1. THE USE OF COMPUTATIONAL MODELLING IN A STUDY OF DOWNWIND SAILS Peter Richards and Gordon Mallinson University of Auckland New Zealand

2. Auckland - The City of Sails

3. Home of The Americas Cup

4. The Yacht Research Unit The primary function of the YRU is to co-ordinate and promote research and engineering activities within the University of Auckland that are related to yachts.

5. Images created by Gordon Mallinsons package SeeFD (Son of GRAFFIC)

9. Wind direction Americas Cup Course

10. VMG maximum with a wind speed of 5 m/s True wind direction

11. Wind direction

12. Spinnaker Mainsail Spinnaker pole Boom Spreaders Masthead

13. Velocity Triangles A VTVT VAVA VBVB VTVT VAVA VBVB A (a) Boom Height(b) Masthead

14. Apparent Wind Twist

15. Test Section of Tamaki Twisted Flow Wind Tunnel

16. Schematic of Tamaki Twisted Flow Wind Tunnel Test section Honeycomb and mesh 25m 3m Wind Flow Fans Wooden blocks Pitot tube Flow twisting vanes (Not to scale)

17. Twisted flow?

18. Interaction Test Scenarios Mainsail onlySpinnaker onlyCombined sails

19. Lift and Drag Coefficients - Wind Tunnel

20. Lift and Drag on the Spinnaker - CFD

22. Lift and Drag on the Mainsail - CFD

25. Thrust and Side Forces: Spinnaker, Main and Both

26. Thrust and Side Force: Summed and Both

28. Twisted Onset Flow

30. Thrust and Side Force: Summed, Both and Twisted

31. CONCLUSIONS Both straight and twisted flow around a pair of downwind sails have been modelled. Under some sailing conditions there is significant interactions between the sails, however under normal sailing situations this interaction is quite small. With twisted flow the thrust and side force coefficients are similar to those for straight flow if referenced to the masthead apparent wind speed and direction.