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By
the same token, a boat’s steering compass and log—once the
essential tools for figuring DR heading and speed—are now often
ignored or misunderstood. Why is my speed-reading 9 knots while the GPS
says 8—do I have a current pushing me, or is the darn paddle wheel
sensor getting fouled with growth again? Is my boat really crabbing along
with her bow eight degrees off my true course, or does my compass need
swinging? Oh, who cares! The truth is, as long as the plotter’s running,
we really don’t need to resolve these issues.
On
the other hand, fishermen crave detailed knowledge of currents, and all
of us could use it to improve our situational awareness, especially around
sharp, fixed objects. And while electronics may have let us get lazy on
the subject, it’s also electronics that can ultimately provide the
best information. In a twist on that old vector math, some integrated
instrument systems and plotters can take SOG and COG and subtract boat
speed and heading to determine real-time current set and drift. Of course
the quality of the results is completely dependent on the sensors involved,
but they’re getting better all the time.
One
of the tougher things a GPS does is computing COG and SOG by comparing
positions at short intervals. Before Selective Availability (SA) was shut
off in 2000, those readings used to jump all over the place, particularly
on a slow boat. Now, with SA off and two kinds of differential corrections
(WAAS and DGPS) easily available, the numbers are excellent. Similarly,
the flux gate compasses commonly installed with autopilots are now being
augmented with rate-of-turn gyros, and can deliver precise bow heading
to a boat’s data system, even during maneuvers. If there’s a
weak link, it’s that darn mechanical paddle wheel speed transducer.
That’s
why I was intrigued by the Ultrasonic Speed Sensor that Airmar was showing
at that same National Marine Electronics Association show. I watched it
being demo’d side by side with a paddle wheel in a variable-speed
flow tank, and it seemed to have distinctly improved accuracy at slow
speeds and responsiveness at all speeds.
With
sensor advancements like these, we’re reaching the point where differences
between readouts might get our attention and where automated set and drift
calculations might be truly reliable. Still, there’s no gizmo in
sight that will alert you to a side-setting current when you’re tied
to a dock, which is why it’s a damn good idea to look over the side
and check.
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