Complex sentence: Aubrey reports to his commander-in-chief at Gibraltar, who sends him and to intercept the American frigate USS "Norfolk" which plans to attack British whalers in the South Seas.
Split into simpler sentences: Aubrey reports to his commander-in-chief at Gibraltar, who sends him and to intercept the American frigate USS "Norfolk". The American ship is under orders to attack British whalers in the South Seas.