Aerodynamic interference of yachts sailing upwind on opposite tacks

When two sailing yachts pass each other on opposite tacks, the downwind (or leeward) yacht passes through the wake of its upwind (or windward) counterpart. To date there has been no study that has quantified the changes in drive force, apparent wind, or distance sailed to windward associated with these manoeuvres, although it is of interest to the development of racing yacht tactics. In this study a transient computational fluid dynamics model of yachts passing each other on opposite tacks is developed. Data is presented for a range of separation distances. For close crossing distances the drive force on the windward yacht may be reduced during the encounter, while the leeward yacht experiences a net increase in drive force. The change in drive force is primarily due to the change in the apparent wind angle induced by the other yacht. A leeward yacht changing its course to take advantage of this shift in the apparent wind can achieve a gain of 1/4 a boat-length in the distance travelled to windward.