BATH — The largest destroyer built for the U.S. Navy cuts an imposing figure: massive, with an angular shape, hidden weapons and antennas, and electric-drive propulsion. But underneath the stealthy exterior resides a style of hull that fell out of favor a century ago in part because it can be unstable. The Navy will soon learn how this modern take on the “tumblehome” hull holds up when the first-in-class Zumwalt heads out to sea in December for builder trials in the rough-and-tumble North Atlantic. Amy Lent, of the Maine Maritime Museum, which works closely with the shipyard, said taxpayers needn’t worry because the Navy and shipbuilder Bath Iron Works have “tested the hell out of it.” “This is an enormous investment.