7.4 Wind Shifts
The second strategic factor is wind shifts. Shifting winds allow us to reach the windward mark more quickly than we can in steady winds.
When the Wind Shifts
When the wind shifts, our close-hauled compass courses change. With each shift one tack is lifted up above its earlier course, and the other tack in headed below its previous course. When one tack is lifted the other is headed, and vice versa. Our goal upwind is to sail each tack when it is lifted. By sailing the lifted tack, we sail a more direct route to our upwind destination (Fig. 5).
Fig. 5 – As the wind shifts, one tack will be lifted up toward the mark while the other tack will be headed away from the mark.
Sail to the Shift
The fundamental upwind strategy is to sail toward the new wind or wind shift. As we will see, this strategy keeps us on the lifted tack. The application of this principle changes with different types of shifts; but the fundamental rule-sail to the shift-never changes.
The best way to keep track of wind shifts is track your close-hauled compass course. Before and during the race keep a record of compass headings and establish a range of highs and lows for each tack. By recording the shifts, you can look for patterns (see Figure 2) and anticipate upcoming shifts.
Of course, reading the compass only tells about a shift after it has arrived. In addition to the compass, observe the wind on the water and watch its effect on other boats. Work to recognize and predict shifts before they arrive.
Types of Shifts
Wind shifts are generally categorized in two types: oscillating and persistent. Oscillating winds shift back and forth, like windshield wipers. Persistent shifts swing gradually in one direction, like the hands of a clock. A shift to the right is a clocking shift, or “veer;” a shift to the left is called a “back” (Fig. 6a, b).
Fig. 6a – In oscillating shifts the wind swings back and forth
Fig. 6b – In a persistent shift the wind shifts continuously in one direction.
From experience we know the real world is more complex than simple oscillating or persistent shifts. For starters we are going to look at strategy in these two textbook types of shifts. After that we will look at other variations.
We will look at wind shift strategy by first looking at the textbook strategies for oscillating and persistent shifts.
The basic strategy in an oscillating breeze is to tack with the shifts. As the wind shifts one tack is lifted so we can point closer to the mark (or average wind) while the other tack is headed further away. When the wind shifts again the advantage will be reversed; whenever one tack is lifted the other is headed. By coming about when headed and sailing on the lifted tack, we can take advantage of the windshifts to improve upwind performance. We use the term staying in phase to describe the process of tacking on the headers and sailing on the lifts.
Sometimes the wind shifts gradually back and forth. This is seen most often when the winds are coming over open water. At other times the shifts hit all at once. We see this when the shifts are coming off shore, or in the northwest winds after a cold front.
On starboard tack a higher compass reading is a lift, a lower number is a header. On port lower is a lift, higher is a header. It is most important to recognize headers since they suggest it is time to tack. Use the phrase Port, Higher, Header as an aid to remember the correlation between compass readings and shifts.
Get in Phase
By collecting wind information before the race and updating information as the race progresses, we should know at any moment whether we are sailing a lifted, headed, or average course. If conditions change, our lifted and headed numbers will have to be adjusted.
Out of Phase
Sailing in oscillations seems pretty straightforward. If the shifts come in a regular cycle and no one gets in your way, it should be easy to stay in phase. Somehow it doesn’t always work that way. If you find yourself out of phase, sail the tack which takes you closest to the mark, or towards the next puff, while you sort things out.
Tacking as the wind crosses the average works great if the wind spends equal time on each side of average and the mark is straight upwind. This is not always the case. When the leg or shifts are not balanced, then the crossover angle for tacking must be adjusted to match. For example, if the leg is skewed to three times as much starboard as port, then we’ll need to sail on starboard for three quarters of the wind range, and port for one quarter.
Similarly, if there are other strategic considerations, our tacking angles may be skewed. For example, if there is better wind or current to one side of the course, we would only sail away from that side at the extreme end of the shift spectrum.
In preparation for our 12:15 start we periodically record and plot our wind information. Using the close-hauled compass course on each tack, we are able to calculate the wind direction. The Wind Graph from our Race Planner shows the following (Fig. 7):
- An oscillating breeze, shifting back and forth.
- Starboard tack headings ranged from 170º to 185º.
- Port tack headings ranged from 260º to 275º.
- We tack through 90º; from 170º on starboard to 260º on port, or from 185º to 275º.
- The wind speed is a steady 8 to 9 knots.
Fig. 7 – The Wind Graph above shows the plot of an oscillating wind over time. Using this information, the tactician is able to plan a strategy to take advantage of the shifts, shown by the checkered boat, above. The black boat ignores the shifts, and falls behind.
Stay in Phase-Tack on the Headers
Coming off the starting line, we are sailing on starboard with a compass course of 180º to 185º. These are high numbers for starboard tack, which means we are lifted. Gradually our course drops to between 175º and 180º. This is an average course, neither headed or lifted, and we can sail on either tack.
Soon the crew reports that our course has dropped below 175º. Other boats are showing similar angles. We are headed. We tack. As we settle on port tack the compass reads 260º-265º, low numbers which mean we are lifted on port tack. As the numbers rise, we are getting headed. Remember: Port, Higher, Header. When our course falls below ouraverage, we tack again and sail lifted on starboard.
As the plot shows , we continue tacking on the headers in the oscillating shifts. Gradually we find ourselves to the left of the middle of the course. We use the time when the wind is at the average direction to sail on port tack, which returns us to the middle. We sail on starboard only when lifted above 180º; we sail on port for any heading between 260º and 270º.
A New High!
Further up the leg, we are sailing lifted on starboard. Compass readings show a course between 180º and 185º. Gradually we are lifted to 190º. At this point lights should flash, bells should ring, and sirens should sound. 190º is beyond our range of oscillations. We are lifted higher than ever before.
We may have to reevaluate the conditions and modify our strategy. What is causing this new reading? Helmsman error? Changing weather? Are we closer to shore? Are there any new clouds? What is going on with the rest of the fleet?
Is this a momentary aberration, after which we will return to earlier conditions? Or is this the beginning of a persistent shift? Perhaps the wind will continue to oscillate, but over a new range.
The key is to first recognize that something new is happening. The next step is to evaluate the change and make plans accordingly. Ideally we would have seen it coming-either from wind on the water, an expected shift near shore, or by observing other boats (in an earlier class-we are leading our fleet, remember).
The Impact of Shifts-Don’t Miss ‘Em
Meanwhile, one of our competitors has sailed off the line on starboard tack and continued one third of the way up the leg before tacking. From there he sailed across the course on port tack, to the starboard tack layline. Ignoring the shifts has left our rival out of phase and sailing headed half the time. On the other hand, he has only had to tack twice! On a two mile beat with 10º oscillations, a boat which sails in phase will be minutes ahead of a boat which ignores the shifts. That’s even after we throw in the cost of a couple of extra tacks!
A little later, we’ll pull out our slide rules and find out just how big a deal windshifts are, but for now, just remember: Track the shifts, and hit ‘em!
Similarly, if there are other strategic considerations, our tacking angles may be skewed. For example, if there is better wind or current to one side of the course, we would only sail away from that side at the extreme end of the shift spectrum.
The strategy in a persistent shift is to sail toward the new wind. If the wind is shifting to the right, then go right. If the wind is shifting left, go left.
In a persistent shift, one tack is continuously getting headed while the other is getting lifted. Our strategy is to first sail the tack which is getting headed, then sail the tack which is getting lifted. Why? The tack which is getting headed is getting worse all the time. It is headed now, but will be headed more later. Sail it now before it gets worse. The tack which is getting lifted is improving all the time. If we sail it now we will be missing a better lift later.
Are you Sure it’s a Persistent Shift?
How hard is it to split with the fleet and sail into a header? Without our pre-race info, weather forecast, and/or observation of other boats on which to build our strategy, it would be crazy. Even with good information and a well thought out plan, it is hard to stick to your guns as the fleet tacks away. If you know what is coming, then go to it. Position yourself to the favored side of the fleet.
How much do you gain by sailing into a persistent shift? It depends how far the wind shifts and how far you are separated from your competition; but in a word – Plenty.
We know from our Wind Graph that the wind is gradually backing. Our starboard tack readings for the hour before the start show a trend: 30º > 25º > 20º > 15º. The port tack compass readings are similar: 120º > 115º > 110º > 105º. Starboard is getting progressively headed, while port is getting lifted. The forecast tells of a low passing to the south. We expect the wind to continue to back (Fig. 8).
Fig. 8 – Wind Graph and strategy for a Persistent Shift.
Sail to the Shift
Coming off the starting line on starboard tack our compass reads 15º. A minute later it reads 10º, and a few minutes after that it reads 5º. With each little header some of our competition tacks away. Some tacked to port on the first header off the line. Others have gradually bailed out as the header continues.
Eventually, with the compass reading 0º, we tack over. Our course on port tack is 90º. We are short of the layline to the mark. Gradually we are lifted, first to 85º, then 80º, and finally 75º. We are lifted to the mark, far ahead of those who tacked out early.
When should We Tack?
The choice of where to tack is a little tricky. Theoretically we want to tack so we will be lifted exactly to the mark. This curved lifted layline would give us the full advantage of the shift without sailing any extra distance. This is a tough call, to say the least. A more realistic approach is to tack short of the layline. Then, as you approach the mark, tack out again, and take another guess. Don’t overstand or you’ll be sailing extra distance. As you get closer to the mark, you should be able to make an accurate layline call.
Another (tactical) perspective on where to tack is to maintain position between the fleet and the shift. Don’t sail to the corner-just get a controlling position.
So Much for the Competition
Coming off the starting line on starboard, we gradually got headed. Some of the fleet played the header as an oscillating shift, and tacked out. They expected to be headed again (on port tack) before tacking back to starboard. This was a big mistake. We knew from our Wind Graph that we were in a persistent shift. We used persistent shift strategy by sailing into the header, towards the new wind.
The boats which tacked out early kept getting lifted further and further on port tack, which made starboard tack look worse and worse. The port tack boats ended up sailing what is known as the great circle route, getting lifted around the outside of the mark.
Too Far Ahead
If the fleet goes right and you think the wind is going left, then position yourself to the left of the fleet. Don’t split completely with the fleet, for two reasons:
First, if you split and you are right, you will end up way ahead, and everyone will think you were just lucky. You don’t want to win by too much; just a comfortable margin that will let everyone see who is out front.
Second, if you split with the fleet and happen to be wrong, you are sunk. (Somehow it is not unlucky to be half a mile behind-you are just bad.) Hedge your bets.
Other Types of Shifts
As we said above, pure oscillating and pure persistent wind shifts are quite rare. There are infinite variations.
One variation is a mix of persistent and oscillating shifts. This mixed condition is characterized by oscillating shifts gradually shifting one way or the other-veering or backing-over
time.Several other types of wind shifts occur. One is a major shift where a new wind completely replaces the existing wind. This can happen suddenly, or after a period of calm. Winds vary in other ways. There are geographic shifts caused by the configuration of land and thermal wind shifts created by the heating of land. There are also differences in the wind due to differences in current (as we will see below), and there are shifts caused by the movement of weather systems.
Often conditions are not a pure form of persistent or oscillating shifts. Instead we end up with a mixed condition, combining both oscillations and persistent characteristics. The strategy in a mixed breeze is to favor the side toward the persistent shift while playing the oscillations. Part of the trick in coping with these mixed conditions is to realize that the range of oscillations is gradually changing. The high and low numbers on each tack will be increasing or decreasing. What was once a header may now be the median, with a new lower header on the way.
When conditions are too confusing to diagnose, the fall back strategy is to sail to the mark. Which ever tack takes you closer to the mark is preferred until there is reason to do otherwise.
Mixed Conditions Example
Here is an example of strategy in mixed conditions (Fig. 9).
Fig. 9 – Wind Graph and strategy for Mixed Conditions.
Our pre-race data is listed in the Wind Graph. It shows mixed conditions. The trend is veering, but we have oscillations as well. Our strategy will have to consider both; we will also have to keep a keen eye out for changes in conditions. One thing to look for is stronger breeze to one side. Is the trend shifting and building (as in this example) or shifting and fading.
. . . And They’re Off
It is interesting to see the different strategies which emerge from these difficult conditions. Some of the fleet will treat the beat as though they were sailing a persistent shift. Others will tack on the headers. Some will try to balance the mix. And still others will be confused and uncertain of how to handle the conditions.
As the fleet moves up the beat in mixed conditions, the apparent leaders will change with each shift. Often it is unclear until the last shift of the leg who will come out ahead.
In mixed conditions you are never out of it. Keep working, keep trying to decipher the next shift. There are plenty of opportunities to catch up (and more than enough chances to get confused). If you find yourself baffled, try to re-group. Everyone will have their moments-if you can keep from going to pieces during your bad moments, you’ll have another chance for good times.
We do most of our racing near shore, where the interaction of the land and water affects our sailing wind. Further offshore conditions are more stable and predictable; but along the coast, wind conditions are difficult to predict.
There are many ways the shoreline changes the wind. First, the shoreline funnels the wind. The wind shifts to follow the shoreline. Second, offshore winds tend to shift more perpendicular to the shoreline (Fig. 10a). Third, winds shift around obstacles such as hills, buildings, and thermal domes in areas with lots of pavement (Fig. 10b).
Fig. 10a – Offshore winds tend to shift more perpendicular to the shoreline.
Fig. 10b – A city or hill can create oscillating shifts, with bigger shift near shore.
Fourth, the heating of the land creates thermal winds-sea breezes-which blow towards shore during the day.
Fifth, the thermals create turbulence and mixing which can pull the upper winds down to the surface. These upper winds are generally shifted to the right of the surface winds.
The effect of these geographic changes can be either persistent or oscillating. In offshore winds, there will commonly be a mixed effect-with puffs coming from shore lifting the tack which is parallel to shore, and with those lifts being stronger the closer you get to shore.
The thermal effects on an onshore wind usually create a persistent shift from the prevailing wind direction to the normal sea breeze direction (Fig 11).
Fig. 11 – As the land heats up, you get a persistent shift as the prevailing wind shifts to the sea breeze direction.
These shore effects are described in more detail in Chapter 13: Weather later in this book.
Weather System Wind Shifts
A Major Change
As weather systems move or weaken, one wind can replace another. A prevailing wind may be pushed aside by a thermal, or a new weather system wind may arrive. Whatever the cause, there are occasions where a new and different wind appears and all the earlier information becomes irrelevant.
If we can anticipate this change based on forecasts or observations (new clouds etc.) and be in position for it, there can be tremendous gains. But beware-There is a danger in chasing the predicted conditions and expecting a shift which does not arrive as scheduled. Summer weather systems often stall or even disappear as the get to the coast.
Another type of weather system wind is a localized late afternoon squall. These can turn the entire fleet on its head. These squalls create two opportunities-one as the squall hits, and the other as the squall passes.
Be prepared. If you are ready and can continue to race through the squall while others scramble, huge gains are possible. Sail toward the new shift to avoid windward work once it arrives, and be ready to shorten sail. A squall drill should be part of your crew training.
If the squall is localized, then after the squall passes conditions often return to the conditions which existed prior to the squall. If the squall is part of an advancing cold front, then conditions behind the front will be entirely different from earlier. By recognizing the type of squall you may be able to anticipate conditions during and after its passage.
Wind shear is a condition where there are layers of wind-one at the surface, and the other aloft. Wind shear is uncommon. It occurs most often over smooth cold water early on a spring day, or at night.
There is a boundary layer of cool air on the surface, and a different wind aloft. Sail trimming with the wind 30º, 45º or even 180º different from the deck to masthead can be baffling, to say the least.
Generally the upper wind will become dominant. Fundamental wind strategy says sail toward the new wind-in this case sail toward the upper wind, as it will eventually displace the surface wind. If you are a small boat in a mixed fleet, don’t forget to look at the mastheads of larger boats. Sometimes you will be able to find a wind shear (and a clue to the expected wind) which does not show at your masthead (Fig. 12).
Fig. 12 – Wind shear is the stacking of two winds, one on top of the other. The upper wind usually displaces the lower wind.
Don’t confuse wind shear with wind gradient. Wind gradient is the tendency for winds at the mast head to be stronger than those at deck level. It exists almost all the time, and is more dramatic in light air, less pronounced in heavy air.
The Impact of Windshifts
It is often difficult to predict the wind. Is it worth it to try to figure out what the wind is going to do next? How much difference does it make? Here’s an example:
The Impact of Oscillations
We’ll start first with a boat sailing upwind, with a tacking angle of 90°. Using trigonometry (yikes), we find that the distance sailed is 1.42 times the straight-line distance. To sail to a mark one mile upwind, the boat will have to sail a total of .71 miles on port tack, and .71 miles on starboard tack.
If the wind is oscillating as little as 5° either side of the median, then performance improves significantly. To sail to a mark one mile upwind, the boat will have to sail a total of .64 miles on port tack, and .64 miles on starboard tack. By taking advantage of the shifts, the distance sailed is reduced to 1.28 times the straight-line distance. To sail to a mark one mile upwind, the boat will have to sail a total of .64 miles on port tack, and .64 miles on starboard tack. A savings of .14 miles!
If the wind is oscillating 10° either side of the median, then performance improves dramatically. To sail to a mark one mile upwind, the boat will have to sail a total of .57 miles on port tack, and .57 miles on starboard tack. By taking advantage of the shifts, the distance sailed is reduced to 1.14 times the straight line distance. A savings of .28 miles!
At a boat speed of 6 knots this represents over 2-1/2 minutes in one mile! That’s right – 150 seconds per mile! These numbers are for a boat sailing the wind shifts perfectly. If your windshift efficiency is only 40% of optimum, you’ll still save one minute per mile! (Fig. 13)
Fig. 13 – The impact of oscillations can be significant.
Current adds complexity to strategic planning. The obvious, and primary, strategic concern is to seek out better (more favorable or less adverse) current. When the current is not uniform across the course, it can be an overriding strategic factor. Currents run stronger in deep water than in shallow, and faster in narrows than in open water. Below points and around bends, eddies can develop. Adding further complexity is the fact that currents change. Correct strategy can change dramatically over a period of hours. Storms and strong winds can distort surface currents and delay tides, sometimes making tide tables useless (Fig 14).
Fig 14 – Currents are created by tides and rivers. They run stronger in channels than in shallows, and can be a major strategic factor.
Wind Driven Current
Currents are not limited to rivers and tidal basins. In the Great Lakes, for example, currents of one full knot are possible. Currents build when strong winds drive the surface water. After the winds abate, the currents reverse as the water, which has been stacked up at one end of the lake, returns to level (Fig. 15).
Fig. 15 – Wind can create current. Wind driven current can exaggerate or reverse tidal currents in shallow bays. And it can create currents where there otherwise would be none. After the wind has pushed water to one end of a basin, the current will reverse when the wind subsides.
Uniform Across the Course
When the current is uniform throughout the course, it affects the laylines and sailing angles to the mark. If it is running across the course, then it can also change the balance of time spent on each tack (See Fig. 16).
Fig. 16 – When the current is uniform throughout the leg, the biggest impact is on laylines into the mark. Current can also skew the course, changing the balance of time on each tack.
When Current is Not Uniform
When the current is not the same across the course area, then we must seek out the advantage. Differences across the course can shape our strategy. Unless there are dramatic differences in wind conditions a current advantage is key to strategic planning.
Current to one side
Obviously if the current is stronger to one side seek out that side if it is favorable, and avoid that side if it is adverse. A favorable current, running against the wind, can set up a pronounced chop. Look for this. Sail into the choppy water and ride the current upwind. Similarly, smooth water can indicate wind and current running together. Avoid this area upwind (Fig. 17).
Fig. 17 – When there is stronger current to one side, go to it if favorable, sail away if adverse. The advantage can change with the tides.
Current across the course
When the current runs across the course and is stronger in one part of the leg, you want to take advantage of the change in sailing wind caused by the current. Sailing bow into the current you will be lifted; sailing with the current astern you will be headed. For example, in ten knots of wind and one knot of current the sailing wind is shifted 6º. With one tack lifted and the other headed, the effect is a 20% advantage in VMG (Fig. 18).
Fig. 18 – When current is not uniform, take advantage of the shift in sailing wind. In this example we have 10 knots of wind, 1 knot of current, and boats tacking through 90°. The current gives one boat a 6° lift, the other a 6° header. The lifted boat’s VMG is 20% better than the headed boat’s. (VMG is normally .71 of boat speed. The shift is 6° because arc-tan 6°=1/10. A 6° lift creates a VMG of .78, a 6° header creates a VMG of .63. .63/.78=.80. I’m glad you asked.)
Predictable, to a Point
Current, whether tidal or river generated, is predictable. Tide tables and current charts should be studied, and their predictions compared with observation. When the current runs strong it is often more reliable and predictable than the wind; a small current advantage translates into big gains.
In tidal areas the advantage can be fleeting-or reverse-over the course of a race. Obviously we need to pay attention to changes in the tide.
Changes with Wind Conditions
The wind can upset current predictions, particularly in shallow water. A strong wind blowing over a long period can overwhelm tidal effects, pushing surface water and delaying or reversing tides. When the winds abate, the current distortions will remain until the water has had a chance to return to level by flowing in the direction opposite the earlier wind. Winds can also create currents where there otherwise are none, as mentioned above.
Effects on Sailing Wind
Current changes the sailing wind for a boat. The sailing wind is the sum of the true wind over the bottom and the current. The net effect of current on the wind reinforces the effect or the current. A favorable current creates a favorable change in the sailing wind, and an adverse current makes for an unfavorable change in the sailing wind (Fig. 19). Details are explained in Chapter 13: Weather near the end of this book.
Fig. 19 – The vector of the current is added to the true wind to create our sailing wind.
Develop Local Knowledge
One key to success in current is to develop local knowledge. Keep records of how the current runs in various wind and tide combinations. Our strategic plan is only as good as the information it is based on. Accurate current information is critical to good strategic planning.