbrella fabric. The thing is about ten feet by ten feet and is made up of several panels with full width zippers at each end. We are using a Yamata staright-stitch industrial machine, which has taken us a very, very long time to get set up properly. So many dials to twiddle! We are using DB 92 bonded polyester thread and Innnova DBx1 #18 needles. It seems to be working. Our greatest problem, actually, is controlling the speed of the machine. It takes an extremely light touch on the treadle indeed. One slip and it runs away, breaking and snarling threads and sometimes needles too. That is one humongous motor!
Is there a better thread/needle combination? Various trouble-shooting guides we have seen say "Use proper thread or needle" or "Replace needle with correct size" or "Choose correct size needle for thread and fabric." No argument here. But how does one determine what the correct sizes are?