Re-looking at the real nav stuff, as I found myself a bit lost at times
First stop is
Dead Reckoning
Usually done by keeping a record of your speed and direction.
A quick test revealed that SH3 differs from the real world. This I think was mentioned by someone else, but here it is, for anyone else trying to decipher the Nav problems.
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SH3's world is 43,200 km (width) x 21,600 km (Height) - These are 2x the dimensions of the large map.
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1 Knot = 1.8 km per hour. - The real world has 1 knot = 1.852 kph
Funny enough the distance covered seems to be irrespective of (not affected by) the sea conditions

- This is still under testing.
Next on the list are the local and GMT times and anything else that come to mind.