I an sure that this question has been discussed but was unable to find anything..... I have an Autohelm ST50 and a Garmin GPS38. While out yesterday I recorded a difference of 2knots, with the knotmeter being the slower of the two. What do I go by...... Dave
- Status
- Not open for further replies.
D
Doug T.
Currents
You had a 2kt helping current. (Or your knotmeter is sludged up and not performing properly.) What you go by depends on what you're using the speed indication for. If you want to know "when will I get there", then the GPS is the thing to use. If you want to know "how well is my boat performing relative to past performance" then the knotmeter will tell you more.
You had a 2kt helping current. (Or your knotmeter is sludged up and not performing properly.) What you go by depends on what you're using the speed indication for. If you want to know "when will I get there", then the GPS is the thing to use. If you want to know "how well is my boat performing relative to past performance" then the knotmeter will tell you more.
B
Bill O'Donovan
Explained
By "helping current," Doug means that the current was moving the knotmeter faster than if there was no current at all. Hence it looked faster on one instrument than the other.
By "helping current," Doug means that the current was moving the knotmeter faster than if there was no current at all. Hence it looked faster on one instrument than the other.
D
Doug T.
Actually,
... just the opposite. The current was moving the boat faster than the knotmeter indicated. If it's after high tide, and the tide is ebbing, and you're simply drifting with the tide (no wind at all), your knotmeter may say 0 knots, yet your GPS may say 2 knots... or however fast the tide is moving. Add some wind and go in the same direction both will say you're going faster, but they'll still have that 2 knot difference. Everything's relative...
... just the opposite. The current was moving the boat faster than the knotmeter indicated. If it's after high tide, and the tide is ebbing, and you're simply drifting with the tide (no wind at all), your knotmeter may say 0 knots, yet your GPS may say 2 knots... or however fast the tide is moving. Add some wind and go in the same direction both will say you're going faster, but they'll still have that 2 knot difference. Everything's relative...
B
Bill Ebling
Knot meter Calibration??
You may want to make sure your knot meter is calibrated...you can use time across known distance at constant boat speed....with no current or leeway on flat water. Check the manual of your knot meter for details. If the knot meter is calibrated the delta that your looking at (if integrated for sufficient time) is current and/or leeway
You may want to make sure your knot meter is calibrated...you can use time across known distance at constant boat speed....with no current or leeway on flat water. Check the manual of your knot meter for details. If the knot meter is calibrated the delta that your looking at (if integrated for sufficient time) is current and/or leeway
D
Del Wiese
averaging interval
I have an older GPS and the period over which it averages and displays speed is fairly long, 10+ seconds. The speed of boat changes more rapidly than this, albeit not by much, but enough to cause disparity in the readings. On a calm day, no current, and under power so speed is quite constant, the two tend to agree. And this is the way I calibrate my knotmeter, I think the GPS is quite accurate under these conditions.
I have an older GPS and the period over which it averages and displays speed is fairly long, 10+ seconds. The speed of boat changes more rapidly than this, albeit not by much, but enough to cause disparity in the readings. On a calm day, no current, and under power so speed is quite constant, the two tend to agree. And this is the way I calibrate my knotmeter, I think the GPS is quite accurate under these conditions.
M
Mike DiMario
Boat Speed
Dave, I agree with Doug's opinion. The GPS indicates the boat's average speed over the ground and the knotmeter indicates the boat's speed through the water. Assuming that both instruments are calibrated and/or accurate, the difference is the effect of the water's movement. If you have done any hand plotting, the water's movement is plotted as a vector onto the boat's movement over the bottom. Theoretically, the GPS indicated speed will be the length of the vector sum or speed made good. The direction of the vector sum will be the course made good. Together they are called velocity made good (speed with direction). Hope this makes sense! good luck Mike DiMario
Dave, I agree with Doug's opinion. The GPS indicates the boat's average speed over the ground and the knotmeter indicates the boat's speed through the water. Assuming that both instruments are calibrated and/or accurate, the difference is the effect of the water's movement. If you have done any hand plotting, the water's movement is plotted as a vector onto the boat's movement over the bottom. Theoretically, the GPS indicated speed will be the length of the vector sum or speed made good. The direction of the vector sum will be the course made good. Together they are called velocity made good (speed with direction). Hope this makes sense! good luck Mike DiMario
K
Ken
Knot meter/GPS
Having both is a real plus. You can check boat proformance, find out the speed of current and calculate arrival times. I have had about 5 boats now most of the time the knot meters have had problems and in some cases stopped working or never worked. I feel that as long as you have a compass, chart, working sounder and lots of beer I'll be okay (most for when you get their) Colombus had alot less.
Having both is a real plus. You can check boat proformance, find out the speed of current and calculate arrival times. I have had about 5 boats now most of the time the knot meters have had problems and in some cases stopped working or never worked. I feel that as long as you have a compass, chart, working sounder and lots of beer I'll be okay (most for when you get their) Colombus had alot less.
D
Dave Royce
Thanks
Thanks for all the responses. I will be using the GPS alot more. Being in a tidal area I guess the currents do play a big part in knotmeter being accurate.....
Thanks for all the responses. I will be using the GPS alot more. Being in a tidal area I guess the currents do play a big part in knotmeter being accurate.....
D
Doug Thom
GPS interfere with Knotmeter?
This week while calibrating my Raytheon knotmeter I found that the knotmeter gave different readings depending upon the proximity of the Magellan GPS. When I moved the GPS away from the Raytheon instrument the knotmeter reading increased a full knot. Voodoo?
This week while calibrating my Raytheon knotmeter I found that the knotmeter gave different readings depending upon the proximity of the Magellan GPS. When I moved the GPS away from the Raytheon instrument the knotmeter reading increased a full knot. Voodoo?
- Status
- Not open for further replies.