The Effects of Heel, Jib Overlap and Fractionality on Upwind Sails
Jim Teeters, President, SYRF
In 2005, the Sailing Yacht Research Foundation (SYRF) executed an experimental program with the Wolfson Unit at the University of Southampton wind tunnel to explore the aerodynamic forces on upwind sails with particular focus on the effect of heel angle. North Sails supported this program.
Various Velocity Prediction Programs (VPPs) used for handicapping races and designing boats were incorrectly predicting the effects of stability on performance. It was observed in IMS handicapping, that optimized designs reduced stability and traded the resulting rating credit for larger boats with little net change in rating. The larger boats with less stability proved to be rule-beaters. In America’s Cup programs under the IACC (later ACC) rule, it was also observed that initial designs were quite wide in the pursuit of form stability. Ultimately, the successful designs coalesced on quite narrow hull forms based on observed success of narrow boats and improved design tools.
The apparent reason for these trends, was an incorrect set of VPP algorithms that predict the aerodynamics of upwind sails. Specifically, the algorithms over-predicted the loss in sail driving force with heel angle. IMS designers took advantage of this. In America’s Cup racing, the rule never measured and rated stability so there was no incentive to trade low stability for speed enhancing characteristics such as length, draft or sail area. The designs pursued form stability in the belief that beamier boats would be faster.
Wind Tunnel tests were performed on a 1:6 scale model of a 30ft yacht which was previously used for a 2001 study performed by US Sailing. Testing was performed in the low speed section of the No.1 Wind Tunnel at the University of Southampton. A short footed main and small code 1 jib were tested at varying heel angle from 0° to 30°, and apparent wind angles from 20° to 55°. Tests were performed at a corresponding wind speed of 13.6 knots and sails were trimmed for maximum driving force for each combination of apparent wind angle and heel angle. Other sheet settings were tested to simulate sail sets that were appropriate to a de-powered mode, i.e. maximum drive at a specified limit of heeling moment or force.
The results from the testing by the Wolfson Unit quickly illustrated that the loss in driving force with heel was much smaller than that predicted by the existing IMS VPP. Flow visualization in the tunnel demonstrated that the wind was deflected upwards by the heeled deck edge of the model boat. This effectively blocked some of the flow from the high pressure side of the main under the boom to the low pressure side. The result would be greater forces on the main and therefore greater driving force.
A follow-up study was conducted by Professor Horst Richter of Dartmouth College using the RANS code FLUENT. This study also provided visualization of the effect of the heeled deck edge creating a vortex of wind that partially blocks the flow under the boom.
The results of both investigations have helped explain the difference in what has been predicted by VPPs of that era and what has been observed in the field of real racing sailboats. These results have also guided further development of the Offshore Racing Rule (ORR) VPP.
"The Story So Far" - SEAHORSE Article July 2007: SeaHorse JLY07.pdf1.55 MB
Research Report 2005 - Wolfson Unit #1838: 1838 MP .pdf10.31 MB
Research Report 2001 - Wolfson Unit #1630: 1630 MP.pdf12.36 MB
Wind Tunnel Photos: Floor 1.zip10 MB
Wind Tunnel Photos: Floor 2.zip8.5 MB
Wind Tunnel Photos: Overhead.zip8.8 MB
Results Spreadsheet: P1913_SYRF_Reesults_2_JRT.xls403 KB
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Author: Jim Teeters
Title: The Effects of Heel, Jib Overlap and Fractionality on Upwind Sails
Publisher: Sailing Yacht Research Foundation
Electronic Retrieval Location: http://www.sailyachtresearch.org/19-library/42-the-effects-of-heel-jib-overlap-and-fractionality-on-upwinds-sails
The Effects of Heel, Jib Overlap and Fractionality on Upwind Sails by Jim Teeters is licensed under a Creative Commons Attribution 4.0 International License.