 Why is Brandt interested in The Comerford?
With him came a boatload of scientific apparatus and equipment, including a number of things that looked like oxygen tanks, which were now stored in the forward hold. "But this storm, sir!" Nelson avoided Curtis' friendly eyes and slipped out from under his arm. Nelson's estimate, I'm checking back," he declared, frowning at the two papers and hastily rechecking his own figures. Bearings north west by three west, U. S. Cruiser Comerford from Cay 364." "Seems to me," Curtis said dryly, "this invention can harm your friends as much as your enemies."
 He wants to use its zone of silence to apprehend the Carthusia.
 He is holding the ship ransom as revenge for what American has done to Germany.  He is holding the ship ransom for Boarts—black diamonds.  He wants to use its zone of silence to apprehend the Carthusia.  He wants to use its zone of silence to trick other ships into crashing on the islet.