!!!...Virtual wind tunneL...!!!

Models air flow at 100% scale

Virtual Wind Tunnel (VWT), was originally developed jointly by Dartmouth College Thayer School of Engineering and North Sails and is now run by the Stevens Institute of Technology. VWT is the first (and still the only) computer simulator to accurately model wind flow on downwind sails at 100% scale. When first developed, North Sails used the world’s two leading low-speed wind tunnels (The University of Auckland's Twisted Flow Wind Tunnel and Oracle’s IACC Twisted Flow Wind Tunnel) to verify it’s accuracy. The results have given us great confidence that North’s “VWT” predicts results with more accuracy than wind tunnel sail testing of any kind. VWT provides powerful insight into flying sail shape, sail drive, shape stability and ease of trim.

Unique to VWT is its ability to integrate with Membrain allowing both upwind and downwind sail shapes to be analyzed with unprecedented accuracy.

Virtual Wind Tunnel run of a Ker 39 medium jib and main showing detailed analysis of the hull and sailplan interaction

Analysis refining the upper shape of a J2 jib studying air flow around leech. Note that headstay and mast are included in the model.

Pressure coefficients used to refine sail shape on A1 asymmetric spinnaker. Note smooth elliptical pressure profile indicating an efficient distribution of loads.

!!!...Wind tunnel testinG...!!!

North Sails has pioneered modern wind tunnel research for sails using two related-but-separate methods. The first is a traditional low-speed tunnel with a unique twist. The other is virtually amazing.

Twisted Flow Wind Tunnel 

The University of Auckland's Twisted Flow Wind Tunnel (TFWT), developed in conjunction with North Sails, was created as a research tool to specifically simulate wind flow over yacht sails. It is the only commercial wind tunnel specifically designed for testing yacht sails. Using unique twisting vanes, the wind tunnel accurately models the gradient wind structure seen by a yacht moving through the water. The Twisted Flow Wind Tunnel has been used extensively by America’s Cup boats, Volvo Ocean Racers, Open 60s teams and other successful racing and cruising projects for over 10 years.

The scale of  TFWT allows relatively large scale model testing without blockage problems. Sail performance tests are typically carried out on a 1:14 scale model fitted with remote-controlled winches. Turbulence, velocity profile and twist are all re-created in the wind tunnel to ensure an accurate simulation and an exclusive real-time VPP which takes a lot of the guesswork out of trimming sails in the tunnel. Forces from the dynamometer are converted immediately to boat speed and a heel angle for a particular true wind speed, and the model is then heeled to the correct angle. This is extremely important for the stability-based VOR 70 Rule, as the heel angle has a significant effect on the performance of reaching sails which cannot be predicted by linear theory.

Virtual Wind Tunnel

The Virtual Wind Tunnel is the first (and still the only) computer simulator to accurately model wind flow on downwind sails at 100% scale. When first developed, North Sails used both the University of Auckland's Twisted Flow Wind Tunnel and BMW/Oracle’s ACC) to verify its accuracy. These comparisons and real-world performance have given us confidence that the “VWT” predicts results with more accuracy than typically encountered with wind tunnel sail testing of any kind. In addition to sail forces, the VWT provides insightful information to our designers relative to flying sail shape, sail drive, shape stability and ease of trim. 


The images below display a sail development process utilizing the VWT to customize a sail shape. The original design on the left was altered based on the predicted flying shape from the pressure’s provided by the VWT and then analyzed with Membrain. The flying shapes and flow images are what allows the designer to know that he is producing a faster, more efficient shape. In this progression from left to right, an increase in efficiency of 2.3% more drive force was realized through this process. Maximizing the low pressure on the leeward side of the sail (more red & orange colors) is what we are after. You can see  clearly, the increase represented below. 

!!!...Hull datA...!!!

Uses hull CFD to define hull drag and appendage lift for North VPP


Hull Data one of the most powerful free surface Hydro CFD programs in the world. Hull Data is a Potential Flow, Free Surface Code used for sail and rig develeopent for America’s Cup teams, TP52s and Volvo Ocean Racers. The results from Hull Data are used to generate hydro resistance files for the North Sails VPP.

Shown ABOVE and BELOW is a hull being run through a series of heel angles to produce a matrix of hull drag to be used in the VPP. In this example, Hull Data is testing a hull at a speed of 5.5 meters/sec., a heel angle of 25 degrees and a yaw angle of 2 degrees. 

!!!...DesmA...!!!

Rig and sail modeling


Desman creates a complete rig/sail model in a three-dimensional environment. In Desman, North designers can specify mast size, rigging position, rigging attachment points at the deck and trimming locations. The modeled sail/rig system incorporates the mechanical properties of the spars, standing rigging, running rigging and sails in terms of moments of inertia, sail and spar surface area, materials stiffness and resistance to stretch. Later in the process, Membrain (described later) uses the Desman model to determine deformation under load for the sail and every piece of standing and running rigging, right down to stretch in the sheets and halyards.

Modeling genoa staysail and A2 spinnaker

Richel / Pugh 82 Highland Fling

Right: Volvo 70 reaching simulation.

Left : Wild Oats model for rig tune. 

How is 3DL Made.....?

sails begins with a three dimensional CAD/CAE design file created by a North Sails sail designer. North's proprietary design software creates a custom "mold" file for each individual sail. Because a 3DL sail stretches less than competing 2D paneled or “string” sail for a given amount of yarn, North designers can more accurately define the desired "flying shape" because the computer molded shape is that much closer to the resultant flying shape.

sophisticated computer program reads the design file, then instructs an articulating mold to assume the designed shape. Shown here is the underside of a 3DL mold with actuators controlled by a highly sophisticated computer program.

After a base layer of Mylar film (made from Mylar sections joined together with modest shaping to lie reasonably smoothly over the 3D surface of the mold) is draped over the mold and tensioned, a 6-axis fiber head suspended from a computer controlled overhead gantry then applies structural yarn onto the surface of the base film, precisely following the 3D curve of the mold surface. The fiber head "draws" a pattern in yarn that matches anticipated loads in the sail. All structural yarns are applied under uniform tension and adhere to the surface of the film to ensure they remain in place prior to being locked by the lamination.

Once the yarns are laid, a second film is positioned on top of the base film and yarn, tensioned, and then covered with a large vacuum bag that compresses the laminate at approximately 1,800 pounds per square foot. This second film contains a secondary mapping of yarns to handle incedental loads off the primary load lines.

The gantry head is then removed and replaced with a carbon element heat “blanket” that cures the pressurized laminate by imparting a carefully controlled amount of heat through the laminate. This causes the laminate to conform tightly to the mold in a manner similar to a shrink-wrapping process. After curing, the sail is allowed to cure further for a full five days prior to shipping and/or finishing.

When the laminate has cured, corner reinforcements, bolt ropes, batten pockets and protective patches are applied by experienced sailmakers. Because of the inherent material efficiency of the 3DL manufacturing process, a finished 3DL sail can be up to 20% lighter than a conventional paneled or a “string” sail of equivalent stretch. Or, it has a wider wind range (larger sweet spot) for a given weight.

..How the modules work togetheR..

Flowchart of software suite tool interaction

..Desing ResourceS..

No other sailmaker can match the experience and resources of the North design team...

• 65 sail designers representing over 350 years of combined experience

• 5 full-time software engineers dedicated purely to the development of our sail design software

• 28 years of leadership in computerized sail design.

A legacy of leadership...

• North was the first sailmaker to perform computerized structural analysis in sails.

• North was the first sailmaker to analyze upwind sail shapes with computer air flow simulations

• North was the first sailmaker to implement 3-dimensional computer modeling.

• North was the first sailmaker to develop and use an integrated suite of sail design software

• North was the first sailmaker to develop and use computer aided laser sail panel cutting, dramatically improving accuracy and consistency in sail production.

• North was the first sailmaker to test downwind sails using a “twisted flow” wind tunnel, which simulates differences in apparent wind speed and apparent wind angle between deck level and the upper part of the rig.

• North was the first sailmaker to develop an accurate wind flow modeling software for analysis of downwind sails. Virtual Wind Tunnel accurately analyzes stress, strain and wind flow at 100% scale.