Designers can tweak boat till last moment

By ADAM GIFFORD

The hull for what could be Team New Zealand's next America's Cup defender is almost complete, but design work for the keel, sails and other features will continue for another six months, thanks to computer-based design systems.

Team designer Nick Holroyd said the hull will emerge from Cookson Boats on the North Shore this month and be taken to the team base in the Viaduct Harbour for deck hardware and appendages to be fitted.

"We had the last tank session in March and the hull design was fixed in early April," Holroyd said.

The design team then shifted its attention to tweaking keels and sails.

"We can change keel configurations up to the last declaration date, which is a week before the cup starts," he said.

America's Cup-class yachts weigh 25 tonnes, 80 per cent of which is in the keel.

While hull design relies heavily on testing designs in tow tanks - about 20 quarter- scale models were built here during the past couple of years and airfreighted to England for testing - most of the keel design can be done on computers.

Holroyd said that was because the tank tests could only measure the bulk forces acting on the model. They could not tell the designers the specific influence a rudder, keel or other feature had on those forces.

"Where computational methods come into their own is not so much in producing the final measurement of drag, but in your ability to go into the data set you create from the tank test and look at what is happening inside the fluid," he said.

Team New Zealand now has three people in its computation fluid dynamics department. They use SeaFX software from British firm AEA, a spinoff of the Atomic Energy Commission, to test possible shapes.

Team New Zealand's design computers are supplied by SGI. One is an 8-processor SGI Origin 2000, with 6 gigabytes of memory which, Holroyd said, allows the designers to break the hull or keel surface into two million cells.

"In each of those cells we store the fluid property, the pressure, the turbulence parameters, maybe eight variables per cell," he said. "We then solve a set of simultaneous equations describing the fluid flow.

"Even with the large SGI box it can take 24 hours to solve the equations."

At the end of the computational run the designers have a data file which is loaded into viewing software.

"That allows us to get into the fluid flows and visualise what is in there."

What the designers look for in particular are the vortexes created at places like the back of the keel and the top of the mast, which contribute to drag.

The extra computing power means the design team can test more design variations faster and cheaper. Holroyd said that by the time racing starts, the design team would have analysed more than 100 different designs in three dimensions.

"We aim to design a new appendage set which will make the boat half a minute or more faster round the course than those used last time - that is the absolute minimum." he said.

Holroyd spent 18 months in southern England during the tank-testing phase and the close relationship he developed with AEA helps when his team is pushing the software to its limits.

Team New Zealand work, particularly that of PhD student Stephen Collie on spinnaker design, may eventually find its way into the SeaFX product.

"A spinnaker is like a barn door up in the wind flow. There is a big eddy structure which is unsteady, so predicting the behaviour of downwind sails is very difficult from a computational viewpoint."

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