The Napa County Sheriff’s Office is reporting a violent weekend on Lake Berryessa. Two jet skis, driven by a man and his sister, collided on Saturday morning. The man suffered a major head injury and needed 15 stitches, while the woman escaped with a minor leg injury. Then on Sunday afternoon, a man’s hand was partially severed by a boat propeller. The man was airlifted to a hospital in Vacaville and was rushed into surgery. Family members say he was standing near the front of the moving boat and fell overboard.