By JB Braun, North Sails Design Services, 2016

Today, the world's leading sailmakers utilize proprietary Fluid-Structure-Interaction (FSI) modeling software to overcome the inaccuracy of VPPs in predicting downwind sail performance. However, as such software is valuable proprietary property for sailmakers, the benefits of this technology have only been realized by the design community and not the handicapping community. Modern handicapping rules rely upon velocity prediction programs (VPP) to predict a boat's performance both in and above the water. Currently, aero VPPs depend heavily upon lift and drag coefficients determined from experimental wind tunnel results that are plagued by difficulties in accounting for the scaling effect, flying shape, and wind profile. As a result, aero VPPs are limited to accurately predict downwind aerodynamic performance.


It is the goal of the Downwind Aero Moments and Forces project to improve the accuracy of modern handicapping rules in understanding downwind aerodynamic performance. By collaborating with those sailmakers using FSI modeling, this project will produce a set of results comprised of all aerodynamic moments and forces for a range of downwind sails, angles, and wind speeds utilizing RANS code coupled with finite element analysis results. These results will be made publicly available and will serve as a valuable validation source for the improvement of VPP.



Using fluid-structure interaction (FSI) modeling, the project simulated three yachts that represent a range of performance for typical modern racing yachts. A modified TP52, Swan42, and McCurdy and Rhodes 48 were simulated with A2 and A3 sail plans and configurations, with an S2 configuration also being simulated for the latter. Sail designs were created using North Sails’ “Global DesMan Design Base” and were then used by North’s design suite to create the 3D finite element analysis (FEA) models. Using North’s FEA tool “MemBrain,” these models were coupled with OpenFOAM to get a converged solution, with sail trim adjusted between each iteration to achieve neutral luff pressure. Simulations were meshed to identify the differences in sail forces and moments through a range of sail designs and boat performance. Sailing conditions were taken from the current performance targets provided for each boat, with leeway set to zero in all cases, and simulations onset flow defined at 10 meter elevation at true wind speeds of 8, 12, and 16 knots. Additionally, a heel sweep and traveler sweep were done in TWS 12 without changes to the sail trim, other than rotating the main’s foot angle +/- 5 degrees throughout the traveler sweep.

Mainsails TP52 Swan42 M&R48
Luff 20.86 17.24 16.94
Leech (straight-line) 21.13 18.25 17.84
Foot 7.33 5.76 5.77
Head 1.380 0.175 0.160
IMS Area, m2 96.67 61.34  58.05
Downwind Sails A2 A3 S2
TP52 Swan42 M&R48 TP52 Swan42  M&R48   M&R48
 STL  9.00 7.25  5.799   9.00  7.35 5.79   5.79
Luff 25.616 21.822 19.236 24.966  21.250 18.692 19.000
Leech (straight-line) 21.790 18.655 18.040 23.470 20.165 18.620 19.000
Foot 14.402 11.026 9.408 13.868 10.573 9.414 10.444
SMG 13.854 11,402 10.466 11.242 9.113 8.390 10.646
IMS Area, m2 275.86 191.06 159.3 237.67 162.30 111.64 167.92


The results spreadsheets include lift and drag coefficients for the total sailplan, as well as the gennaker and mainsail separately. Additionally, individual forces, moments, and center of effort locations are also listed for combined and individual components. The coordinate system is taken from a user-defined origin. Forces are reported in Newtons, moments in Newton-meters, and dimensions in meters.

M&R48 VTK Files:

Swan42 VTK Files:

TP52 VTK Files:


The FSI modeling tools used for this project are the proprietary property of the world’s leading sailmakers and are therefore not readily available to the public. Through this project, handicappers will realize some of the benefits from the significant financial investments that have gone towards developing these modern tools. By providing a set of FSI-based results, the scientific and sailing community will gain a comparison point against which to make significant improvements to current VPPs.


If you use any of the data provided on this page, please use the following information for your citation.

Author: Braun, JB

Title: SYRF Downwind Aero Moments and Forces

Date: 2016

Publisher: Sailing Yacht Research Foundation

Electronic Retrieval Location:

Downwind Aero Moments and Forces by Sailing Yacht Research Foundation is licensed under a Creative Commons Attribution 4.0 International License.

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