Boating and Sailing.pdf - Moja ladja

Chapter 16: Mastering the Tides
In other areas, the tide movements are so gradual and so minimal that they go unnoticed
by most boaters. But even minimal tides can have a big effect on boating. In shallow areas,
a tidal drop of six inches can sometimes bare hundreds of acres of bottom, and a few
inches of depth sometimes make all the difference in whether or not the draft of a boat
will be excessive for entering some waters.
Playing the Tides
Tides affect both the depth of the water and the current in coastal areas, and both factors
are important in operating many types of boats.
Tide heights listed in tide tables are given as plus or minus heights from the zero line,
which is the average low tide at that spot year-round, over the previous 19 years. A tide
indicated as –1.0 is expected to be one foot lower than the mean low, and one listed as
+4.0 will be four feet higher than the mean low.
The greater the variations from zero—that is,
the taller the wave—the stronger the tide flow.
Because the earth has a slight wobble in its
rotation around the sun that repeats on 19-year
cycles, the tides today won’t be exactly the same
until 19 years from now. But for “government
purposes,” it can be said that tides repeat on
yearly cycles in terms of height and current—that
is, the December full moon high tide at the mouth
of the Chesapeake Bay this year will have about
the same height and volume on the December full
moon next year, although the full moon will fall
on a different date.
So what does this mean to you?
Let’s look at an example. If you attempt to cross
a bar with a controlling depth of 3.0 feet and there’s
a –0.5 tide and you have a three-foot draft, you
are going to be doing the sandbar boogie for a
while. On the other hand, if you try to cross the
same bar on high water of a +5.0 tide—or even a
+0.5—you will have no problem whatsoever.
Knowing both the time and the magnitude of the
tide can be critical to coastal boaters in many
instances.
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Boat Bytes
Note that mean low water
is not mean sea level or
just plain sea level, often
referred to when measuring
altitudes ashore. Sea level
is very near the average
between mean low and mean
high tide levels—it’s not a number
used regularly by boaters.
Boater-ese
Controlling depth is the
maximum draft of a vessel that
can pass over a given spot in a
channel at mean low tide. For
example, a vessel with a 4-foot
draft will go aground in a channel
with a controlling depth of
3'11'' at mean low tide.