degrees off. Obviously, if you have a gyro or GPS compass onboard these vectors will be much more accurate. One other note, if you have radar


and overlay its output on the chart plotter this same issue comes into play. The heading from the fluxgate is used to orient the radar images so they will frequently be rotated relative to the chart. This can be corrected by changing the relative bearing setting. Sorry for the asides but I felt this information was worth passing on. So, back to using the COG vector:


What is it? And, how does it work? Figure 1 shows a COG vector as a green line. The plotter constantly recalculates the direction the boat is moving, relative to the ground, and displays a line from the boat’s current position outward. Because this calculation is done using the boat’s actual GPS position it takes into account her speed and heading through the water, current, windage, wave action and anything else affecting movement. It is also an average of several readings, so if the vessel is turned to a new heading it will likely take thirty seconds for the COG line to settle. On most chart plotters the COG vector can be set to display an infinite distance. I like mine set this way so I can zoom out and see if we are going to lay a distant object. At the time Figure 1 was captured,


there was virtually no wind or current. At the same time I took the picture in Figure 2, it shows visually where we were going. The orange line I added to this picture shows the boats heading – note it matches nicely with the COG shown on the chart plotter. And, yes, I was aware we were headed for the rocks - I headed toward the point intentionally to make it easier to see the agreement between the picture and the plotter. The next day, we were heading


home and had a fairly strong current (about 1.5 knots) coming from our port side. We were trying to get around Point Migley on Lummi Island, but the current was sweeping us into the island. In order to correct for this I had to steer 15 degrees to port. Figure 3 shows the chart plotter view of this – the orange line shows our heading and the green line shows our COG (I added the orange line later). Figure 4 shows how this looked from the cockpit.


I like to use the COG in the same way I use tell tales – I steer the boat using a visual reference point beyond the bow and I look at the tell tales, the COG or both, every couple minutes and make small adjustments as needed.


The real beauty of using the COG


vector is that it constantly adjusts to changing current, wind, etc. So if you steer a course that keeps the COG pointing to where you want to go, you will get there in the shortest, fastest way possible. Obviously, this statement assumes ‘normal’ conditions: for example, if there was an eddy going in your direction on the other side of the channel it might be faster to go over and use it. The point is this, using the COG vector allowed me to easily


adjust for changes in the speed of the current and keep the boat on a straight line over ground. I like to use the COG in the same


way I use tell tales – I steer the boat using a visual reference point beyond the bow and I look at the tell tales, the COG or both, every couple minutes and make small adjustments as needed. If sailing, these adjustments will require commensurate sail trimmings, and depending on how energetic I feel, the time between adjustments might be extended. I also leave the COG vector turned on all the time so I can see where our boat is actually going. Doing so lets me see, and easily adjust, for unexpected currents. Hopefully this information will


make your travels in our current filled waters a bit less challenging.


Mike Huston teaches sailing for San


Juan Sailing in Bellingham, WA. He owns “Illumine,” a Jeanneau 43DS.


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fuelpolishing@olypen.com 48° NORTH, JANUARY 2011 PAGE 71


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