Originally Posted by flandria
Boasun.... Yes, you are correct. However, things are a little more complicated and uncertain when you add drift (current) and leeway (especially when traveling under sail). After a few miles, depending on conditions, your actual course vs. DR course may start to diverge noticeably. I'll omit small "discrepancies" in steering the course... Your point that you may well be without celestial observations for a prolonged time is certainly valid and thus one should always be prepared for "the worst" and keep a DR log up at all times, regardless of other gadgets available.
That's true. I cross the straights of Fla. and then the Yucatan Channel all the time under sail. The currents are on the paper chart and I make a vector, calculate the set and drift, average my steering course based on compass
swing. The trick is to do it often, twice an hour when you are really in the throws of the strongest currents and running down a big sea. I DR and look at a hand held GPS
a couple of times a day to see how close I am to correct. Never been more than a few miles off which is close enough to get with in sight of land, at which point you are piloting by points of land, LOPS off of land marks etc.