You're absolutely right. A rhumb line
across Casco Bay will be a constant compass
heading, but one from New York to Ireland will change slightly as yo go along. I once learned all about the details of why that is so from the USPS's Junior Navigation course, but I have to confess that it escapes me now. I just remember it was so.
Originally Posted by AdamLein
Casco Bay is not the Atlantic Ocean. As far as I can tell either from isogonic charts (which are just about the smallest scale charts out there) or from the roses on Chart 13290, the variation does not change significantly throughout the bay, being 16°00'W for most of the bay and 16°15'W in the northern, almost inland parts. 15' is not relevant at those distances.
However go from NY to Ireland the Atlantic is in the way no? RTFT.