Hertfordnc,
Couple of recommendations,
1) pull the paddle
wheel and clean throughly, grease if desired, make sure it spins freely.
2) Reinstall, with the slot alignment, Arrow forward.
3) double check the calibration with the
GPS, at slack tide when there is little wind, using two runs 180 degrees apart averaging the speed between the two runs to compare to the
GPS average Speed of the two runs.
When I did this is took a bit of little time, but it ended up giving me the confidence that the reading is closed to accurate. I aways pull my paddle
wheel and intall the dummy plug when I leave the boat. I do this to keep the growth to a minimum. Where I keep my boat, It will foul during the week while I am at work, within a week!
Best of luck