Boating and Sailing.pdf - Moja ladja

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Part 4: Nature’s Triple Play: Wind, Weather, and Tides
Bridge clearance is listed on marine charts, with the height above mean high water given.
So, if you want to pass under a bridge that has 20-foot listed clearance with your sportfisherman
boasting a BC of 19'6'', you aren’t going to be able to get through at the peak
of a spring tide that’s a foot taller than normal high tide (more on spring tides later in this
chapter). On the other hand, you can get through that bridge with a 19-foot listed clearance
on the bottom of a –1.0 low.
Bet You Didn’t Know
In most areas there’s a normal progression through four tides, but some days have only
three tides. How can this be?
As the 50-minute-per-day regression takes place, the P.M. tide gradually advances past
midnight, thus leaving one 24-hour period without four tides, even though the tidal
movements maintain their steady procession.
Tides in the Estuaries
Tides follow wider, deeper channels first. That’s why tide flows show up at major coastal
passes long before they arrive at docks and boat ramps several miles inland.
In a bayou fed by several tidal creeks, you may see that the inflow begins on the main
arm and is already flowing strong there while it’s dead or even going out on the smaller
feeders.
The tide comes into an estuary in a plume that you can see on the surface when the water
is calm. It has a rounded leading edge, and it stays separate for a time from the residual
water left in the backwaters on the previous low. Differences in salinity or temperature
probably account for the edge.
The life in the estuaries responds to this pulse—you’ll see fish start to jump and gulls start
to whirl on tide movements. For anglers, it’s often a good time to wet a line.
Boat Bytes
Knowing when currents will
be strongest in a pass or
inlet can be critical for sailboats,
which have less
powerful engines than
powerboats.
There are times, too, when the current tables can actually
predict sea conditions to some extent—if a strong
flow is coming out of a big pass and a wind is blowing
in, you can be sure that there will be steep, nasty waves
in the passage.
In a powerboat, you may want to stay at anchor until
the flow reverses and begins to go with the wind,
even though you’ll then be working against the
current, because the water will be much smoother.