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Subject: Re: DR vs EP vs MPP vs Fix vs Running fix
From: Chuck Taylor (ctaylor@XXX.XXX)
Date: Sat Feb 09 2002 - 20:15:34 EST
Yves Arrouye wrote:
> I am puzzled by the comment about plotting the DR being the "legal track
> record" used in case of collision etc. Do you mean that from the DR, and the
> time, and everything written in the log book (what sails you had, what
> course you did, what the wind was, etc.) one would then work a new plot of
> EPs that are as good as possible using all the time needed?
Yes, from the log book and the DR, one ought to be able to reconstruct the
actions of the vessel, i.e. course steered and speed run (through the water).
The actions of the vessel, not the actions of the wind and current, are what
determine liability in court. Looking at it from the positive side, if you can
prove that your actions were correct, you have a better chance of being
exonerated from liability for damages. To reconstruct the position at a given
time, one would need to factor in current.
This brings up another point. Suppose you take a fix at 0800. Then at 1200 you
take another fix. If you haven't plotted your DR without current in the
meantime, how do you determine set and drift for the period 0800-1200? I mean
the *actual* set and drift, not the predicted set and drift. I was taught that
the proper way to determine set and drift was (using this example), to compare
the 1200 DR with the 1200 fix. If you have factored predicted current into your
DR, then it becomes a bit more involved to determine actual current.
> Because AFAIK,
> if you're having 9 knots of current for a few hours, your DP is going to be
> quite far off your real position.
No one is saying you should ignore current. The question is whether or not it
should be in your DR. I am suggesting that current should be reflected in an
EP, but not in a DR.
Chuck Taylor
Everett, WA, USA
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