Sail Design: Art or Science?

Two different boats, two different sail programs, and yet a surprising number of similarities: among them, the fact that even in this era of gee-whiz computer modeling, the human element still plays a major role in a sail’s ultimate success.
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Two different boats, two different sail programs, and yet a surprising number of similarities: among them, the fact that even in this era of gee-whiz computer modeling, the human element still plays a major role in a sail’s ultimate success. 

Equally important is the role of real-world sailing experience. Whether you’re preparing an entire sail wardrobe for a major offshore event or cutting a jib and main for a new production racer, practical knowledge remains as important, if not more so, than theory. A flying shape, for example, that has been optimized for a two-mile beat on flat water aboard a fully crewed one-design daysailer would likely be disastrous for a singlehander crossing an ocean—and it takes time on the water to know this. 

Open 60 Acciona

When it comes to experience designing for International Monohull Open Class Association (IMOCA) 60 sloops for races like the current Vendée Globe and the Transat Jacques-Vabre, few firms can boast the track record of Britain’s Owen Clarke Design (OCD). Not only have principals Merfyn “Merf” Owen and Allen Clarke built an impressive eight Open 60s over the years, but Owen sailed aboard the very first Open 60, Warren Luhrs’s Thursday’s Child, from Australia to the United States in the early 1990s. 

 A Computational Fluid Dynamics image shows air flow over an Open 60 jib and main. Image courtesy of North Sails

A Computational Fluid Dynamics image shows air flow over an Open 60 jib and main. Image courtesy of North Sails

OCD has been intimately involved in every aspect of Open 60 design, pioneering a number of innovations along the way, including the now-standard three-forestay rig arrangement. Owen Clarke has also performed numerous studies on Open 60 hulls and rigs using a combination of computer modeling, tank testing and sailing trials. During the run-up to the 2008-09 Vendée Globe, for example, they conducted extensive two-boat testing aboard the sisterships Ecover 3 and Aviva to evaluate rig and hull performance in the real world. 

In creating a rig and sail wardrobe for its latest Open 60, Acciona 100% EcoPowered—which is currently racing in the 2012-13 Vendée Globe with Spaniard Javier “Bubi” Sansó at the helm—OCD drew heavily from its work with Mike Golding’s Gamesa, formerly Ecover 3, which is also in the 2012-13 Vendée. According to Owen, this decision was largely based on the latest iteration of the rules governing IMOCA 60 design, which set limits on mast height and righting moment that are nearly identical to those on Gamesa, effectively stopping “a lot of the development that we would have otherwise done.” 

However, the fact that Gamesa and Acciona share a similar hull form and rig geometry does not mean they carry identical sails—just the opposite. When it comes to offshore singlehanded sailing—a setting in which, obviously, a single sailor is doing everything from choosing how to navigate weather systems to changing headsails—the sail wardrobe has to be carefully matched to the sailing style of each skipper. 

“It starts with a description of the sailor, about how he likes to sail the boat,” Owen says. “For instance, looking at the downwind sails, you’ve got sailors who prefer to keep spinnakers up longer and to run deep, and they’ll just deal with the consequences of having to make a change at higher windspeeds. So their sailplan is quite different, with massive kites...Then you’ve got other sailors whose belief system tells them, ‘We’re shorthanded, we don’t really want to be dealing with this.’”

 Skipper Javier Sansó

Skipper Javier Sansó

In the case of Acciona, that meant bigger sails to accommodate Sansó’s more aggressive approach, or “sails at maximum measures” as Sansó puts it, with an eye toward carrying a full main and the boat’s biggest, 4,600 ft2 kite in winds of 22-24 knots. “[At that point] depending on the sea state the autopilots really start to struggle. From that we could go to our gennaker or fractional kite,” Sansó says. 

Further complicating the equation is the race itself. In the current Vendée Globe, for example, skippers are being required to make do with fewer sails: nine in all, compared to 10 in 2008 and an unlimited number in years past. Sail requirements also vary dramatically depending on the route, with a trip around the world making different demands than a race confined to the Northern Hermisphere. 

“Average windspeed on the Vendée Globe is 13 knots from about 110 true wind angle,” Owen says. “But if you’re doing a singlehanded transatlantic race upwind, average wind direction is pretty much on the nose all the way and the average windspeed probably 17 knots…so you have to optimize the boat for two different races.

“It’s about efficiency and not necessarily outright pace,” he adds. “Inevitably…there’s going to be some gaps in your wardrobe, so it is about weather modeling and understanding your boat well to limit those gaps to improve overall performance and efficiency.”

That said, Sansó emphasizes that you still want your sails to be as fast as possible. “Sometimes you are very fast at 15 knots, but you could go half a knot or one knot faster with a better sail shape,” he says. “Half a knot slower is 12 miles in one day, so it is extremely important to have the right shape.”

 Designer Merf Owen checks out a headsail

Designer Merf Owen checks out a headsail

Not surprisingly, given the many factors involved, finding the sweet spot for a particular sailor aboard a particular boat in a particular race takes time, both in front of a computer and on the water. Designers like Owen spend countless hours not just tweaking computer models but running and re-running past races based on weather data. Sansó says it was only after a full year of data logging over the course of some 15,000 sea miles—testing the boat under different sail combinations on various angles of sail—that he was able to reach an “optimum sail design and wardrobe.”

Finally, there is the working style of the particular sailmaker being used for a particular job, and the fact that different lofts function differently, according to their own unique culture. 

For French sailmaker Incidences, which built the sails for Acciona, the approach tends to be more intuitive, heavily from the thousands of offshore miles logged by its staff. According to Owen, “They design the sails with computers, but there’s not the same kind of [state of the art] programs that others use to develop polars, etc. It’s actually more seat of the pants, which is not to say that it’s wrong. It’s just a different approach, and it’s probably an approach that many sailmakers will recognize.”

No matter the loft, Owen says a perennial challenge is getting sailmakers to find the right balance between top-end performance and shapes that will work well over the long haul. 

According to Owen, “Just about every sailmaker we’ve ever worked with has come into shorthanded sailing not knowing how these boats are sailed. They always make the same mistake, even though you tell them not to. What you keep saying to them is that this is a shorthanded boat, and the guy sailing the boat is pretty tired or using the autopilot, so it’s quite hard to keep the boat in a groove. What you need is a really forgiving headsail shape, not too flat an entry [which will stall too easily]...And of course they’re not interested in that. They do [inshore] windward-leewards all the time where you try to make the boat really efficient upwind and point and everything. We’re not interested in the boat pointing, because we’re in the ocean, so we’re always footing. And you’re footing with an autopilot, or with generally a tired sailor.”

As for headsails, Owen notes that someone sailing alone across an ocean or around the world can’t continually trim a spinnaker sheet like a larger crew sailing around the buoys or for only a few days. Nor does a solo sailor have the option of constantly changing headsails in response to every minor variation in wind speed or direction—it would simply be too exhausting. Instead, a solo sailor needs sails that work well over as wide a wind range as possible, even when not in an optimal state of trim.

This is an area in which it appears that Incidences and the many short-handed racing veterans on its staff excel—more than justifying their “qualitative” approach. Sansó certainly seems happy. “Our sail designer Christophe Cudennec and the team from Incidences have done an incredible job,” he says. “Christophe knows Open 60s well, and his sail designs have won the last three Vendée Globes, and have occupied most of the podium places. So his input has been very important.” 

Chalk one up for experience!

The J/70 project

 J/70 sails are designed to be forgiving as well as fast. Photo courtesy of J/Boats

J/70 sails are designed to be forgiving as well as fast. Photo courtesy of J/Boats

In contrast to the Acciona project, when North Sails started working up a suit of sails for the new J/70, it began not only with a clean slate, but with a rig that had not yet been built and was still lacking in specifics. 

One of two sailmakers brought in by J/Boats to produce sail wardrobes for hulls Nos. 1 and 2 (Quantum was the other), North enjoyed an inside edge because its Southern Spars subsidiary had been hired to design the mast. In contrast to the Acciona project, designing sails for the new 22-foot one-design sloop also required the construction of only a jib, main and asymmetrical spinnaker, as opposed to the various staysails, jibs and spinnakers required for an ocean race. 

Nonetheless, the similarities to the Acciona project far outweigh the differences. Again, technology played an important role—all the more so, because there was no physical boat to work with for much of the early design process. And again, experience and real-world know-how are what ultimately drove the program. Equally important was sailing style, or philosophy, as was the case with an Open 60—albeit the philosophy of a veteran boatbuilder trying to appeal to what it perceived to be the demands of its customers in the marketplace. 

The North Sails program began in late 2011 with the building up of a virtual model of the new boat, based on data supplied by J/Boats. At this point, Southern Spars had not yet created a mast (among other things J/Boats was still trying to decide whether the boat was going to be equipped with a backstay or not) so North’s sail designer Doug Slocum says they input the parameters for a number of other sportboat spars to establish a point of reference. 

By January 2012, based in part on performance data generated by the North modeling, Southern Spars was closing in on a final spar configuration, and North began polishing its sail design—running tests on the various virtual sails it “built” to see how they performed across the wind spectrum. 

As was the case with the designers working on the Acciona project and Incidence, North didn’t just rely on a computer program to do the thinking, but brought in members of its one-design team as well, including West Coast sailor Chris Snow—sailors who regularly interact with customers to ensure their sails work as advertised out on the race course. Together they would go over Slocum’s computer models, tweaking everything from draft depth to the way the sails react to different amounts of mast bend and forestay sag—which can be closely predicted once you have the exact parameters of the spars. 

“The details of the flying shape are dictated from experience, and not just the experience of any one designer so much as the global experience of North Sails, because we have a lot of good designs we can start from,” says Slocum. “In a case like this it wasn’t just one person working on the new flying shapes. For this boat we had a team saying things like the flying shape is alright there, but maybe the top quarter of the sail is too flat or too round.”

Again, the goal was not just out-right speed, but leveraging the cumulative time on the water of the entire North team in an effort to match the expectation of both J/Boats and J/Boats’ customers.

“The sails and the boat all have to be designed and tailored to the market they’re trying to hit. In this case, J/Boats has said multiple times that they want this to be a boat that all three generations could get on and sail comfortably—grandfather, father and son,” Slocum says. “They don’t say we’re looking for a fine entry or a forgiving entry. They’re saying they want the boat to be lively but manageable, so it needs to be able to depower well, but still be able to move well in light stuff.”

These conversations continued during the J/70’s sea trials in March 2012, with the North Sails team, Quantum and a number of other sailmakers all weighing in on how the boat was performing out on the water (see sidebar). The end result was a slight adjustment to the distribution of the roach curve, to keep it from powering up too aggressively in the gusts. Otherwise, the sails performed pretty much the way the computers said they would—in contrast to the bad-old-days before computers, when you could never be sure what you had until it was flying.

 The details in this J/70 Finite Element Analysis diagram can help with rig as well as sail design. Image courtesy of North Sails

The details in this J/70 Finite Element Analysis diagram can help with rig as well as sail design. Image courtesy of North Sails

After that, it was a matter of playing with the sails out on the water and seeing how they performed not only in the hands of the pros, but in the hands of ordinary J/Boats sailors. Feedback from these experiences has since been used to further tweak North’s computer models, a process that is ongoing.

“[Computer modeling] allows you to get very close on the first set of sails you make,” Snow says. “Then you’re going to go out and sail the boat, you’re going to work with the tuning of the rig, you’re going to race the boat, and then you’re going to fine tune it...You may get a gut feeling that ‘I think the sail is a little too flat,’ so you design a fuller sail on the computer, run the flow and membrane programs and see how the sail compares to your previous iteration, just to make sure what you think you’re going to see is what you actually see [on the water]. Then you go ahead and make the sail and give it a try. It basically takes a lot of the trial and error out of it.”

Currently, the sails that Slocum, Snow and the rest of the North team put together are being put to the test in the crucible that is competitive winter regattas like Key West and Charleston Race Weeks. According to Snow, he and other members of the North Sails one-design division will be keeping close tabs on how their customers do in the races, and listening closely both to how they are making them perform and what, if anything, should be changed.

“It’s really good to listen to the customer that is in the middle to upper part of the pack,” Snow says. “If they give you good feedback on the sail, that they felt they could make it go fast, that it was relatively easy for them, and you can see that they’re getting good results with the sail, that is a real winner.”

All Aboard the J/70 Express

 J/Boats president Jeff Johnstone (in yellow) prepares one of two J/70 prototypes for sail trials

J/Boats president Jeff Johnstone (in yellow) prepares one of two J/70 prototypes for sail trials

While North and Quantum were the two sailmakers whose products were onboard for sail trials in March 2012, they are hardly the only ones designing and building sails for the new J/70—which should come as a surprise to no one given the fact that the boat appears destined to become a major force in one-design racing for years to come.

They are also not the only ones J/Boats listened to when it was putting the finishing touches on both the prototype and the future of the class.

“We have many sailmaking friends who’ve spent a lot of time in our boats and classes, so when it comes to introducing a new design like the J/70, we like to get a small group together that functions as a sounding board for class rules, particularly the sail portion of the rules,” says J/Boats president Jeff Johnstone. “For the J/70, we’ve had as many as five different lofts providing input on the class rules.”

Johnstone notes that, “After only two to three weeks of sailing—which also included time with sailmakers like Doyle, Ullman and UK—we were able to lock in the class sail sizes and cloth weights. In retrospect it was probably the most efficient way to finalize the rule writing and at the same time provide new J/70 owners with a refined sail package right out of the box.”

Acciona photos courtesy of Jesús Renedo/Acciona



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