Test it first
Pete,There are measured miles available to test your speedometer (Speed through the water). It depends on where you live of course. If you have a Raymarine, is the paddle wheel removable? Ours can be pulled form the inside with a quasi stop and insert a plug. This allows us to clean it.Our Raymarine also has a calibration mode. You need to be in a non current, non wind situation then you push the button, do a measured mile and push it again.Also, you do know that your GPS reads speed over the bottom and your "speedometer" reads speed through the water. There seems to always be a difference between the two. Sometimes I think it is due to the moving current and other times because the instruments are finicky.I get ours close. I use the GPS for trip planning but the "speedometer" for measure of how I am trimming my sails. It is faster reacting and as long as it reads higher when I am doing something right and lower when I am doing something not so right, what's the difference?Good luck,Mike D.p.s. IMHO, since I am not familiar with the H26 in specific, you may get more help on your unique situation if you put the year of your boat and the Name and type of your instrument.