Originally Posted by hellosailor
"when the display shows 2 knots my handheld GPS will show 5"
In other words, your waterwheel is VERY SLOW.
When is the last time you removed the impeller and cleaned it out? Sometimes you'll find enough growth in there to freeze it up in just a week or two. I'll pull the log whenever the boat is being left at the mooring, and if it has been left in, it gets cleaned (old toothbrush) picked (dental picks) and if there is barnacle crud on it, you may need to soak it in vinegar as well.
Before you decide something needs calibrating--pull it and see if it isn't just crud.
thanks for taking the time to respond. I have never removed the impeller and cleaned it out! But the speed log is easily accessible and I remove it after every trip. So I dont think the problem is dirt! But having said that, I will try and remove the impeller.