After spending hours going through the forum, I still have my question. For an installation on a sailboat with a fiberglass hull, why is a double pole circuit breaker required in the PV+ and PV- wires between the panel and the MPPT? The DC distribution system is grounded with the battery negative to the main grounding plate in the hull.
Not only that, but why is overcurrent protection needed at all? The panel fault current is much lower than the MPPT allowable input current, and the wires are oversized even for the MPPT input current rating.