NavList:
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WasRE: digital compass, etc Now Lining Lubber Line
From: Paul Marcuzzo
Date: 2000 Sep 12, 6:29 AM
From: Paul Marcuzzo
Date: 2000 Sep 12, 6:29 AM
Hi Lu, is that written up anywhere that I can reference? I am in the process of writing the PowerPoint presentation for the Coast Guard Auxiliary's Advance Navigation Class and would like to add a slide or two on this. But I'll need some references. Thanks ! Fair Winds ! Paul Punta Gorda, FL to dock - N 26? 53.471' W 082? 03.515' to Corona Locker - N 26? 53.472' W 082? 03.516' -----Original Message----- From Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Lu Abel Sent: Tuesday, September 12, 2000 1:25 AM To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM Subject: Re: digital compass, etc Let's step back to the original 360 or 720 degree turns. The microprocessors in electronic compasses can record the compass reading fractions of a second apart. The compass software assumes you've made a smooth turn (ie, changed your boat's direction at a uniform rate of speed), so any deviation from that in the compass readings it records is, well, deviation. It sounds as though your Furuno goes one step further and tries to figure out if the compass's lubber line is aligned with the boat's keel by having you take a bearing on a fixed object. By having you input the bearing, it can correct the compass reading for a misaligned lubber line. While many courses and texts give methods for developing a deviation table for a convention compass, they rarely mention that the resulting table can be used to detect a misaligned lubber line. If one has determined deviation by taking compass bearings on one or more distant objects, the sum of the westerly deviations should exactly match the sum of the easterlys. If not, the compass's lubber line is misaligned. A way to see this visually is by plotting the deviation table on a piece of graph paper. Draw a horizontal axis across the center of the paper, divide it into 0 to 30 degrees. Then plot westerly deviations as positive values and easterlys as negative. When the points are joined together result should look like a sine wave centered around the horizontal axis. A lubber line misalignment effectively shifts the horizontal axis upwards or downwards, making more of the sine wave be on one side of it than the other. Why am I so hot on lubber line misalignment? Because I bought a brand new, high-quality 36' sailboat some years back. Immediately made a deviation table for its compass which I checked periodically. Wasn't till several years later that I heard about summing the deviations and, surprise, my lubber line was off by 2 degrees! That's a big error in all but he shortest courses, so I've become a misionary for doing this oft-ignored check. Lu Abel At 11:51 AM 9/11/00 -0400, you wrote: >I've got a Furuno electronic compass on my boat (connected to the radar). >This is swung by turning the boat slowly thru 360 degrees. I also have a >autohelm 3000 autopilot this is swung in a similar manner. >Except that in the autohelm it is necessary to then point the boat on a >fixed bearing and remove any offset in the deviation. >This is not necessary with the Furuno compass. How can the Furuno "know" >any offset ? > >Aubrey. > >At 08:39 AM 07-09-00 -0700, Patrick McVey wrote: >>I don't mind though slightly off subject. You're doing what's called, >>"swinging the compass." The sensor (fluxgate I believe) and microprocessor >>are testing the sensitivity of all 360 degrees and making a correction >>table for each point of the compass (probably all 360 points). The >>sensitivity varies due to large pieces of >