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
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.