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Old 20-02-2012, 19:37   #1
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GPS Disorientation

So during a passage race off shore this weekend we were in really bad haze for a while out of site of the shores which were only 5 miles away or so. We were becalmed although I knew we had about 1/2 knot boat speed as we were making a tiny wake. The skipper started turning the boat back and forth and with the tiny breaze we had eventually got us stuck in irons. He then started making noises about "the bloody" gps being wrong and which way to go etc...

I finally went to look and the boat was headed 270 on the compass, boat speed was 1.8 knots but the "lead line" projection of course was leading out at 90 degrees i.e. off the bow of the little icon which was also headed 90.

I started laughing at the skipper which is not a good thing to do 5 miles off shore but then had to explain to him why the gps was "all screwed up."

We were heading 270 mag, but the gps doesn't know heading. It only knows direction. It aligned the little boat icon on 90 @ 1.8 knots because that is where the boat was going in the 2+ knot current.

He has a speed log but it is uncalibrated. It reads about 1 knot when we are doing 4+ gps speed. I have asked him a couple of times to go out and calibrate it and took the opportunity to remind him again why it is good to have boat speed available.
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Old 20-02-2012, 20:06   #2
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Re: GPS Disorientation

Guy sounds like a hell of a captain. Ask him to chime in on the celestial navigation thread.

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Old 20-02-2012, 20:23   #3
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Quote:
Originally Posted by rebel heart
Guy sounds like a hell of a captain. Ask him to chime in on the celestial navigation thread.

He's a decent guy and most important willing to learn.

Not to start another navigation war but this is a great example of reliance on the box vs. basic navigation witth a mag compass. He thought because there is a waypoint set the icon should point to the waypoint and the gps speed should have been reading negative! <sigh>

The other mistake was violating rule number one. Keep the sails full, especially in light air. Getting out of irons was a pita...
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Old 21-02-2012, 00:45   #4
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Re: GPS Disorientation

Good grief. I just assume that anything under 2 knots is going to read zero or be wildly inaccurate. Keeping a paper log, pencil marks on a chart and a Plastimo in the bulkhead means I usually know where I am even in fog at midnight.

Which had better be well offshore!
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Old 21-02-2012, 01:00   #5
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Re: GPS Disorientation

True Confessions:
After several days in a Force 8 - 9 storm in the Bay of Bengal, I noticed a 90 degree difference in the compass and COG. I was tired and thought something was broken. Fired up the standby GPS and the standby, standby GPS, same thing.

We were pointed South, but moving East...

In my tired state it took me a while to realize that we were actually going East.

Bill
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Old 21-02-2012, 05:13   #6
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Re: GPS Disorientation

not trying to be smart or ofend anyone but looks a lot fuss about nothing.in this case you do not need even sextant. did you had a chart. i do not believe that someone going out (even 5 miles) and do not have at least local water chart and navigation triangle or any protractor. if you had chart you coud easily find course to steer and how far you are of shore. you had your gps positin, find that position on the chart, take protractor from your position to the shore and you will get course and distance.
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Old 21-02-2012, 05:22   #7
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Re: GPS Disorientation

Quote:
Originally Posted by Ex-Calif View Post
He's a decent guy and most important willing to learn.

Not to start another navigation war but this is a great example of reliance on the box vs. basic navigation witth a mag compass. He thought because there is a waypoint set the icon should point to the waypoint and the gps speed should have been reading negative! <sigh>
A funny story.

And what your cap did was much worse than simply "overrelying on the box". He was not even using "the box" in a competent manner. COG is not heading. And SOG is not boat speed. It is deeply wrong to use them as cheap substitutes for each other, and here's a good example of why.
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Old 21-02-2012, 05:30   #8
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Re: GPS Disorientation

I have my charts marked with direction and speed of tides at all the tide diamonds and the time relative to my home port. As I'm on a drying mooring I usually have a fair idea of when high tide is there, so a check on the nearest diamond gives me a fair idea of expected drift.
The Solent has local tides up to 4kts, about the same as my boat speed. I need to know these things.
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Old 21-02-2012, 06:30   #9
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Re: GPS Disorientation

Quote:
Originally Posted by Ex-Calif View Post
He's a decent guy and most important willing to learn.
That is the most important thing. You can make all kinds of mistakes, and be ignorant of nearly everything, but if you're willing to learn then eventually you will get there. Those who think they already know everything, and so are unwilling to learn, are the ones that I can't abide. When they DO make a mistake (and everyone does from time to time) they refuse to acknowledge it, fix it, or learn from it.
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Old 21-02-2012, 16:49   #10
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Quote:
Originally Posted by Dockhead

A funny story.

And what your cap did was much worse than simply "overrelying on the box". He was not even using "the box" in a competent manner. COG is not heading. And SOG is not boat speed. It is deeply wrong to use them as cheap substitutes for each other, and here's a good example of why.
I haven't spent much time at the binacle and he has the Garmin 620. I wanted to find out if the unit provides vmg and explain it to him but didn't spend the time on it. I was busy trying to get the boat out of irons and the sails full.

Does the 620 provide vmg somewhere?

I was aware and we talked a lot about currents as they were a huge factor in the race that day.
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Old 24-02-2012, 10:08   #11
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Re: GPS Disorientation

The "track" button helps, given time.Not immediate, but might serve to distract a trusting helm from the other less accurate fields.. I say "button" to save detail,but most gps iterations have it.
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