In open water, i.e., not in a channel, the ship on the right is privileged, the one on the left is burdened. In this case, Porter was still burdened because once she made her turn, the tanker was to her starboard. From the tanker's point of view, Porter was approaching from her port side. It's very similar to several cars simultaneously approaching a 4-way stop intersection. The guy on the right has the right of way.

Then there's the General Prudential Rule: regardless of who has the right of way, the easier to maneuver ship has the obligation to stay out of the way of the clumsy and awkward vessel.