Quite normal for calibration to change as the hull gets a bit furry. Recalibrate now while its clean and make a note of it in the log so you can return to that setting. I found best way is either to calibrate against a traditional trailig log which because its streamed astern is not influenced by the hull so more accurate. The second method is to find somewhere with neglegable tides and calibrate to distance run either against GPS log or radar range. Much more accurate to calibrate over say 5 miles run that try and do it on speed.