SAN FRANCISCO (July 29, 1997) America's Cup detractors take great delight in
claiming that watching a match race is about as interesting as watching paint dry.
But the joke's on them. What they don't know is that an integral part of the technology program required to produce an America's Cup winner is about watching paint dry. And it's darned fascinating stuff! Just take a look at the accompanying photos. (Click on the pictures to see larger images.)
AmericaOne kicked off its extensive model-testing program the week of July 21 with a series of wind tunnel tests of several keel designs. The tests were conducted in the Kirsten Wind Tunnel at University of Washington Aeronautical Laboratory in Seattle, Wash. Towing tank testing starts in August at the Naval Surface Warfare Center/Carderock in Maryland.
"The wind tunnel and towing tank tests will establish a current leading-edge baseline for the AmericaOne Design Team," said Bruce Nelson, principal designer for AmericaOne. "These tests are critical to helping us gather and access competitive data, and will serve as a standard for all future testing throughout the program to ensure that design decisions are based on accurate and consistent data."
At the University of Washington facility, several keel configurations were examined under the direction of aerodynamics engineer Ken Visser, of The Boeing Co., and naval architect Claudio Fassardi, of Scientific Marine Services Inc. (SMS). These same appendages will later be fitted to two 25-foot-long hull models for the towing tank tests under the supervision of marine engineer Frank DeBord, president of SMS. The combined results of these two sets of tests will provide the design team with a very complete understanding of the forces acting on the hull and appendages of modern IACC designs.
Wind Tunnel Tests
The wind tunnel testing provides three forms of data:
- Forces and moments
- Surface pressures
- Flow visualization
The force measurements include the lift and drag components of the appendages using a wind speed of 128 mph, which simulates 9 knots of boat speed in water. The surface pressure testing utilizes pressure-sensing devices to determine pressure distributions on the surfaces of the appendages. The flow visualization testing involves using paint and watching it dry or smoke, which allows the designers to actually see, or visualize, the flow patterns as the air passes over the surfaces of the appendages.
The process requires multiple tests to accommodate the many possible combinations of heel, yaw and flap (trim tab) deflection. The yaw and flap angles are adjusted in small increments between the tests, then the procedure is repeated over and over again. These tests allow the design team to identify optimum angles and flap deflection, thus reducing the number of tests required in the towing tank, Visser explained.
Adds Fassardi: "In the wind tunnel we can determine how each component contributes to lift and drag. The force measurements in the towing tank provide the total integrated results for the combined hull/appendage system, but not the individual components. By combining the two forms of testing, we can break down and understand the performance of each component."
A wind tunnel is used at the outset rather than a towing tank because it's much more efficient. Many more tests can be conducted per day in the wind tunnel, as opposed to a towing tank, where the water must settle before the next test can be conducted.
Towing Tank Tests
In August, Frank DeBord of Scientific Marine Services will supervise a series of tests in the towing tank at the Naval Surface Warfare Center to determine the total hydrodynamic resistance of several hulls, with the appendages from the wind tunnel, as they are driven down the tank attached to the towing carriage.
The forces and moments between the model and carriage are measured with load cells to determine the total lift, drag and heeling moments. In addition, the characteristics of the waves created as the 25-foot models are towed the length of the tank are analyzed to determine the component of resistance associated with the wave-making characteristics of each hull.
This provides the design team with further information and insight regarding the performance of each design in the presence of the air-water, free-surface interface. This is an important element of sailing performance that cannot be modelled in a wind tunnel.
As in the wind tunnel, many combinations of speed, heel, yaw, flap and rudder angles are tested, resulting in well over one hundred tests in the test matrix for each model. And once the data are collected for each model, they then need to be extrapolated to full-scale hydrodynamic performance through a series of corrections for scale effects due to viscosity.
The more accurately the forces associated with each component are known, the more accurate the extrapolation to full-scale performance, thus the wind tunnel tests serve to improve the quality of the tank test results. The resultant full-scale hydrodynamic data are then used within a Velocity Prediction Program (VPP), where aerodynamic forces are applied to predict the full-scale sailing performance at various wind speeds and angles.