The spreadsheet model (about 24kb) calculates the apparent wind given these parameters. It is interesting to see how much the apparent wind angle increasingly differs from the true wind as the yacht moves from a beat onto a reach, and how the apparent wind speed dies as the yacht moves onto a run. The graph shows that, closehauled, at the sort of speed that an IOM makes in the middle of its No.1 rig, the apparent wind is about 30 degrees off the bows when the true wind is about 45 degrees off.