SAWDOC
regular
Reged: 24/02/2008
Posts: 185
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The log on my 26 footer seems to over read by 1 knot when compared to speed registered on my GPS ( please ignore tide in this discussion). having enjoyed the false feel good factor that we are sailing wonderfully for long enough, I am now wondering if i can recalibrate the log to read more accurately. Can anyone advise if this is possible and if so how do i go about it?
(Switch it off and stick to the GPS???)
Thanks
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Damo
regular
Reged: 22/02/2005
Posts: 3078
Loc: k keeper,Portishead
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Is it a consistent over-read? If so just subtract, if you have lost the manual 
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Never be at a loose end with the Yosemite bowline 
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roly_voya
regular
Reged: 05/02/2004
Posts: 1051
Loc: Pembrokeshire Wales
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Should be able to recalibrate it, whatmake is it? try the manufacturers web site for a copy of the manual
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DogWatch
regular
Reged: 10/09/2004
Posts: 9502
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As well as calibration, although your log is over reading, the first thing to do. In a straight line, ease off the log holding nut so you can rotate the transducer and gently rotate and find the maximum speed position. It is not always the arrow point on the fitting. I have marked mine with a dob of white paint.
Then as stated you can calibrate the head..
On nasa this involves holding down a key or two when powering up.. I forget which, but the manuals are on line all over. From memory you can go 50% up and down which should well cover your 1 knot reading.
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Jonny_H
regular
Reged: 15/08/2006
Posts: 1514
Loc: Liveaboard - following the sun...
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The trouble is finding a place / time when there is genuinely no tide or current so you can accurately calibrate it.
We have tried to calibrate ours, but it is very difficult to get it within .5 knots due to tidal effects.
Jonny
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www.freewebs.com/jksailing
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KenMcCulloch
regular
Reged: 22/04/2007
Posts: 1044
Loc: Edinburgh, Scotland
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Quote:
The trouble is finding a place / time when there is genuinely no tide or current so you can accurately calibrate it.
We have tried to calibrate ours, but it is very difficult to get it within .5 knots due to tidal effects.
Jonny
All you need is a measureable half mile where the tide is constant or nearly so for an hour or so and aligned with or against the course you are steering. Make 2 runs, 1 down tide and one up and your log wiil have measured the actual distance over the ground, the tides cancelling each other. Calibrate the instrument accordingly. I recently did this having fitted a new log/echosounder, and last night at slack water neaps went for a sail and found the log and GPS in agreement with a difference of less than 10%. That's going to give me a DR error of about half a mile on a 60 mile passage which I find acceptable.
Edit Sorry I think I was still half asleep when I wrote that... The error on my log as now calibrated is in the region of 5% which gives me an uncertainty of 'a few miles' at the end of a 60 mile passage. Thanks for the correction.
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Ken McCulloch
Border Maid
Edited by KenMcCulloch (16/05/2008 09:32)
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John_Baldock
regular
Reged: 11/08/2007
Posts: 140
Loc: Falmouth
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Sorry Ken, but 10% of 60 miles is 6 miles, not about 1/2 a mile
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KenMcCulloch
regular
Reged: 22/04/2007
Posts: 1044
Loc: Edinburgh, Scotland
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Ahh yes thanks see my edit above. My point is really that over time if you keep a log of your log you can of course refine the calibration forther and further. When I first installed my new instrument the error was about 30% but after 3 calibration attempts I have now got it down to a pretty acceptable margin.
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Ken McCulloch
Border Maid
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flaming
regular
Reged: 24/03/2004
Posts: 1885
Loc: Southampton
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If you have a "full suit" of raymarine stuff all linked together there's a neat shortcut if you can find some tideless water that calibrates the log to match the GPS.
I'd have to read the manual to remember exactly how to do that though.
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You never know, I might be right!
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LadyInBed
regular
Reged: 02/09/2001
Posts: 4004
Loc: Zumerzet
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Paddle wheel error may vary with speed.
ie reads low below 4 knots and high above!
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Beating is sailing for twice the distance at half the speed and three times the discomfort.
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