KOGE MARINA, Denmark — From a distance they look almost like ordinary sailboats, their sails emblazoned with the red-and-white flag of Denmark . But these 10-meter-long (30-foot-long) vessels carry no crew and are designed for surveillance. Four uncrewed robotic sailboats, known as “Voyagers,” have been put into service by Denmark's armed forces for a three-month operational trial. Built by Alameda, California-based company Saildrone, the vessels will patrol Danish and NATO waters in the Baltic