Wednesday, April 22, 2015

Persistent Shifts Part 4

Shifts are often modeled and thought of as sudden changes of direction, but this is not necessarily the case.  In the Sail Racer game we have a scenario where the wind shifts from the weather input are smooth and continuous.  So how do you decide when is the time to tack during a smooth shift?

If you dig out your strategy books you should find somewhere in there that you are supposed to tack when the wind passes the median wind direction.  For example, let's say that you are sailing on an exceptional shift, you are very lifted, pointing almost straight at the mark, but then then you get a slight knock.  You are probably still lifted overall (ie. you are above your median), so don’t tack yet.  Only tack when you have been knocked so far that you are pointing below your median heading because being below your median on one tack means being above your median heading on the other tack.

For a good illustration of how to tack on the median in a smooth shift, check out  this Speed and Smarts article posted on the Destination One Design website from which I pulled the picture below:

This image is not mine, I found it by following the above link.  Please consider subscribing to Speed and Smarts if you like it:

The above picture shows how a boat tacking on the median would make gains on a boat tacking on the maximum.

It is nice to be able to put this theory into practice by playing the wind game immediately after reading it instead of having to wait for the race on the weekend (or for the lake spring to heat up to put it into practice.

I don't know exactly what Sail Racer's algorithm is, but while playing the game the instantaneous wind trace line and a damped-down wind trace line are drawn out in grey.  The damped line is apparently averaging out the wind direction from the last little while.  With some exceptions, like approaching laylines, the Sail Racer algorithm seems to make the computer’s boat tack whenever the line that represents the wind direction crosses this damped average wind trace line.  This essentially means that the algorithm tacks on the median, but we don't know how the algorithm calculates the median.

I have noticed that for whatever reason, that in this game, the wind has tended generally to come back to North and to swing more or less evenly around the direction of North.  If I sail/play the game with that information in the back of my mind, my memory is going back much farther than the wind algorithm’s memory.  So when there are longer phase oscillations and the game begins with the wind direction off to one side, well to the East or West of North, I am usually able to win by being more patient or longsighted than the algorithm.  The algorithm gets impatient (so to speak) by deciding that the current wind direction must be the new normal and so it sets it as it's median and starts tacking on it.  On the other hand I sail the lift to get inside the next shift, that means taking deep breaths and waiting with fingers crossed for the wind to normalize back to North.  

In the screen shots below I am the red boat and black is the algorithm.  I took a screen capture of these races because they show an example of playing the long phase oscillation as a persistent shift by sailing on the lifted tack and waiting for the wind to (hopefully) come back to North (my long term median) before the end of the leg.  The computer in the black boat, blindly following the algorithm, assumes that the initial wind direction is here to stay and that that should be the median.

Looking more closely at the first screen capture you can see that when the race started the wind was already to the right of the rhumb line (slightly East of North) by looking at the grey wind trace lines.  I made a plan based on my theory that the rhumb line (North) was the true median.  The computer’s boat (black) took the North-Northeast direction to be the median wind direction and so she tacked on the short phase oscillations thinking it was a neutral phase.  Over the course of the leg, the wind eventually returned to my predicted Northerly direction, so my strategy of treating this scenario as a persistent shift paid off, I got well inside the long phase persistent shift.  However I want to emphasize that then crazy shifts came through (shown by the squiggly faint wind trace line), I tacked on them, I didn't just blindly follow my plan with no regard for the short phases that were extreme enough to pay for their tacks.  This is what I have been building up to with these blog posts, the idea that overall you may have a persistent shift strategy based on the big phase, but superimposed on that, you can still tack on the extreme oscillations that come through in the small phases as long as you get back to your plan as soon as possible and as long as your tacks pay for themselves (remember the bit in the previous blog about how costly tacks tend to be).

In the first example where the persistent shift was more subtle, I still tacked on the big shifts, but I made sure to get enough separation from black to be able to cash in my left side advantage if my prediction came true.  When the prediction did come true, I did not know exactly how long this big phase would last, so once I had big paper gains, I crossed back in front of the other boat then tacked directly to windward of her to make it more or less impossible to come back and pass me.  You can tell more clearly that I did the same defensive cashing in move in the second screenshot race.  This is an effective move not because of my wind shadow (no wind shadow in this game), but because tacking to windward of black removes leverage that she needs in order to gain or lose on wind shifts.

Paper gains mean the theoretical advantage that you have if nothing changes.  Until you have cashed in your paper gains, because of your leverage, a change in the form of a bad shift could make your gains vanish.  Cashing in means reducing your leverage: sailing ahead (or behind) your competition.  Once you have cashed in you have reduced your leverage and so you are not as susceptible to undesirable shifts.

What I have been trying to illustrate is that in situations with combined long and short phase oscillations, that is, with real world messy persistent shifts, you often have competing strategies: to get inside the (big) persistent shift and to tack on the (small) oscillations.  The trick first to identify whether there is a high likelihood of a persistent shift that could be capitalized on.  Next you need to get a feel for which oscillations are big enough that tacking on them pays for itself and which ones are a waste of time and leverage.  Another thing to keep in mind is where you need to go next so that if you find yourself in a neutral (macro) phase, you can act purposefully.  If you are happy with your placement on the course during the neutral phase you could spend your time tacking on small shifts.  If you are ahead your time in the neutral phase might be well spent cashing in any gains you may have by reducing your leverage.  If you have losses rather than gains, leverage is your only hope of recovery and it is laylines that are the enemy as time runs out waiting for the miracle shift to get you back in the game.  In that case you could decide in the neutral phase either to eat your losses by cashing in/consolidating your loss or to double down and go for more leverage.  Another use for the neutral phase particularly at the beginning of the leg is to start executing the overall strategy.  That could be getting inside the persistent shift, getting out of bad current or sailing to where there is better wind, for example.

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