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Understanding Wind Shear
Less than two weeks ago at Storm Trysail Club's Block Island Race Week, it wasn't uncommon to see sailors on dozens of boats tapping on their instrument displays and scratching their heads. Throughout the week, tacticians and pretty much anyone on board who was paying attention to the wind angles, had to grapple with an interesting phenomenon that's fairly common in many early-season racing venues. The confusion these sailors were experiencing was based on reading radically different apparent wind angles from one tack to the next. This anomaly wasn't only playing havoc with tacticians, who were trying to determine tacking angles, but also with helmsmen and trimmers who were trying to hit their boat's target speeds and angles, especially upwind.
While this situation was most noticeable on boats with larger rigs, the effect has been seen even on boats as small as Solings. And it is not trivial: the amount of variation we observed on board Bob Towse's Reichel/Pugh 66 Blue Yankee was as high as 20 degrees. Observing from the back of the boat, helmsman Steve Benjamin explained his dilemma this way: "On starboard tack, the instruments were telling me that I was sailing upwind at an apparent wind angle of 38 degrees, while on port the angle was close to nine degrees!"
The phenomenon we were all encountering is known as wind shear, which occurs when the angle of the wind relative to the boat is different at deck level than at the masthead. Now there is actually a very normal and well-understood shear effect associated with the frictional resistance of wind travelling at the water's surface. Sailmakers generally know enough about it to take that into account when they design and build sails. (Note that your sails will generally have a certain amount of twist built into them to accommodate this effect.)
The pronounced wind shear we observed in Block Island was in response to cold surface water and an overlying warm humid air mass. What happens is that the warm, humid air travels basically in accordance with the ambient air mass, while the cold air in the boundary layer immediately over the water is heavier, and thus more resistant to flow. This resistance is what causes the difference in direction and often velocity of the wind. It's not by mistake that the bigger boats at Block Island Race Week generally dominated their classes in the event; their taller sail plans were able to better utilize the stronger breeze higher off the water.
Besides tacticians and helmsmen, sail trimmers must also be very attentive to wind shear as it often means having to set up the sails with different profiles on one tack versus the other. In the example cited on board Blue Yankee, we found that we usually had to sail on starboard tack with both the mainsail and the headsail very twisted (meaning that we eased the fairleads aft for the genoa and generally eased both the genoa and mainsail sheets to open up the upper portion of the sails' leeches). On port tack, we found it was faster to take out that twist by moving the lead forward on the genoa and sheeting the mainsheet more tightly. Only in this way would the wind be at the proper angle of attack throughout the vertical profile of the sail plan, as evidenced by the telltale flow on the luff of the genoa and off the leech of the mainsail.
Is the effect of wind shear persistent? The answer is generally no, since as the air warms up at the water's surface throughout the day, it will rise and mix with the overlying air mass and the two will combine into one. Knowing this can actually be a powerful tactical tool, since it predicts that the shift at the surface will become whatever the direction is aloft.
Is wind shear confined to Block Island? Definitely not, as the effect can be common anywhere there is cold water overlaid by warm, humid air. This week's Mackinac racers, beware!
Mainsail Twist for Waves by Dobbs Davis
Headsail Trimming Basics by Rich Bowen
Understanding Apparent Wind by Steve Colgate
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