Sales of hammocks, coolers and thermal/insulated containers for camping and other purposes have grown rapidly in the U.S. in recent years, according to research conducted by the NPD Group. Sales of hammocks have more than doubled in terms of volume and value over the past two years, lead by the Eagles Nest Outfitters brand. In the cooler category, where Yeti stands out as the leading brand, hard-sided products rose by 34 percent in the 12 months ended last June, while soft-sided coolers tripled. Sales of thermal/insulated containers rose by 56 percent during the same period.