I think that was my comment in the other thread. The first rule of instruments is they're probably wrong. The 2nd rule is...The paddlewheel being off-center has a big (negative) affect on the entire instrument system. I have seen up to 1kt difference from tack to tack with an off-centerline paddwheel sensor. Set your instruments to GPS speed, then take data on long tacks to confirm what you are seeing isn't simply a speedo issue.