If you don’t use the boom vang in all but the lightest conditions, the boom will rise up as it is eased out and the sail will twist off, spilling power out of the top of the sail. This is okay when you have too much heel and helm, but not when you need power. Not taking advantage of your boom vang also limits how far out you can ease the sail. If the top is twisted off, it will run up against the upper shrouds and wrap itself around the stay, which prevents you from letting out the sail as far as you would like. The golden rule of thumb: Use enough vang to keep the top batten parallel to the boom. If it is too tight, the top telltale will stall.