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Calibrating speed Meter

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ssteakley
Captain

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USA
467 Posts

Initially Posted - 03/31/2003 : 21:04:09

I have a standard Horizon Speed transducer and meter. I am trying to calibrate the speed meter. I used a hand held GPS to get it close, but I am wondering which unit would read faster? The GPS or the Transducer/Meter?. The instructions say to use a GPS or have another boat run along side to determine the correct setting (assuming their speed meter is correctly calibrated).
thanks for the help,
Steve

<img src="http://im1.shutterfly.com/procserv/47b3dc32b3127cce94d57c8cff3b0000001010" border=0> Moon Chaser #385WK <img src="http://www.catalina25-250.org/forum/forum1.gif" border=0>

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Bryan Beamer
Master Marine Consultant

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USA
1038 Posts

Response Posted - 03/31/2003 : 23:48:19

I also use a hand held GPS to calibrate my Standard Horizon Knot meter 2 or 3 times a year. I don't know enough about a GPS but I do see the GPS change speeds while sailing more than the knot meter.

I usually do my calibration while motoring on calm waters. That way I can keep a consistent speed for a longer period of time.

Bryan Beamer
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Daylight Again
C250wk #495

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John G-
Admiral

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793 Posts

Response Posted - 04/01/2003 : 00:53:17

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
I also use a hand held GPS to calibrate my Standard Horizon Knot meter 2 or 3 times a year. I don't know enough about a GPS but I do see the GPS change speeds while sailing more than the knot meter.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

A GPS and a knot meter are measuring two distinct things. The GPS is measuring your progress across the face of the earth. The knot meter is measuring your speed through the water. With current, against you or with you, these speeds should be different. It is possible to sail through the water at 2½ knots and go backwards. 
<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
I usually do my calibration while motoring on calm waters. That way I can keep a consistent speed for a longer period of time. <hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

Only in perfectly calm waters with NO current should your GPS and knot meter read the same thing. Which means there are ony two times a day you can calibrate your knot meter with the GPS. It’s better to find a measured mile between marks to set the knot meter. Most large harbors have one. You can use the difference between your knot meter and GPS to calculate whether you have a favorable current or not. In racing use your knot meter to set your sail trim use the GPS to help decide which side of the course is favored.

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n/a
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163 Posts

Response Posted - 04/01/2003 : 08:00:30

Another way to check, if you know the exact distance between your bow pulpit and any point near the helm, is to have someone on the bow holler when a buoy or daymark passes close aboard and time it until it passes your stern mark. Once you know your speed in ft/sec it's easy to calculate your speed in knots or mph.

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Derek Crawford
Master Marine Consultant

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USA
3324 Posts

Response Posted - 04/01/2003 : 11:26:06

For what it's worth, as an hard core racer I keep my knotmeter reading as high as possible (we call it the "clickmeter" and we've had it read 12.4 knots...) The reasoning is that I'm not really interested whether I'm going 4.5 or 5.5 knots - what I need to know is, when we retrim the genoa or the main, does our speed go up or down. With it at a "wide open" setting it becomes a much more sensitive instrument. Now, if you're a cruiser going from A to B on the coast, the knotmeter should be accurate - it makes dead reckoning much easier! On the lake I know where I am just by looking around <img src=icon_smile_big.gif border=0 align=middle>
Derek

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Bryan Beamer
Master Marine Consultant

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USA
1038 Posts

Response Posted - 04/01/2003 : 15:28:36

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>Only in perfectly calm waters with NO current should your GPS and knot meter read the same thing. Which means there are ony two times a day you can calibrate your knot meter with the GPS. It’s better to find a measured mile between marks to set the knot meter. Most large harbors have one. You can use the difference between your knot meter and GPS to calculate whether you have a favorable current or not. In racing use your knot meter to set your sail trim use the GPS to help decide which side of the course is favored.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>

The lake we sail in is only 11 miles long there aren't any mile markers, and there isn't any current. (except the waves from the stink potters)

Bryan Beamer
<img src="http://im1.shutterfly.com/procserv/47b3dc23b3127cce9462f2858f2b0000001010" border=0>
Daylight Again
C250wk #495

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