In May of 2000, and again this September, the Glenn L Martin Wind Tunnel at the University of Maryland tested a series of "IMS style" spinnakers for US Sailing. The tests were conducted by Dr. Robert Ranzenbach and Jim Teeters in collaboration with Per Andersson of Quantum Design Group who provided the sails as well as trimming expertise during the tests.

Centerline asymmetric spinnakers (sails whose tack is restricted to the boat’s centerline) were first permitted in IMS in 1995. The general observation since then had been that these spinnakers do not perform as well as the IMS VPP predicts. The purpose of this test program was to quantify the downwind performance of centerline asymmetric spinnakers relative to both symmetrics and asymmetrics on a moveable pole. Coincidentally, there was emerging contrary evidence that centerline spinnakers actually had a rating advantage.

Three centerline asymmetric running spinnakers were built with sprit lengths of 0, 25% of J and 50% of J. All had 180% girths. These were tested over a range of apparent wind angles that emphasized downwind conditions. All forces and moments were measured and corrections applied for the blockage effects of the tunnel walls on the sails.

This photo shows the test rig viewed from behind with an apparent wind angle of 135 degrees. The wind is perpendicular to the heavy black line on the floor. All controls for the guy, sheets, boom vang, etc. are driven by servos and winches controlled remotely from a station outside the tunnel.

Settings are "dialed" in with visual observation as well as real time monitoring of the forces and moments. In the case of running spinnakers, the sails are trimmed to maximize the driving force, parallel to the boat’s centerline.

Of particular interest in this photo is how much of the luff projects out to windward where it intercepts the wind. This is likely one of the reasons that contemporary centerline asymmetrics outperform earlier designs.

The following graph shows the driving force vs. wind angle for each of the three spinnakers. The solid lines are wind tunnel data. The dashed lines are predictions from the IMS Rule. There are vertical lines that show the apparent wind angles for optimum VMG downwind sailing for each wind speed. The tunnel data has been expanded to full scale for a boat approximately 48 feet long.

click for larger view

The test data shows greater forces than the IMS, in part due to the absence of vertical wind gradient in the tunnel. The IMS Rule incorporates the effect that "real" wind has much less velocity close to the water than at the nominal 10 meter height.

In any case, it is the differences between one sail and another that we expect to derive from this kind of experiment. Beyond 90 degrees apparent, the tunnel data show that the asymmetric on a conventional pole (diamonds) has somewhat more drive than that of a symmetric on a pole (squares), which again has somewhat more drive than the centerline asymmetric (triangles). The differences are relatively small.

The IMS Rule predicts identical driving force at large angles for both the symmetric and asymmetric on the conventional pole and the curves are indistinguishable above 100 apparent. This is similar to the trend in the test data. However, the predictions from IMS 2000 for the centerline asymmetric (bottom dashed curve with triangles) are far less than those for the other spinnakers, a trend not supported by the tests, and an error confirmed by actual race results this past season. This information was used as a guide to the ITC in formulating a new curve for centerline asymmetrics, shown here as the intermediate curve, labeled "New Coefficients" .

Special thanks to Steve Munger and Paul Ansfield (Lake Michigan PHRF) for their financial support of this program