Monday, March 21, 2011

Nav 101–Track Made Good

We dealt with some serious currents in Haro Strait going to and coming from Anacortes.  I’ve known about track made good and how to calculate it since I was about 12 years old but I’ve never seen it so clearly illustrated as it was on this trip.  Father was taking ground school and I used to study his lessons with him in the evening.  I always loved the logical certainty of geometry and conceptually its pretty simple to plot a course but its weird to see it in action.  When the current vector is close in size to the course vector then its really dramatic.  If you’ve already taken navigation then this is going to be pretty elementary and you may want to watch the news instead.  If not, read on.


Don’t get intimidated by big words like “vector”.  That’s just a 64 dollar word for arrow.  The direction of the arrow represents the direction of something – your course or the current in this case.  The length of the vector is scaled to represent your speed.  If you were doing this exercise ahead of time, which would be the most useful way to do it, then you would plot these in a different order but I think its easier to understand how it works if we start from the end and work back to the beginning. 

This morning we started at the exit of Guemes Channel, on the east side of Rosario Strait.  That point is the “W” in “We start” on the drawing.  The first thing we know is the current, in this case it was about 4 knots out of the north so we draw a blue arrow 4 knots long, pointing south.  Don’t get hung up on how long 4 knots is – it could be 4 inches, 4 centimeters or (if you have a really big sheet of paper) 4 yards long.  It just needs to be 4 somethings and then you need to use the same somethings for all the other measurements.  We had to steer significantly north of west in order to actually travel a little south of west, which is represented by the red arrow pointing west northwest.  Our speed was roughly 6 knots so this arrow is 1-1/2 times the length of the blue arrow to keep everything to scale. 

The brown arrow on top is just the sum of the other two arrows.  You could measure it to see what your actual speed over the water was and in this case it would be roughly equal to the boat speed.  The direction is determined by the current flowing down and the boat going generally west. 


I took the picture above somewhere in the middle of the strait.  You can see that the bow of the boat is no way pointing toward the channel which is visible in the lower left corner of the windshield yet that is in fact exactly where we were headed.  If you look closely at the nav screen there are two lines.  The top line is the plotted route with various waypoints in it.  You can distinguish it from the second line because it bends to follow from one waypoint to the next.  The lower line is a predictor line that the software creates based on our actual travel direction.  It takes the direction we are actually moving and projects that out 30 minutes ahead of us.  If you can make all that out the bow of the boats is pointing maybe 20 degrees north of the channel but the predictor line says we are actually moving directly toward the channel.  It was a pretty clear demonstration of crab steering.

Now in the real world you would solve this drawing a little differently.  You wouldn’t know where you had to point the boat in advance.  In fact calculating where to point the boat would be the whole object of the exercise.  In that case you would draw exactly the same drawing.  You would draw the blue line for the current and starting from the exact same place you started the blue line you would draw the brown arrow for your desired track made good with the length of the line representing your desired speed.  Then you would connect those two lines (with the red arrow) to determine what direction to point the boat and how fast to power it.  If you ended up with a speed that was impossible then you would have to adjust the length of the desired track made good vector.  Or in other words, you wouldn’t make as good a speed as you initially wanted.  QED as father used to say.

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