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post #9 of Old 08-02-2012
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Re: Navigation Problem

Wind direction doesn't help you.

Here's an idea:

Mark waypoint A on your GPS. Sail on a constant heading for a fixed length t of time. Mark waypoint B. Let C be the place you would have gotten to if there was no current or leeway. The vector from C to B will give you the combined effect of current and leeway.

For example, suppose you sail due east for one minute at six knots. Had there been no current, you would have ended up 0.1 nmi due east of your starting position. But let's say you measure your new position (B) and it's 0.112 nmi east and 0.012 nmi north of your station position (A).

Let's assume there's no wind and you're motoring. That means the current pushed you an extra 0.012 nmi east and 0.012 nmi north in an interval of one minute. In an hour, it would have pushed you sixty times that, or 0.707 nmi east and 0.707 nmi north. By Pythagoras, it would have pushed you 1 nmi NE in on hour, yielding a set of NE and a drift of 1 knot.

s/v Laelia - 1978 Pearson 365 ketch
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