You can't calculate a Great Circle
route in Magnetic. You can't do a Rhumb Line in Magnetic. You can't even check your magnetic
compass without referring to True North. And even if you simply assume that your
compass is correct because it once was approximately sort of in the ballpark, and don't create your own GC or RL track, but instead rely upon a magic box to hold your hand and do it for you, you still have to correct for Deviation, anyway, which is individual for a particular
boat and a particular magnetic compass
installation and even a particular heading, since Deviation changes with heading. You will probably get where you are going without referring to True North. Sort of like you will probably get to
Europe if you sail toward the rising sun and then turn Right or Left when you get to land. Once again, I refer you to the most widely acknowledged authority on
navigation, American Practical Navigator, AKA "Bowditch". If you are not familiar with this essential book, I will categorically state, and beg anyone to differ with me at the risk of being laughed at by real Navigators both recreational and professional, then you are NOT a Navigator. A real Navigator uses
electronics as a valuable, every day, time-saving, practical tool, but does not NEED them, is not helpless when they fail for whatever reason, and can still navigate without them.
You don't need to be a Navigator to sail in coastal or inland waters. But it would be very reassuring for there to be a Navigator onboard when making a long ocean
passage and a prudent
skipper would agree.
BTW, Bowditch is available as a free PDF download for those who don't think it is worth spending
money for, or who simply don't have room on their
boat for another big book.