a little context to start us off.

As by way of the mighty Don:


Course over ground

Speed over ground

These may sound a little odd because a boat is in the water, why do we care what ground speed is? Well we look at our boat position on a nautical chart, a map, and the map is dry paper on a desk. But the boat moves through the water and the water moves. Sometimes fast. If we come in the Golden Gate at 6 knots and the tide is going out at 4 knots, our speed on the chart (over ground) is only 2 knots. There are little clues in the details! When sailing, the boat can make about half the speed of the wind. So if they report SOG faster than the true wind, either they are tired, groggy or they are cheating: motoring! And if we're proceeding crosswise to the current, our course on the chart is even weirder.

It used to be a math and geometry problem to figure these things out, because we measured speed through the water with various water sensing devices (look it up, where did the word knots come from?). And we measured direction with magnetic compasses. These things are subject to all kinds of local influence and error, which we had to account for when trying to put our position, speed and course on a paper chart.

No more. Now the GPS system looks down on us from the heavens and sees our business just like you see a speck on a map or chart. If you grew up using a AAA map to get to grandma's house, and now you take orders from the little voice in the GPS speaker, you have an idea how our life at sea changed. Sextants, plotters, dividers, course calculators, sight reduction tables and universal plotting sheets are all gone. We're GPS button pushers now.

GPS doesn't see the wind though. For that we still use speed and direction sensors on the mast of the sailboat. But Einstein's relativity gets busy here again. The speed and direction of the wind are one thing when we stand still, but once we start moving, it all changes.

True wind speed. That's the wind you feel when you are planted in one spot. If it's 10 when you're standing still, and you start roller-blading at six knots, straight down wind, you feel a 4 knot wind at your back. We call this "Apparent Wind."

True wind angle. The direction the wind is coming from, as if plotted on a map or chart. This gets even more confusing. Because when we start moving through the breeze we cause the apparent wind to change direction. The faster we go, the more we influence the direction of wind we feel. On a motorcycle at 120 knots, we feel the wind coming from straight ahead, never mind that the true wind is coming from our left side. So when Ed reports true wind angle to us, he's doing some good trigonometry to remove the influence of the boat speed and direction. Good thing he does, so we don't have to do it at home, to make his numbers compare to the weather man on TV.

By the way, it's more complicated than that. When he says "true wind angle" he's also referring to true map directions, not boat compass directions. The magnetic compass we use on a boat (or you use on your dashboard or camping trip) points to the earth's magnetic north pole. Sadly, this is not located at Santa's north pole. It's actually somewhere in northern Canada. And it's moving at about 35 miles a year, toward Russia. Look it up.

So when we talk about true directions on a boat, we not only have to take into account our own boat speed and direction, but also the geophysics of magnetic variation.

So when we say COG, SOG, TWS and TWA, we're explaining important stuff to other sailors about the language and possible complicating choices. To those of you watching at home, we have just made it as simple as possible: these are numbers you can put straight on a map.