Why S-Turns, Roll Jibes and Roll Tacks Are Fast

“S” Turns are used ­downwind to increase the average ­boatspeed. The authors of a study exploring the ­dynamics defined four key moments of the technique: 1) ­minimum “­by-the-lee” heading; 2) heading dead downwind; 3) ­maximum “reaching” heading; and 4) Heading ­dead-­downwind again. These points approximately correspond to the ­minimum and maximum heel, and minimum and maximum rate of change in heel (Vroll). Illustration by John Tomac

I’m about to lose the surf, so I lean in, steering up to a momentary reach. Then, at just the right moment, I roll the boat to windward, carving down to catch the next wave. Surfing, I follow the trough, sailing well by the lee. Sadly, I can’t ride this wave forever, so once again I lean in to head up. Carving S-turns down the course is a hoot. On the upwind leg, I lean in to head up and start my tack. Then as I near head to wind, I snap my shoulders back, rolling the mast hard over me. That sets me up for an aggressive flatten, accelerating my boat up to speed on to the new tack. A satisfying roll tack is a welcome break from ghosting along in light air.

I had always figured that the reason kinetics like these are so effective is because as we roll our boat, and thus rig, through the air, we create extra wind over the sail. Although partially true, it turns out a recent Cornell doctoral thesis shows that it is way more interesting than that. When we roll, our sail induces an invisible spinning tube of air called a vortex. It is this vortex that is the mechanism of optimizing kinetics.

The fundamental feature of a vortex that makes it useful to us sailors is that it is an area of low pressure. The thesis shows that creating and managing the placement of these low-pressure vortices enhances the overall lift on a sail. Lift is the principle driving force that makes our sail work. Airflow around our sail results in a higher-pressure region to windward and a lower-pressure region to leeward behind the sail. It’s this pressure difference that produces a net force forward on the sail, and therefore the sails ­propelling our boat.

The vortex we are ­interested in creating and managing is behind the sail. The low ­pressure of the vortex further decreases the low pressure already there. This lower combined pressure makes for a greater differential between the front and back. With that comes a greater overall lift, and thus force and ultimately boatspeed.   

The study of my reference was designed by professor Charley Williamson, head of the Fluid Dynamics Research Laboratories at Cornell University. Williamson is well-known and at the top of his field. But he’s also a small-boat sailor at heart. Brought up in England in the Laser, he knows kinetics as well as anyone.

Williamson’s specialty in the field of fluid dynamics happens to be vortices. Although much of his work has to do with aircraft and other mainstream applications, he always has his eye on sailing. “I realized these vortices are likely a leading contributor to the kinetics that make a sailboat go faster,” he says, “so I designed a doctoral thesis study to investigate further. I needed a doctoral student who had the background and knowledge to technically fit the bill, but also a strong sailing background.”

Williamson connected with Riley Schutt, who studied aerospace engineering at MIT for his undergraduate degree, then went on to be part of the design team for Volvo Ocean Race and America’s Cup programs. He now works for New York YC’s American Magic and is the head of technology for US Sailing’s Olympic program. Schutt’s thesis models the ­underlying mechanism of what makes these kinetics work. “Our goal is to explain the physics of these kinetic techniques and show the fluid-dynamics ­mechanisms that result in added driving force,” he says.

I recently had them explain to me a few vortex fundamentals that are relevant to kinetics, and here’s what I learned: A vortex is a region of low-pressure because spinning air is pushing outward due to centrifugal force, lowering the pressure in the center. We create a vortex, or enhance an existing one, by moving our sail through the air when we roll our boat. The harder a roll, the stronger the vortex and the larger the area of influence, and a vortex continues to spin for quite a while.

“It will still be spinning and creating its low pressure long after we have lost touch with it,” Schutt says.

A vortex is by nature stationary; it needs external influences to move. Williamson adds: “A vortex will move with the greater air mass that surrounds it, which is, of course, always going downwind. This means it passes quickly when sailing upwind, like cars going the opposite direction on a highway. But when sailing downwind, like a car slowly overtaking another in the same direction, the vortex stays close far longer.”

“If we can control where we can place the vortices, we control where the low pressure is, enhancing this effect,” Schutt adds. “The usefulness of whatever vortex we create is based on how long we can keep it close.” What that means is each kinetic technique takes advantage of the properties of a vortex in a different way. Let’s dive into the specifics.

The S-Turn

Downwind, when we bear away by rolling to weather, we create a vortex with the luff of the sail. The bigger and faster the heel, the bigger and better the vortex. This S-turn vortex is going with the wind and so are we, so it can linger behind the main and be effective for a while. The cars are going in the same direction. To be useful, we need to position it behind the main and then keep it there as long as we can. Because we can’t actually see it, this is not so easy. When asked if can we move the vortex or are we just chasing it, Williamson says: “Both. Because we are creating the vortex, we are in effect controlling and moving it. But after formation, it will travel with the wind and do what it wants, so we are no longer in control.”  

Eventually, we lose both our wave and the vortex, and just before this happens, we roll to leeward to turn up. This roll induces a leech, a vortex that briefly moves behind the sail, then quickly gets swept away once on a reach.  

Is this vortex more efficient bearing off or heading up? “More often than not, the ­bigger vortex with a longer-lasting effect is from heeling to weather as we bear off,” Schutt says.

My personal experience with S-turns confers. I feel a strong acceleration and sustained speed when I bear off and carve down. But when I head up, the feeling is fleeting, so I try to head up hard and fast. Because I am heading away from the mark on this reach, I am anxious to roll to weather again and bear off on the next wave.

Roll Jibes

Although not part of the study, it stands to reason that a roll jibe has similarities to the bear-away of an S-turn. We initiate the jibe with a roll to windward to turn down. This action creates a vortex at the luff that we can keep behind the sail briefly, then jibe the main. With the sail now on the other jibe and the boat heeled, we are set up for a hard flatten. Our flatten creates a second and more significant vortex, and if we bear off at the right pace, like we do in an S-turn, we can keep the vortex behind the sail for a while. Two rolls and the ability to follow the vortex downwind make a good roll jibe highly effective.

Flicking Sails

“Flicking” is a series of short pulses we apply to the rig while sailing upwind. Olympic 470 sailors are particularly good at this. While hiking, we can repeatedly propel our shoulders out. While trapping, we can yank down on the trap wire. The vortex we use is created at the luff, from mast pumping. The boat sails upwind as the vortex is swept downwind behind the main, so its useful life is short. Cars pass in opposite directions quickly. At best, it’s a few seconds in light air, and just a fraction of a second in heavy.  

Since the vortices created are small, we want aggressive pulses because that will make them spin fast, and thus be lower pressure. Since each vortex is fleeting, we need to create a new one as fast as the old is carried away. Any one pulse does not do much; it’s the cumulative effect that creates an overall greater lift. All rigs are different, but I feel as if there is a natural frequency of a mast oscillation in the range of one flick per second that works best. “Sail flicking can be useful in situations where you need to maintain driving force but want to point a little higher,” Schutt says. “It’s like pinching up to the weather mark or coming off the start line in a crowded fleet.”

Roll Tack

Like the flick, we use the roll tack’s vortex created at the mast, and that vortex passes from luff to leech behind the sail quickly. The flattened part of a roll tack creates one large vortex instead of the flick’s series of small ones. Because it is large, it can still be quite effective, even though it is just one.

A roll tack is most effective in light air. Because the steady-state lift generated by sailing is so small, any enhancement is welcome. “The effect lasts longer in lighter wind because although the cars are passing in opposite directions, both are going much slower and spend more time overlapped,” Williamson says. “But it is an important time because it is a significant vortex, and we are downspeed from turning through the tack because we use this large vortex to accelerate back up to speed.”  

My own roll tacks are most effective in light air and moderately effective in medium. Once it’s windy, the boat is easy to get back up to full speed after a tack. I don’t roll at all; I “flat tack.” Rolling just heels the boat, and that dumps power from the sail while the boat slides sideways.

About Rule 42

I would be remiss if I did not discuss the rules limiting kinetics, and there is no doubt that all the techniques we are discussing are kinetic. RRS 42.1 states as part of its premise: “A boat shall compete by using only the wind and water to increase, maintain or decrease her speed.”  Reading this, one would think all kinetics are prohibited, but not so. Rule 42 instead defines and sets the circumstances and boundaries to the level of kinetics allowed. Further, there is a Call Book that umpires use for guidance. Generally speaking, here is how Rule 42 applies to each of our techniques. 

A roll tack or jibe is permitted if the boat does not exit the maneuver faster than it entered. Repeated actions are prohibited. A roll, such as heeling as you head up just before the start signal, then flattening at “go” is accepted as standard practice, as is the S-turn cadence downwind. In both cases, if the heeling is in conjunction with a change of course, it’s all good. But an umpire will flag repeated rolls or a roll with no change of course.

Flicking is generally not allowed, but some classes encourage the athleticism of kinetics as part of the game. When the race committee permits it, we see 470 crews flicking their rigs aggressively from the wire, windsurfers pumping their sail as if it were a wing, and Finn sailors rocking back and forth as physically as their heart rate will allow. 

As Williamson puts it, “A ­vortex imposes a region of low pressure, and this can be a good thing if positioned correctly.”

Schutt’s final thought: “The hope is that sailors can use this knowledge to improve their sailing and ultimately be faster around the racecourse.” The thesis Williamson designed and Schutt carried out reveals the vortex as the mechanism behind kinetics, but it’s up to the sailor to take the next step.

Now that I know there’s a ­vortex behind my sail, I try to create and manage it better. Whenever I do an S-turn, I ask myself: Did I roll to windward hard enough to create a significant vortex? And where is it? Is it l behind my sail, or have I carved down too fast and shot past it? Knowing the why helps me focus my experimentation. My goal is to visualize where that invisible vortex lurks and take full advantage of it.

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