Knot Meter vs. GPS - SailNet Community

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  #1  
Old 03-12-2008
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Knot Meter vs. GPS

Which speed indicator should I believe: a knot meter or a hand held GPS??
My knot meter may show 2 knts, but my Garmin handheld shows 4+ knots. I like the idea that I am going faster, but which is best. ( Or does it really matter... whatever floats your boat... so to speak... )
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Old 03-12-2008
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Quote:
Originally Posted by maboyer View Post
Which speed indicator should I believe: a knot meter or a hand held GPS??
My knot meter may show 2 knts, but my Garmin handheld shows 4+ knots. I like the idea that I am going faster, but which is best. ( Or does it really matter... whatever floats your boat... so to speak... )
Your knot meter is showing "speed-though-the water" and your GPS "speed-over-ground". Completely different things.

You can do the math, but assuming you were going down river at the time, the GPS is showing a 2+ current pushing you along.. Enjoy the ride!
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Without knowing the conditions you were sailing in 4 knots sounds more like a "normal" speed, and so seems more likely to be correct. The knot meter may be under reading because the paddle wheel is dirty or it may just need calibrating.

You can check them both by sailing between two known points and timing how long it takes and seeing if one or neither of them is correct.

Both ussually have some sort of damping control to even out the reading, so it might be worth looking at that. Was the GPS reading steady or fluctuating a lot?

Cheers

Ian
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So, my speed between point A and point B is best defined by the GPS?
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Old 03-12-2008
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Maboyer-

A GPS will give you Speed Over Ground—SOG. The SOG is your actualy speed relative to two fixed points on earth... A knotmeter will give you Speed through the Water... which is a totally different thing.

For instance, if you're sailing and you have two knots of current against you, the knot meter may say six knots, where the GPS will say four. Likewise, if you're sailing with a two-knot current, the knotmeter may say four knots and the GPS six.
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I think the GPS is a more accurate tool for speed indication, but it's not as instantaneous as a knot meter (because you have to wait for the signal to return to the GPS from the satellites). The GPS is calculating your speed based on the distance traveled over ground, but the knot meter is calculating you speed based on the amount of times its wheel is turning in the water (directions of currents can affect those readings).
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GPS is more accurate and precise. The knot meter is important whenever sailing in a current. Then, you need to compare the GPS to the knot meter.
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Run a measured mile and time it. Compare that with your knot meter to set in correctly. The GPS is always correct; it just isn't giving you the information you want to check your knot meter.
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You can also do a controlled run on a calm day in what you know is a "no current" environment where you theoretically should get the same reading between GPS and speed log. If not, there should be an electronic adjustment you can make to the speed log that will allow you to "sync" the two to each other. While on the controlled run mentioned above, adjust the speed log until it reads what the GPS is indicating. When you've done that, turn around and run back where you came from and you should have the two agreeing with each other. Repeat the process until you have them agreeing within a few tenths of a knot.

It's a good idea to make sure the paddle of the speed log is clean and turning freely before you start the calibration exercise.
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I use the GPS to tell me how fast I am going between two points. I use the knot meter as an indication of the effect of changes in sail trim.
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