Monday, March 23, 2015

Persistent Shifts Part 2

In this post I present some more thoughts about persistent shifts before I get into a few more complicated scenarios in upcoming posts, then at the bottom I have a short glossary for some terms, not all of the terms come up in this particular post, but the ones that do, I have put in bold.

As we get into a couple more persistent shift posts, I should mention that along with the invalid assumption that tactics are not a major consideration, for simplicity I am also assuming that all wind shifts happen across the entire race course evenly, so the wind direction varies only with time and do not vary according to your position on the course.  As we know in reality your neighbour can get a shift that never comes to you or that takes its sweet time.  Maybe that should be another blog post.


The real world examples of opportunities to use persistent shift strategies that I have in mind include, for example, on the prairies when a large thunder cloud passes by the lake, or on the coast it could be a slow and transition from the gradient wind to the thermal wind.


The time scale of a shift or a pattern of shifts is also very important.  When I coached in the 2015 Jensen Beach Laser Masters Regattas, there was a very slow persistent shift on the scale of multiple hours that could be seen clearly on the weather prediction websites as their website’s predicted direction arrow turned steadily around the clock through the hours and days.  Sure enough over a day of racing the course often shifted by 40 degrees, but this overall trend was generally on too large of a time scale to be used as a reliable strategy on a particular leg of the course.  The winning strategy came from playing the many and sometimes drastic smaller phase wind shifts that did not correspond to the overall macroscopic trend, they had their own causes separate from the trend that the wind prediction websites picked up.  More examples of time scale considerations are to come in the next posts.
In the diagram, I have drawn a scenario with a start line and no windward mark, so this is like a training scenario where the boat that can cross ahead of the others is in front (no worry of overshooting a mark).  The course that I have drawn (again built from arcs of a large circle as I described in the last post) has two boats A and C starting on starboard and one boat, B starting on port at the pin.  After a while of sailing the identical course and tacking at the exact same time A and B are still the same distance up a course that would have been square to the start line and square to the initial wind direction, but that reference is no longer relevant.  The red ladder rungs show the effective position of boats A and C half way up, then I have drawn ladder rungs for A, B and C at the end.  A is quite a long way ahead of C just from having started at the end of the line closer to the eventual wind direction.  B also starts off with this potential advantage, but ends up sailing with the wrong strategy.  B is probably thinking: “Great, I am on the lift!  I am on the lift!”  And if the wind was going to eventually come back right, B would eventually end up miles ahead, but since the wind just keeps on (persistently) pivoting left or backing, it is bad news for B.  By half way up B tacks to rejoin the other boats and solidifies her losses.

My main point here is that if you decide that the wind will shift one way or other, it is urgent to get to that side so that when the wind eventually comes from that direction you are the upwind boat.  There is nothing much new in this post, but I just want to lay a solid foundation before I start bringing in some weirder scenarios.
Here we are waiting for wind and sheltering from the sun at one of the 2015 Jensen Beach Masters Regattas that I mentioned


Backing:             The wind is backing when it shifts left or clockwise.  This can also be thought of as a lift on port.    Using the terms ‘veer’ and ‘back’ is a great way to communicate clearly with your training partner and coach about the wind shifts experienced during a split tack. 

Ladder rungs:    Lines drawn perpendicular to the wind or to the rhumb line of a windward leg (the rhumb line is hopefully lined up with a good guess of where the wind will generally come from in the race).  Ladder rungs can be drawn on the water with boats passing or climbing from one evenly spaced ladder rung to the next as they zig zag up the course.  Ladder rungs can also be drawn extending from one or more boats to show which boat is further to windward or further up-course.  The distance along a ladder rung is leverage. 

Leverage:            The distance between two boats if you measure only along a ladder rung, or said another way, leverage is the he across-course component of the distance between two boats.  I have also heard leverage called lateral separation.  The opposite of leverage would be distance directly upwind or downwind.

Median wind direction:    The middle wind direction.  This is not the official definition, but it is what I mean, in Laser racing most calculations are done by intuition, even if you have a compass there is only so much mental math you can do while doing everything else.  I have not differentiated in these persistent shift posts between the statistical median and the statistical mean or for that matter the mode.  Technically if you want to calculate the median you take the two extremes of the wind direction and average them together. 

Noise:                   Quick, relatively insignificant changes in the wind direction or in some other signal.

Phase:                  In an oscillating breeze, once you have established your median wind direction you can tell that the wind is in a ‘right phase’ when the wind is to the right of the median and in a ‘left phase’ when it is to the left of the median.  You can also be in phase or out of phase with other boats if you are on the same tack as them or on different tacks respectively.  In strong fleets, staying in phase with the fleet is usually the same as staying in phase with the lifted tack, for example being on starboard in a right phase.  One fairly common scenario is to have smaller shifts coming through at a fast (higher) frequency at the same time as larger shifts that are oscillating at a slow (low) frequency.  In this scenario we have a smaller phase and a larger phase coming through at the same time.  Sometimes the smaller phase can be thought of as noise.

Pressure line:      Increase of wind velocity usually visible as a dark line of approaching rippled water

Puff:                     Localized increased wind velocity usually visible on the water as a patch of ripples

Rhumb line:             The imaginary line straight from the last rounding mark, or from the start line to the next rounding mark.

Strategy and Tactics:       These terms are often used interchangeably, but I am differentiating between them in these blogs.  Strategy is how you plan and respond to conditions like wind patterns, wind strength, waves and current before you take other boats into account.  Boats at the front of the fleet often free to execute their strategy.  Tactics are considerations relating to other boats and they should also factor into your plan.  Dirty air, leverage and rules situations are examples of tactical considerations.  Boats in the middle to back of the fleet often have trouble executing their strategy because of all of the tactical situations that they get caught up in.  In this blog entry, I am trying to focus mainly on strategy.

Veering:               The wind is veering when it shifts right or counter clockwise.  This can also be thought of as a lift on starboard.  Using the terms ‘veer’ and ‘back’ is a great way to communicate clearly with your training partner and coach about the wind shifts experienced especially during a split tack.

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