Sailing in Shifty Wind
|Wind is never
constant. This is not news to sailors, who are constantly dealing with
shifting wind and wave to maximize performance. On this page,
we'll discuss wind shifts and how to extract maximum performance from
them. Another important but less understood phenomenon - wind shear
- is discussed here.
Wind shifts occur at all time scales - from
seconds to days. By
definition, wind shifts of less than the reaction time of the boat must
be ignored. Shifts on a time scale much longer than the race are also
only of academic interest. We're interested in shifts with a time scale
from say 10 seconds to the length of the race (hours to days).
Long Race - Optimal Routing
Optimal routing is commonly used on long distance races. This technique
uses weather prediction (in the form of GRIB
files) and polar
curves to determine the fastest path to the finish line (here
is a free GRIB file
many sailors, Expedition
is the optimal router of choice.
GRIB files generally predict wind every 3 hours on a grid 30
miles on a side. If your race is on a scale commensurate with the GRIB,
you should certainly consider doing an optimal route to set strategy.
If possible, the route should be redone during the race to factor in
actual present position and changes in forcast.
For the smaller scale, prediction is less important than knowing the
current conditions and how local knowledge will affect them.
fast - Targets
fundamental technique for optimal sailing up and down wind is the Target
Boatspeed idea. Polar curves allow you to determine the
boatspeed at which your Vmg is maximized. There is good reason to use
boatspeed and not Vmg to set the optimum sailing point (see the
This technique implicitly sets the true wind angle to be sailed. If the
wind shifts, heading changes the same amount to as to maintain target
boatspeed (and therefore, true wind angle).
Changes in true wind speed affect the shape and magnitude of the polar
therefore both target boatspeed and true wind angle. But in general,
sailing targets means sailing a fixed true wind angle (as determined by
target boatspeed) relative to the
Targets themselves say nothing about wind shifts. When shifts
added to the mix, there are techniques to get extra performance beyond
targets alone (here).
in the right place - Lateral
Winning a boat race doesn't depend only on speed. Cunning plays an
important role too. When sailing upwind, if the wind doesn't shift and
two boats have the same performance, they can be said to be even when
they are on the same "ladder rung", a line perpendicular to the true
wind. In the figure, A and B start off even because they are on the
same ladder rung. Then the wind shifts 10º to the left, and A
suddenly 25% of the lateral seperation ahead of B, and didn't have to
sand a single bottom to do it. This mechanism
is why being on
the "right side of the fleet" is so very, very important.
So how do you know the wind is going to shift left instead of right? By
playing the percentages. By observing wind direction, you get
feeling that the average is, say 225º and gain a sense of how
shifty the breeze is. Then, because wind always oscillates (more or
less depending on the weather pattern), you will know that when the
gauge reads 230º, the wind is 5º right, and you can
to go left in a while. You can also read the wind shear tea leaves (see
In addition to tacking, lateral separation can be manipulated by
pinching in the headers and
footing in the lifts (doing the Wally - see below). If you don't overdo
it, Vmg isn't affected much, but lateral separation can be changed a
lot. If you're headed and pinch, your separation is reduced on the
boats to to the favored side and simultaneously increased on those to
the unfavored side. When the wind shifts, you suffer less damage from
the favored boats and gain extra on the unfavored boats.
|Shifts on the small scale - Doing
When the wind will oscillate at least 1 cycle before you get to the
windward (or leeward) mark, then your objective is to make the best
speed up (down) the AVERAGE wind direction, not the present wind
direction. This change in perspective moves the little squares on the
polar depending on where the present wind is relative to the average
In the figure, the wind is shifted 10º right from
moving the Vmg points. This means that the optimum Vmg point has a
faster than standard Target on the lifted tack and lower than standard
Target on the headed tack. This change actually makes your Vmg HIGHER
than standard, because the curve rotates up relative to the Vmg point
on the rotated polar. Therefore, if you Wally (sail the faster target
speed when lifted and the slower target when headed), two things happen:
Why not just tack? There may be tactical or strategic considerations,
but more importantly, tacking costs distance to weather. If the shift
is too quick or too small, you won't make up for the loss. Wallying
gains distance to weather and Vmg when you can't or shouldn't tack. Here is a paper on Tack Analysis - How to find out
how much your tacks are costing, and how to optimize them.
- You go upwind (up the average wind) faster than
you would if you just sailed to your basic target speed.
- Your separation automatically moves in the
direction to reduce damage from boats on the favored side, and increase
gain on boats on the unfavored side.
How to Wally
Let's say you're sailing in an oscillating northwest wind with shifts
lasting long enough to allow tacking.
There are two advantages to doing this:
- As the wind starts to lift above mean, you
foot. The greater the lift, the more the foot.
- When the lift hits maximum, you're going your
- As the wind swings back to mean, you begin to
return to your target speed.
- When the wind hits the mean, you're at target
speed, and you tack onto the new favored board.
- You're always
going faster upwind than you
would if you just sailed your targets.
- Your lateral separation automatically changes
gain more on those on the unfavored side and loose less to those on the
favored side when the wind shifts back.
to large scale - Vmc
What about non-upwind/downwind sailing, like in distance
races? In the figure, you can see that bearing off below the rhumbline
increases your speed toward the mark
Of course, the disadvantage of Vmc sailing is that you go off
rhumbline, and will eventually have to get back. In the upwind case,
you can tack. But in the offwind case, you cannot. If the mark is days
away, the wind will likely change and allow you to consolidate back to
the rhumbline. If the mark is minutes away, maybe no Vmc sailing. If
the mark is in between, it depends on circumstances.
Whether or how much to Vmc sail depends on your knowlege of the future.
If you have a GRIB file, and you trust it, you will be able to
determine whether or how much to stick your hide out. If the wind is
going to shift enough to tack, or
at least enough to get you back to the rhumbline, then you can Vmc with
If you use optimal routing, the granularity (time steps and size of the
grid) of the underlying GRIB file
forces prediction on a scale of 30 miles and 3 hours.
Vmc sailing let you milk extra performance out of wind shifts that fall
into the smaller time and distance scale. And you have knowlege of the
that will help you decide whether and by how much to Vmc sail.
If you don't have an optimal route, Vmc is a way to get an extra 10%
performance that nobody else even knows about.
Whether or not you are playing an optimal route, Wallying takes
advantage of time scales in the minute range.