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Greg wrote

> Let me re-phrase the original question a little differently: On an
> ocean passage (at least as I understand it...) you basically navigate
> based on course and speed (i.e. DR positions) until you get a fix, at
> which time you "refine" your DR to a known position (I'm also assuming
> no electronic navigation aids here too).
>
> So if you find that your fix is a fair ways off from your DR position,
> do you alter your course (and maybe speed too) to compensate - or just
> carry on as before and take the resulting error out each time you get a
> good fix?
>
> Not sure what the technique is on the ocean, for air navigation we're
> always adjusting heading (and maybe speed if needed) based on
> real-world conditions, but it's really a different problem because
> there's usually a reference for the true course line (ground
> contact/map reference for visual conditions, and electronic guidance
> for instrument conditions).

Again, I won't speak for others.  I certainly calculate EP.  I trust GPS and
Loran as long as I can see landmarks on the shore (I have seen both get
hinky too many times).   Otherwise I want other methods as backup.  In
fact, GPS is backup to my other methods. I love the sport of sailing and
traditional navigation.

I guess one way to calculate set and drift is after the fact with a fix
(locking the barn door after the horse is stolen).  A common sense test: why
would tide and current information, and methods of correcting course for set
and drift be published if empirical methods were the norm? Why bother
calculating it it all (except for curiosity) if you don't intend to use the
information?  I've seen DR position off by 1 mile in 7 on Lake Michigan,
while EP was off by only  0.2 nm.

I'd bet the prudent cruiser and professional sailor pay attention to even
little things like wind-induced surface currents and correct for them.

Bill


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