Katharine McPhee's Hawaii Trip Is Bumpin'

Katharine McPhee's idle time looks just as good as her 'Idol' time -- in fact it looks hotter, due to her butt-hugging swimwear in Hawaii.

Kat's on vacation and Thursday she was ready to swing into the weekend ... courtesy of some chill time in a hammock. Her bf, David Foster, wasn't around, but she seemed to be in a deep FaceTime convo with someone. Based on the Blue Steel looks she was flashing ... it could've been David.

Looks like Katherine knows how to work her angles. All of them.