GPS vs knotmeter

I have noticed a pretty dramatic difference between the speed my Datamarine Corinthian knotmeter reports vs what my GPS is reporting. Sometimes that difference can be as much as two or three knots (sailing with the waves). As I go faster the difference gets greater. The GPS always reports a higher speed. I'm sailing on Lake Michigan so normally currents are pretty localized and in the areas I sail in (Chicago to Racine, Wisconsin) are less than a knot if they exist at all. How close should the readings from these two instruments be? What are others experiencing? Is my knotmeter wheel just gunked up or perhaps it needs replacing? Maybe nothing's wrong?

Gary
s/v Imi Loa 1983 E28+

If the GPS is *always* reporting a higher speed, something is not kosher with your KM.
Have you done a constant-rpm two-direction calibration run? Handier if you happen to have available a local "measured mile"... But any such run will do as long as you have room to do a compass reciprical (to cancel out current influence).
Have a stop watch handy or any good timepiece with seconds.
I used to try and calibrate the old original-equipment Datamarine instrument on our boat, but the potentiometer was evidently crudded up and the numbers would jump around, making it a near-futile effort.
It was much easier with the new Raymarine ST60, a couple years ago. We ran both directons, and used a GPS to get the speeds.
The GPS is still the best choice for calculating ETA's, and the boat's KM is still best for sailing input.
These opinions worth what you are paying...

Loren in PDX
Olson 34 #8

I had my Corinthian professionally calibrated, which got the error down to less than a knot, but they seem to be inherently inaccurate. They are best used to see instantaneous changes in speed after sail adjustments, etc.

I am thinking of adding the knotmeter module to my Garmin 3005c this winter. It will measure speed both over the ground (GPS) and through the water and will use the former to calibrate the latter through software (assuming you moving through water without a current). Quite an advance in technology.

calibrate!

Loren is right-You need to calibrate your knotmeter-there are instructions in the paperwork for your model-or the supplier should have them.

Be careful about currents on Lake Michigan. If you want a big surprise, go to:http://www.glerl.noaa.gov/res/glcfs/msfcur+60.html-and watch the forcast over the 60-72 hour time period.

They change alot, and often, and they are often well over 1 knot. In the last Mac Race, the current directions were exactly as forecast, but the speeds were higher-we saw over 2 knots between the Manitous and Gray's Reef!
Yes-there are currents on the Great Lakes. I still think you have a calibration problem, though!
cheers,
S

You do have the GPS set for kts, not MPH ? Obvious, but I had to ask.

BTW, in my Toyota truck the speedo always reads 10% higher than the GPS, turns out that the tires are smaller than they should be




-sven

is it clean?

Calibration is a must- but you might want to double check that it's clean first... Check for growth and see if scrubbing it doesn't help as well.
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