Twisted Flow Wind Tunnel

New Zealand study advances understanding of downwind sails

In 1993 North Sails undertook a testing program in the Wind Tunnel at | the Wolfson Unit within the University of Southampton in England. The focus of the study was the (then) new Whitbread 60 class where asymmetric spinnakers were to be used seriously offshore for the first time. Mike Schreiber IPRG and Tom Schnackenberg (NSNZ) tested a series of shapes, but they found that the model sails did not twist like sails do on real boats. The reason was that the flow in the tunnel at Southampton was not twisted, whereas the apparent wind real boats see is twisted.

Because the water's surface has "grip," and because there is more wind at the top of the mast than at deck level, both Apparent Wind Speed (AWS)and Apparent Wind Angle (AWA) change as you move up the mast. When the Team New Zealand campaign for the 1995 America's Cup began, Schnackenberg, the design coordinator. was aware that there was still a lot of untapped potential boatspeed in downwind sails. By doing most of the design development in the wind tunnel, quickly, at a fraction of the cost of full scale testing. However, the air flow in the tunnel had to simulate the changes in AWA and AWS with height.

Team New Zealand sought the help of the Mechanical Engineering department at the University of Auckland to design the tunnel. After testing several 1/10 scale tunnel designs, it was decided to use a series of vanes just upstream of the tunnel. Burns Fallow upwind sail designer for NSNZ and Team New Zealand was given the task of getting the tunnel built. Team New Zealand funded most of the tunnel, but NSNZ and University of Auckland made significant contributions in both time and money.

The tunnel is a creative blend of highly technical components and "Kiwi ingenuity." The balance scale that measures the loads on the model sail was designed by the Wolfson Unit, and their engineers flew to New Zealand to set up and calibrate the instrument. An elaborate Speed Controller monitors electrical current to the fans to ensure that the air is pushed through the tunnel at a constant rate. The "open jet" tunnel is 30 meters long and can blow up to 22 knots of breeze through a 6meter­by­3­meter test section.

Once the construction and setup phase was over, it was then up to NSNZ's downwind sail designer Michael Ickert to make the most of his new design tool. Ickert tested more than 300 gennaker and spinnaker designs for NZL­32 and NZL­33 in the tunnel. All model sails were designed and cut using North Sails design software and M90 Laser Plotter, the same used on our full scale sails. However, they had to wait until both boats were sailing in San Diego to calibrate the performance of the sails in full scale. They quickly discovered that the sails Michael said would be fast in the tunnel were fast in real life. It is noteworthy that this process worked in reverse as well. In San Diego they recut an existing Gennaker and it proved to be quite a bit faster than it was before. NSNZ got Michael to perform the recut on the model sail and the performance in the tunnel was enhanced by the predicted amount.

In the early rounds it was obvious that the black boats were extremely fast upwind, meaning that speed potential had to be sacrificed downwind. This placed added weight on a successful downwind program. Without question, the twisted flow wind tunnel was a significant contributor to the speed edge that NZL­32 enjoyed. This is just the beginning, however The tunnel is currently being used to analyze gennakers from other Amenca's Cup syndicates. Tom is also Olympic Coach for the New Zealand team in Atlanta, so the tunnel is being used for VPP work and sail development for Olympic classes.

America's Cup, apparent wind angles were never deeper than 120 degrees because of the light San Diego conditions. The upcoming Grand Mistral and Whitbread races will provide a platform to study this unexplored region. -Burns Fallow