There’s not much inside the Navy’s newest ship, and that’s exactly how they like it. 338 feet long, 93 feet wide, low and blocky, USNS Spearhead is basically a thin aluminum shell wrapped around four diesel engines, rudimentary control facilities for its 40 crew plus 312 airline-style passenger seats. The rest of the $250-million, twin-hull catamaran vessel, christened this weekend, is empty space … with an expansive flight deck on top.