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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 07-18-2007
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BTW, the GPS bearing is not always accurate, since it depends on the motion of the boat to determine the bearing. The handheld GPS I carry as backup has a fluxgate compass built into it, and I'd rather use that than the GPS bearing.

For instance, on a GPS Chartplotter, you can see this when the boat is being set by a strong current. The direction the boat is pointed and the direction the boat icon on the chartplotter screen can be very diffferent...
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  #22  
Old 07-18-2007
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  #23  
Old 07-18-2007
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camaraderie is a jewel in the rough camaraderie is a jewel in the rough camaraderie is a jewel in the rough
...and for the rest of us...the GPS tells you what course to steer to your waypoint and corrects you as you go along for any deviation on your ship compass... Knowing how to do all the other stuff with your compass and course is fun but not really necessary any longer. I simply check my course with a course protractor and leave the rest to the 2 GPS's that are always operational.
BTW...I am not being glib and I have taken and passed the USCG Aux course in navigation BUT that was before we had GPS. I DO think coastal navigation skills are more important than celestial ones but today you can sail without them if you have a GPS and can plot a course on a chart and enter waypoints. You can even do without a traditional compass these days snce the GPS has one.
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THAT'S IT! We're done now. Here, the Electronic Sailor and I part ways. Big time. Steering your boat by your GPS compass is a bunch of tomfoolery, particularly in coastal waters. You are relying on a device that is subject to update interruptions, like the one in my truck that decided to take last Thursday off, although reported for duty Friday morning with no signs of hangover. The magnetic compass is reliably on duty twentyfour/seven.

Furthermore, Camaraderie's Guide to Automated Sailing continues to ignore, or diminish the importance of, those skills essential to being a competent mariner. If you are relying on your GPS for steering, knowing it is correcting for set and drift, you probably have little or no idea what the set and drift are. You do not know because you are altering course continuously to adjust for them. To the inexperieced, this is bassackwards and risky. In fact, it may come as a surprise to them that there is such a strong current present.

Perhaps the biggest troublesome aspect of this mindless sailing is that all of your decision making is being made by a little black box and how well and accurately the destination Lat/Long was plugged in. One, count 'em, one error in initial programming throws the whole navigational exercise off, ot the window, and maybe on the rocks. Garbage in/Garbage out you know. That grinding sound under the keel you do not.

There is, in fact, no such thing as a GPS bearing. It is a computed course suitable for a DR track. A bearing is an angular measurement taken between two terrestial points. Furthermore, your "GPS bearing" is a Great Circle course, not a rhumb line. If you do not know the difference between those two you may be quite surprised by what your GPS is telling you to do and what the line you drew on the chart is telling you. Anotherwords, the "GPS bearing" you are reading is not the course you will steer the entire way from departure to destination. It is what is called, the "initial course" or departure course. If you assume that this "GPS bearing" is your course to steer, and you hold to it, you will not arrive at your destination. Bowditch articles on catography, specifically mercator charts, Lambert Conformal, and polyconic will be of help, and the meat and potatoes will be found in the sailings where rhumb lines and GC are defined. The polyconic projection will be of interest to Great Lakes sailors as those charts use it, versus the more common coastal use of mercator.

Cam's suggestions provoke in me the same feeling one would get on the Greyhound bus, if the bus driver pushed a button and got up, walked aft to use the head, all at seventy miles an hour. Cam also knows that most sailors are going to have one GPS only, not the two or more he recommends and that his offshore arguments against celestial do not apply inshore against traditional practises of piloting. And the stark reality is, when the fog rolls in, the GPS gets wet, a dumb look is less than what is required when gazing upon your magnetic compass for the first time seriously.

To others reading this, I would encourage you to learn everything you can about your magnetic compass, it's use, it's adjustment, and maintenance. It is the first tool in the navigator's arsenal, regardless of what some "autosailors" may think. Stay alive-learn the practise of navigation.
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  #25  
Old 07-18-2007
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I'd have to agree with Sailaway... GPS is just an aid to navigation...one of many... not a panacea to it. It should be used in conjunction with traditional coastal pilotage skills.
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—Cpt. Mal Reynolds, Serenity (edited)

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  #26  
Old 07-18-2007
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I'm sure Cam will defend his case eloquently.....

I like steering by compass rather than the number only heading my autopilot and chartplotters display. One thing I would like to see is a compass that is NOT magnetic. What value is there in maintaining the whole true/magnetic confusion now that most of us correct for it anyway - or at least are forced to go back and forth frequently? It seems to confuse things. Why not just have all compasses be adjusted to show True north?
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pamlico,
you can buy a gyro compass for about $5000 and then you'll need to power it.

The whole purpose of this conversation is based on the fact that you can rarely, if ever, get all of the error out of a magnetic compass. Once you did, you'd change locations and the variation would change and there'd you'd be again.
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SEMIJim will become famous soon enough SEMIJim will become famous soon enough
Quote:
Originally Posted by sailaway21
And while I was joking with Cam, I am entirely serious about the use of charts and navigation. Your instincts are correct. Your wife's attitude, with all due respect, will get you killed.
No it won't, cuz I'm gonna learn it . (She's bad with math and admits it.)

Quote:
Originally Posted by sailaway21
Here's a sample question from the USCG exam for Third Mate-Oceans:
...
Here's another. (give you a hint-ignore the Gyro info, it's extraneous)
I believe I know the answers. Should I post 'em here?


Quote:
Originally Posted by sailaway21
Answers will be found in Off Topic in the AFOC thread.
I looked in OT. "AFOC thread?"

[Edit]Never mind. Figured it out

Quote:
Originally Posted by sailaway21
Enjoy!
I did! Thanks! (Dunno yet if I got 'em right, but I enjoyed trying to work it out, anyway .)

[Edit]Hah! Got 'em both right! Do I get a gold star?

Thanks,
Jim

Last edited by SEMIJim; 07-18-2007 at 10:51 PM.
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Old 07-18-2007
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Hah...knew that would get you old stick in the muds going.
Do Navy ships still steer by magnetic compass? Hmmmm...must be something to this electronic stuff.
I don't care WHAT you call the course the GPS tells me to steer...bearing, great circle route etc. ...what I am saying is that if I enter a waypoint and consistently follow my GPS's directions for steering to that way point (which are constantly being adjusted), I will arrive within 100 ft. of that waypoint more quickly than if I had drawn a course, computed variation and deviation and current set and drift and then steered the proscribed course with a magnetic compass and used coastal navigation skills to check things along the way.

To be clear, I am not denigrating coastal navigation skills. Indeed on the ICW route the GPS is largely useless and following charts and using range markers, and taking quick plots to the next mark with my protractor and using my liquid compass is what I do.
I am just saying that it is possible to quite skillfully and safely do the type of sailing most of us do...short passages, coastal hops without even mentioning deviation or using the liquid compass.

I am not saying to throw out the compass and the charts so rest easy dear Sailaway....just saying there is an easier way that is quicker and more accruate. Here's one that will get you going...If I had to choose between having a "real compass" on board or a GPS (with backups) ....I would choose the GPS. Which would you choose??
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Quote:
Originally Posted by camaraderie
Here's one that will get you going...If I had to choose between having a "real compass" on board or a GPS (with backups) ....I would choose the GPS. Which would you choose??
Ohhhh boy. Oh boy, oh boy, oh boy, oh boy...

With all due respect to both side of the debate: I honestly do not know. Yet . Give me some time on the water--perhaps a lot of time, and I may be able to answer that question. I do know I'll be plotting the course with chart and Weems Protractor (tho I could do it all in the GPS). And I plan to steer the course with the compass. (Tho it hasn't been swung yet, so far as I know.) But we'll not be departing until that GPS is operational. And the boat comes with a hand-held GPS, for backup. (And I'll be having an extra set of batteries for that.)

Jim
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