I used a colorful 5/16 inch line poly line I got at Home Depot for a main sheet for a while and it worked just fine. I since replaced it with something of high quality from the marine store. The new line has a better "hand" (feel) and will not fade like the old less expensive utility line. I find the jib sheet should be slightly smaller, perhaps 1/4". How the line handles in the jib sheet cleat is most important; you want to be able to get it in and out easily and not have it slip. You don't want the jib sheet too heavy as it will be a drag on the jib shape in very light air.