Tom often used Redil's instruments to judge the performance difference from one tack to the other. But before using the instruments for this kind of job, it's important to check that they're accurately calibrated. Nick felt that Rebel was sailing both faster and at a closer angle to the wind on port tack. So we had to be sure that both the wind angle measurement at the top of the mast, and the boat speed, were reading correctly. Normally the boat speed can be calibrated against a measured mile, but that kind of calibration wouldn't tell us if the boat speed read differently on each tack. That's because calibration against a measured mile is almost always done under engine, with the boat upright. Different speedo readings on the two tacks occur when the boat is heeled, and because the paddlewheel sensor isn't quite on the centreline, or lined up straight with the hull. Fortunately, David had foreseen this problem in advance, and had brought along the solution. The Autonnic Speed 2000 is an electronic trailing log, with its own battery power (see photo with main text). The trailing log doesn't suffer from the problems that might make the through-hull paddlewheel read differently from one tack to the other. It would stream out behind us exactly the same on both tacks, and provide th perfect check on the speedo.

We also had to check that the masthead wind angle instruments were set up straight. If the wind indicator is skewed to one side, then it will read a different sailing angle on each tack. Rebel would appear to be sailing closer to the wind on one tack, even if she was sailing the same angle on both. The best way to check this is by going to the masthead to have a look (see photo). I went aloft and lined up the indicator as closely as I could with the centre line of the boat below. Once I was happy that it was straight, Nick read off the sailing angle on the display in the console. When I thought it should be reading zero ¬head to wind - the instrument indicated a wind from 6° off the starboard bow. To get the true reading, we had to include this as a correction - so we deducted 6° on starboard tack and added it on port tack.