— April 2003
By Ben Ellison
Mind the Vectors
|Part 2: Fishermen crave detailed knowledge of currents.|
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.
This article originally appeared in the March 2003 issue of Power & Motoryacht magazine.