:avi: Just pay some repect if you havent,I always wish sailors of allied nations of The US good luck on any cruise.
But to help anwer your question,It may be the Ballast,the Nimitz,AKA one of those fat mothers in the USN,also does the same thing,heck ships in the Savvanah River near where I live do that too,The Crews mess around with the Ballast ,making adjustments before the cruise,in order to get it adjusted for their current load or mission in this case.
Oh and any Austrailian Military members who read this,good chioce in the Sea Hawk ,I love that Chopper,very versitile and has many uses!