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.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27