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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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  #31  
Old 07-19-2007
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Magnetic compass, not even close.

Now i know, and you know, that you are plotting these GPS positions on a chart. But you failed to mention that, thus possibly leading some to believe that you just follow the little arrow on the GPS thingy. Which is a pretty good way to run into an island or the like, what with your GPS doing all your navigating for you. See, for those who don't see where this is going, the GPS is calculating your set and drift like a labrador retriever, adjusting course all the time, but it doesn't know that the set and drift have now placed you in a spot where you have to sail over land to get to your waypoint. Now your labrador wouldn't do that to you. At least not if you treat him right. But your GPS, it don't know land for nothin'.

And, if it's so damn accurate, why don't you take me up on my Ford truck bet? Course we could use your new R/V if ya wanted!(g)

I suppose it is futile to discuss the differences between charted positions and GPS positions. In particular, the positions of such as those nasty little rocky isles so detrimental to good bottom paint adhesion. And, regarding accuracy, you don't have the slightest clue as to how accurate your GPS is. The only thing you have is what the GPS tells you it thinks it's doing for accuracy today. To confirm it you'd have to take a couple of bearings off a lighthouse or two.

As to your question again. I'd surely take the compass. I'll dispense with noting what other navigational gear I'd also take ahead of the GPS. I'll leave it with the thought that you are one good boarding sea away from having less than Columbus, following your logic. A wet compass works fine, wet batteries not so well.

Gimme a call about the Ford. I still got enough of the spray paint left. We'll have to find another judge. Junior's in jail for awhile; somethin' about the pursuit of deer at night utilizing bright lights and firearms. Guess that means you can stay in his trailer, though. Park the R/V right out front next to the hydrant.
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  #32  
Old 07-19-2007
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LOL...Well I went trolling and caught a big FISH!! (G)
I'm not gonna get into an extended argument here cause you KNOW that 90% of the boaters out there are doing what I just described except they are steering by their magnetic compasses then correcting to where the GPS tells them to go ...OR following the line on their chartplotter...both of which are more dangerous than doing it YOUR WAY or MY Way....I guess I've just been lucky the last 6 years...not even one close call navigationally that I can think of and I don't have a deviation card! (But I do love my CPlath Venus...it is just a work of art.)
No boarding waves soaking my electronics or GPS failures either...and we've sailed in some pretty nasty stuff. I don't want to rely on my compass cause all my electronics cause false readings...especially those big subwoofers!! (G)

Enter GPS waypoint...double check course on the CHART....Steer to the Waypoint and check the chart along the way. Easy...Prudent...Accurate.
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  #33  
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Cam why are you causing troubles.

Everybody knows you would not rely solely on GPS on your own boat and that you are taking up being devil's advocate for electronic navigation just to have fun on this thread. Admit it! lol.

Differing nav gear and techniques are all important. The more you know, the better off you are going to be. Navigation is a process, a way of reaching out with your mind into the future and out across distances so that you can "feel" your way forward in your mind's eye. Navigation is the way to perceive many possible courses of action and to discern a path forward for yourself and for your boat so that you end up with the result you desire.

What is situational awareness ? (wiki link)

Situational awareness is having the image in your brain match up to the real world in and around your boat. It's that comfortable feeling you get when a lot of information matches up in your head and it all seems to be saying the same thing, a feeling of confidence you get from having a lot of things come together so that you can move forward. It is also the subtle feeling of suspicion you get when one of your perceptions doesn't match up and you start to realize something has changed, that you need to investigate more to see what is going on and get the image in your head to match back up to reality again.

GPS is a single amazing tool. But it is just one tool, and the human mind lusts for much more information than this one tool can provide, even if you have 10 GPS receivers onboard. Navigation is about a lot more than having a hyper accurate fix. Being able to navigate is being able to know what is apt to happen next, to see into the future and divine a way forward that ends in success. It is a process and a history of experiences that gives you the ability to act correctly to achieve an end. And I think a good navigator on a boat is probably a good navigator in a lot of other endeavors as well. Philosophically I think that a good navigator is someone who can be molded by whatever powers of nature exist in this world to become a learned participant, clever and able to outwit circumstances, willful and self-directed enough to slip forward through adversity. Much like the water we sail on.

By way of analogy (and I have used this one before) - Having an accurate GPS fix is like knowing exactly down to the penny what every stock is selling for on the stock market. It's great to know, but it doesn't mean you're going to make any money.

Navigation is the ability to snatch fortune from an all too often unfortunate world.

EDIT: (you confessed before I even finished my post! LOL)

Quote:
Originally Posted by Cam
LOL...Well I went trolling and caught a big FISH!! (G)
We knew it all along lol ....

Last edited by wind_magic; 07-19-2007 at 01:08 AM.
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  #34  
Old 07-19-2007
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Windy...they've been trolling for me on the Celestial thread so I figured I'd really shock 'em!! (G) You're right about situational awareness and I do use many inputs when I navigate...but it is fun to ask what would you do without even if that situation never arises.
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  #35  
Old 07-19-2007
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Quote:
Originally Posted by wind_magic View Post
Everybody knows you would not rely solely on GPS on your own boat and that you are taking up being devil's advocate for electronic navigation just to have fun on this thread.
Actually Cam probably is using one of those Magellan units that talks to you and tells you where to turn. You're in an RV now, right CAM?
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  #36  
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Magellan??? Wasn't he a Portagee? I've got the new version...the GIU on my RV. Only problem is that since I installed it...things keep breaking ad my water hose doesn't work!! (G)
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Quote:
Originally Posted by sailaway21 View Post
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.
I disagree, but not for the reasons you cite. "GPS compass" or "satellite compass" (such as this one I am considering purchasing as a supplement to an autopilot: http://www.comnavmarine.com/html/cmnv2340.htm) is a reasonable way to steer a steel vessel as an alternative to a fluxgate compass. I have such a steel vessel, and while I have a KVH AC103 fluxgate compass properly situated already (plus a Ritchie Globemaster the size of a human head, a pelorus, a compass in my wristwatch, plus two handheld GPSes, a Raymarine plotter and metal pins in my leg), I think it's prudent to have a "GPS compass" to drive the autopilot. Unlike the fluxgate compass, such a beast will alert me when it loses a lock, giving me plenty of time to switch to one of several Plan Bs.

Naturally, one would plot DRs and lat/lons by other means, including sextant noonsites, compensated compass bearings and all the other means at one's disposal, following the axiom that the prudent mariner uses all the tools available to him, without necessarily putting 100% faith in any one tool.

Equally naturally, one would NOT go in on autopilot to a "suggested" waypoint via GPS, which would be dangerous if not culpable, just like setting "cruise control" would be in downtown Manhattan.

Frankly, it is the discrepancies between these various tools that alerts me to anomalies. I had a strong deviation appear recently in Lake Ontario in the Globemaster (the only thing I usually use on a light-air, sunny day) and sure enough, a check of the paper chart showed I'd sailed right over some large chunk of iron ore (or something) that was marked on the chart. Now "big compass screwer-upper" is a fresh GPS waypoint...one that can be used to confirm if the GPS is working: no compass wobble at that spot means the waypoint has drifted.
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Absolutely, Val.
In this age of miniturization it boggles the mind that the gyro compass has not been reduced in size and power consumption to the point of availability to the small boater. The ability to take accurate true bearings from a gyro repeater versus relative bearings from a pelorus, if equipped, is of immeasurable advantage not only in navigation but collision avoidance.

It might take the coastal fisherman more than a few minutes to give you his Lat/Long because he does not use that reference daily. For him, 25 miles SSE of Sandy Hook is sufficient. But then, he probably knows where the magnetic anomalies, affecting his compass, are and has a real good idea of compass error.

Appropriate skills appropriate to the situation. Over-reliance on any one navigational tool is ill advised. Not having a working knowledge of compass correction because of the presence of GPS is "over-reliance". And let's be honest; it takes about the same amount of time to learn compass correction as it does to wade your way through the GPS owner's manual.
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More USCG questions

3. Your ship is proceeding seaward down a channel. A pair of range lights are in line when viewed over your stern, and the masts of your ship are in line with the range. Your heading at this time is 152 pSC. From your chart you determine the direction to this pair of range lights is 317 True, and that the variation for the locale is 17 West. Which of the following is correct deviation for your present heading?

A. 32 East
B. 12 West
C. 2 West
D. 2 East


4. Enroute from Rio de Janiero to Montevideo, the ship is "swung" to take a round of bearings from the standard compass. With Ilha Rasa about ten miles distant, the swing is made and the following data noted:

Ship's Head pSC...........................pSC bearing on Ilha Rasa
000...........................................036. 5
030...........................................037. 0
060...........................................037. 5
090...........................................038. 5
120...........................................039. 0
150...........................................040. 0
180...........................................040. 0
210...........................................039. 0
240...........................................038. 5
270...........................................037. 5
300...........................................037. 0
330...........................................036. 5

After the swing has been made, the ship resumes the voyage on a course of 210 pSC. Which of the following is the correct deviation when the ship is on course 210 pSC?

A. 2 West
B. 1 East
C. 1 West
D. 2 East

Note: the above question is a deliberately tricky USCG question designed to determine whether you know your theory of compass correction. It is not a "practical" question. (anotherwords, lots of people get it wrong-never figuring out the point to the question)


5. You want to make good a true course of 210. The variation for the area is 9 West and the deviation of your compass is 2 East. The wind is Northwest and you should allow 3 degrees for leeway. What is the course to steer per steering compass?

A. 206
B. 217
C. 214
D. 220


Answers in Off Topic AFOC thread.


For a fuller understanding of variation and deviation see Bowditch arts. 709-711.
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Last edited by sailaway21; 07-19-2007 at 10:00 PM.
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  #40  
Old 07-19-2007
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I think I've got #3 and #5, but I haven't yet studied compass correction, so #4 is a mystery to me.

Thanks again for the tests

[Edit] Tricky indeed. Like I said: I haven't yet studied compass correction, but ISTM ... well, I don't want to give it away. But I think I may know the answer to #4, after all.

Jim

Last edited by SEMIJim; 07-19-2007 at 10:53 PM.
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