You need to thin out the fruit on grape vines pretty hard. You also want to thin vines down to a few productive ones which are health/strong. Likewise, you prob want two main vines running out either side, and run them the full length of the poly. I can't remember the standard spacing for the fruit clusters, but it's something like one per nine " of vine. You'll get two or three forming round the same spot, pick the best, and remove the rest. Same basic idea as with say apples. The plant is going to produce X weight of fruit by partitioning resources between green growth, and fruit. If we didn't thin the apples beyond what they naturally drop in June/July. Then we'd get lots of small apples, and likely more scab as well. If we thin them, then we get fewer large apples. The overall weight of apples won't actually change much. The plants don't care about producing nice fruit for us. They just want to pump out as much seed as they can for the next generation. That's also why some can become biannual, with bumper crops one year, and almost nothing the next. Plum trees are a classic example of that kinda thing. Nothing for a few years between frost, or stormy wet weather. Then the tree is breaking major limbs because the thing's loaded with fruit. That is its big chance to get as many seeds out as it possibly can. Look up Maritime Gardening Why Fruit Trees and Bushes Have Good and Bad Years.
