I and my wife will be spending Shabbat Hagadol (this year the 31st of March) on a ship. The evening before the ship will be leaving one of the more northern ports of Norway, and navigating through fjords on that Saturday before emerging just north of Trondheim as the sun goes down. The internet says that three stars will be visible from Hammerfest at 9:29pm, after a sunset longer than is halachically thought possible. But I am also aware of the halachic indecision when it comes to observance of the Jewish calendar in the polar regions. So specifically, what times do I observe when the ship leaves port on Friday evening (in the polar region), whether or not I know my ship will be traveling far enough south to make a halachic difference on Shabbat?