Oscillating Shifts
In the first two persistent shift posts I presented some scenarios that were pretty much geometrically perfect, but in the messy real-world boats
rarely sail on arcs of perfect circles. So I want to
take some time to cover oscillating shifts with the aim of eventually combine oscillating
and persistent strategies. In the next blog post and by following the link to the game at the bottom of this post, we will get into using some real wind data.
In the below, I have drawn the courses of
boats A and B, let’s say they are sailed by prairie sailors from a small lake
or coastal sailors who sail exclusively on a small bay in Victoria out of the way of the current. Either way, they find themselves on a big race course competing against the sailor in boat C who is more
used to big courses on bigger water (though conveniently still with negligible current). Boats A
and B know to tack on the shifts, why wouldn’t you?
 |
| In this strange diagram, I have colour coded the tacking angles of four different wind directions with a legend of sorts in the bottom right: 4 degrees left, four degrees right, 10 degrees left and 10 degrees right. I then built the courses of the three boats based on whether they were on starboard or port during which of the phases |
Back Stories and Scenarios
To make sense of this diagram, I have made up some back stories...
Boat A came out to the race course a couple of minutes early and
figured out that of the two oscillating wind directions, when the wind is in
the direction that I have coloured orange, it is a veer (lift on starboard) and
when the wind is in the yellow direction the wind has backed (port is the
lifted tack). In the time that A has been paying attention, she does not encounter the red or maroon wind directions. Naturally A starts on
the lifted tack and every time A gets knocked, she tacks: fair enough.
B looked at the windward mark and realized that in the
orange phase she was pointing closer to the windward mark on port, so maybe B
started on port or maybe she started at the barge on starboard and then immediately after the start she
tacked onto port. B would have been happy
after the first shift when she was lifted onto the yellow course even closer to the mark, but then
when she was knocked back onto the orange course, B tacked to stay in phase
with the shifts. So B continues tacking
on the shifts which usually works back home on small courses on say, Chestermere Lake or Cadboro Bay.
Maybe C had been out on the course checking the wind for 45
minutes before the start which let her notice that although there are bunch of
6 degree oscillations that come and go, there are bigger 14 degree oscillations
that come and go more rarely (C didn’t need to know those exact numbers, just the feeling of bigger and smaller shifts and a sense of where they come from). So C builds a strategy that she should only play the
big oscillations and forget about the little ones: stay in phase with only the big shifts. Maybe C didn’t even do 45 minutes of
research with a compass, maybe C just eavesdropped on the race committee
channel that was playing at high volume from the pin boat or a coach boat and C
overheard the following conversation:
“Markset, are you happy with the position of Mark 1? You look a little right to me“
“Affirmative, we have been doing a wind trace and as well as
some small shifts, there are 14 degree oscillations perfectly about our rhumb
line at this bearing”
“Good work Markset, then, we will leave Mark 1 there”
So C thought: ‘…perfect oscillations about the rhumb line
eh? In that case, I’ll just try to stay on
whichever tack takes me closer to the windward mark’
When I showed this diagram to my partner Maura, who is a
strong Radial sailor, she was uncomfortable because she thought that I was in
danger of encouraging people to bang corners.
For this reason I have not drawn in the windward mark. With some imagination, we could put this
diagram and scenario in different contexts.
Firstly we could have the windward mark just at the top of thr diagram with C on or just below the layline getting ready to round.
Secondly we could say that the diagram only shows the first
third of the beat and C continues on to cross A and B for another four minor
oscillations before she tacks again on the next major oscillation. In this case C would continue to make similar gains as the big phases roll through.
Alternately, if we put the windward mark half way up the
diagram for a short course, A and B would win because C would have to tack back onto the layline
before making it to the big right shift that she had been calling for and
without the benefit of having tacked on the small shifts.
Segue about Sailing in the Middle of the Course
There are many (primarily tactical) reasons not to sail in the middle of a big
fleet (as A and B would have done with this strategy). Some of the reasons are
that:
-The combined effect of dirty air from many boats makes it
generally slower if you are not well out in front
-Puffs and shifts often come in from a side and dissipate and can’t be used by the
time they make it to the middle
-In the middle, with boats to both sides of you, no matter
which way the wind goes, lose to someone
-In the case of the above diagram especially, the boats in the middle have tacked a lot. In the diagram I have made tacks cost nothing, but A and B have done 7 or 8 more tacks than C so in reality they should be even further behind. In almost all boats in almost all conditions, tacks are significantly slower than sailing straight unless the tactical gain from playing the shift pays for the cost of slowing down through the tack. However in the above diagram, you see that A and B’s strategy actually hurt them even before taking into account the cost of the tacks.
To continue on with that train of thought for a bit… a tack
often has a double cost if you had been purposefully executing a strategy. For example:
You had been sailing out towards pressure or out of some adverse
current, but then you tack on a nice shift. After tacking on that shift you have to throw in another
tack to get back to heading out towards the puff or current relief.
The cost of tacking can be even bigger if you bring in the
tactical consideration that while you throw in this ‘hitch’ (tacking and tacking
back) the rest of the boats around you that didn’t tack will be continuing out
towards the favoured side. This lateral
separation on the race course becomes leverage against you if you are sailing
towards a shift. If you were sailing to a puff, throwing in the hitch means you
spend less time out in the stronger wind of the puff at the edge of the course. If you were sailing out of
adverse current, the hitch keeps you in that current for longer. So if you throw in the hitch, it had better
gain you a bunch of ground to pay for all those disadvantages.
The Payoff
To emphasize the slight difference in course position resulting from the different strategies in the above diagram, at the top of the diagram, I have drawn thin black ladder rungs. I drew these ladder rungs perpendicular to the rhumb line
because according to the race committee the course is perfectly square. But there are other ways of thinking about the relative position of the three boats. If we don’t take the rhumb line into
account and stay in the present, we have the red ladder rung (red for the red
wind phase) corresponding to the instantaneous wind direction which shows that
C is even further ahead of any boats to the left of her. I have also drawn in a brown ladder rung
corresponding to the average of the red and maroon wind directions, because C is not
differentiating between these smaller wind fluctuations and is calling them one phase. According to this brown ladder rung
corresponding to the major right phase, A is even further ahead of the boats to
her left. It is worth noting that
because C is directly upwind of B, she has no leverage on B, so the different
angles of the ladder rungs have essentially no effect.
A Last Scenario
In a fourth and final scenario for the diagram, we can imagine
that there is no major phase, that is, there is no back-and-forth to the larger
oscillations, instead the 14 degree right shift was a one-time event. For example C saw a wind line on the right
that A and B overlooked and so C ignored the oscillations, put all her eggs in one
basket and pinned it out right. The wind
line magically came in at the right time and C just beat A and B to the
mark. In this scenario where the 14
degree wind shift was a one-time event this is effectively a persistent shift
(and this scenario would make C is a corner banger). One of the big things that I am trying to illustrate is that by comparing the fourth scenario with the first, you can see that that the winning strategy for a persistent shift is the
same as for a long phase oscillating pattern that takes longer to oscillate through its
major phase than the length of one beat: here we come back to the importance of time scale. Often in a very slow
oscillating shift, the legs of the race must be raced like persistent shifts. This one of my justifications for talking so much about oscillating strategy in a series supposedly about persistent shifts.
For your homework today try playing this game:
When I was searching around for online software or games
that could help me illustrate these racing scenarios, I came across this interesting
site. It claims to use a real weather
station as the input for the game’s wind:
“Wind oscillation is real from ultra-sonic weather station at NannyCay marina BVI. So, no wind - no game.”
The point of the game is to try to beat Sail Racer’s
strategy algorithm. There is no dirty
air, no Racing Rules of Sailing or collisions, the boats are always sailing
close hauled at the same speed except for out of a tack, but tacks are very
inexpensive, a mere 10% of boat speed for one second. So it is almost completely a strategy game,
though you do have the computer’s boat to check in with and copy if
you are a defensive-minded tactician.
Enjoy! The next blog will use an example from this game.
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