Boating and Sailing.pdf - Moja ladja

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Part 3: Getting from Point A to Point B: Marine Navigation
Variables of Variation
The outer circle of numbers on the compass rose indicates true north, the inner circle
magnetic north—the North your compass points to. They’re not exactly the same in
many parts of the globe, because the magnetic centers of the earth are not exactly the
geographic centers—another of God’s little tricks to keep us occupied. The difference
between magnetic and geographic north is called variation.
The difference can be significant in some areas, and particularly for longer voyages it
must be included in calculations or you could wind up many miles from your intended
destination.
As another example of nature’s sense of humor, the variation does not remain constant. It
changes a few minutes (sixtieths of a degree) or so each year. It takes a long time for this
minor change to be significant, but you don’t want to try an open ocean crossing with
hundred-year-old charts.
The concern with variation is gradually disappearing due to GPS navigation. But if you’re
in the middle of that 2,000-mile run to Tahiti and your electronics go out, you’d better
know something about variation.
What You Don’t Know Can Hurt You: Bottom
Conditions
One of the objects of safe skippering is to stay out of areas where the bottom is too close
to the top, and charts tell you how to do that with their depth indication.
But there are times when you must enter shallow areas, as when you want to have lunch
on the beach, and knowing whether the bottom along the beach is soft sand or hard rock
can be critical to the safety of the lower units of both your boat and your passengers.
You also need to know the bottom conditions when you look for a spot to anchor. Soft
mud and grass make for tough anchoring, as does smooth rock. Hard sand is the preferred
anchorage—it’s soft enough for the anchor to dig in yet firm enough to hold
against heavy strain once it does.
How do you find out what’s down there, short of donning a mask and fins? Again, you
look on the chart:
◆ “Grs” indicates a grassy area where fishing may be good.
◆ “Hrd” indicates a sand bottom that’s good for anchoring.
◆ “Rky” is rocky, maybe good for bottom fishing, not good for anchoring.