Mind the Vectors
— April 2003
By Ben Ellison
Mind the Vectors
|Don’t let electronics get between you and the primal dynamics of navigation.|
One of the most unforgettable moments—highlights would be the wrong word—at the annual NMEA conference last fall was a boating accident. A bunch of us were enjoying an alfresco breakfast at the Sanibel Harbor Inn and Resort in Fort Meyers, Florida, when the balmy peace was disturbed by a loud crash coming from the little marina. It was closely followed by shouts and then sobs. A few of us investigated. I even took a picture of the incident (above) and for the first time felt like a predatory journalist, because what we saw was a traumatized skipper and his family in a scene of the yachting dream gone suddenly sour.
All this man had tried to do was back off a pier onto a flat-calm Sanibel Sound. What he had not figured in was the roughly 1 1/2-knot current quietly pushing him onto the dock, and--much worse—onto the sharp bow of a big steel tour boat tied up beyond. Then he’d compounded his mistake by trying to power out of it. It must have been an excruciating moment when that steel bow blew through the big starboard window and into the main cabin where his teenage children were gathered, and then ripped the cabin corner post out of his vintage sportfisherman.
Now let me emphasize that I have no desire to embarrass this man. In fact, I regret to report that, judging from his family’s response to the accident, his dream boat is probably finished. Sure, he should have been aware of the current, but I bring the whole subject up because I fear that he’s just one of too many new boaters who’ve failed to develop a solid sense of the fluid dynamics that can so affect close-quarters boat handling, not to mention offshore navigation. And a prime culprit is our wonderful electronics! I’ve observed even experienced skippers having trouble understanding where current fits into all the readouts on their bridge.
So let’s step back a moment to the days before Loran or GPS could constantly plot our true position. Back then, anyone who ventured beyond the horizon or into a fog bank soon developed a gut-level understanding that the water they were moving through might very possibly be simultaneously moving them somewhere else. They sought guidance from current tables, even though such data is pretty spotty, particularly along coasts complicated by islands and capes. And, since current activity is invisible without a fixed reference, they were always eyeballing the subtle wakes around navigation buoys and the like (lobster pots, pesky as they are, work nicely for this).
Such pencil-and-chart navigators learned to think in terms of the vector math that is considered grade school stuff today. Vectors are simply graphic expressions of direction and speed that can be added together to figure out the true motion of something that is traveling in two directions at once. At the heart of traditional navigation is a holy triangle of such vectors. One describes a boat’s heading, or course, and speed through the water and is the essence of dead reckoning, or DR. Another defines the direction and velocity of the water itself, known as set and drift and usually caused by tidal forces. And the third, their sum, shows what the boat is actually doing; i.e., course and speed over ground (COG and SOG).
The illustration on page 46 shows how an old-school navigator might guesstimate a current vector to correct his DR and figure out where the heck he really is. There are more elaborate techniques for adjusting course to compensate for set and drift, but they hardly matter these days. A GPS can deliver accurate (at least on average) COG and SOG every second and, better yet, can constantly plot your real track over the ground on a digital chart while displaying spot-on bearing and distance to a desired waypoint. Keep your boat in the middle of that nice highway display, and you don’t really need to know beans about current.
This article originally appeared in the March 2003 issue of Power & Motoryacht magazine.