Ball fenders are ideal for larger boats because, when properly inflated, they are harder to crush on impact. They are also better for boats with a lot of bow flare because they are wide enough to keep the lower hull sides from hitting the dock. The caveat? If left inflated, they take up more space, which makes them better for boats with large stowage capacities. What size should you get? For round fenders, West Marine recommends 2 inches of diameter for every 4 to 5 feet of boat length. So, a boat in the 30- to 35-foot range should use 21-inch-diameter ball fenders; a boat with a length overall of 50 feet or longer should use the larger 27-inch size.