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  Topic Review (Newest First)
02-18-2009 04:47 PM
Valiente Well, we are beginning to derail the thread, but that is what I was getting at: how the behaviours of sailors relying solely on GPS are on occasion premised on unsupportable or transitory data.

I prefer to think of GPS are "a really good suggestion with elements of a guess", and I don't use the plotter aspect, preferring to transfer lat/lon to a paper chart, and then to look at the paper chart, and then look out the pilothouse window, and then back at the compass, and then back out the window, and then I figure out if I actually am when I'm supposed to be.

And yet every summer you can see gloriously clean Bendytoys in Lake Ontario with a guy at the helm in a Tilley hat staring at a huge helm plotter display, more or less oblivious to his immediate surroundings.

I really enjoy sailing past such people in my monstrous steel tank about five feet away, and fouling their wind! I grin and wave and say "Nice day, skipper! How's she runnin'?" as I haul ass ahead. One guy actually dropped his drink when the shadow of my sails fell across his display.

Yeah, I'm a bit of a ***** sometimes. But it's actually a worse problem than idiots driving with a sandwich in one hand and a cell phone in the other, because of kayakers, canoeists and so on that these dopes aren't looking for.
02-18-2009 10:52 AM
sailingdog Valiente—

Unfortunately, too many GPS chart plotter users believe that the icon of the boat is actually where to boat is in the real world. What the icon of the boat actually represents is an assumed physical location, subject to the accuracy limitations of the satellite system being used, on a map makers interpretation of the real world, which may be based on information that is decades old. There's way too many qualifiers in that for me to rely on it. Mark I eyeball is always my primary navigation tool when in coastal waters... Dead reckoning, CN and GPS are all to be used when in open waters.
02-18-2009 09:23 AM
Valiente I think the "special case" is nominally mid-ocean places like the Tuamotus, and so on, an area both sometimes inaccurately charted, or inherently dangerous due to charted but awash reefs and atolls.

Sable Island comes to mind, as do places like the Torres Strait, the Barrier Reef and like this: Pacific Ocean - Elizabeth & Middleton Reefs (this whole site is great for weird little islands and reefs.)

If you chart a course and keep a running fix from CN, divergences due to current or steerage (like XTE with GPS) will eventually appear and can be corrected. Sailors used to plot courses to avoid land, but GPS sailors will, by contrast, tend to want to sail past land for the visual confirmation or because it's a more direct route. "Waypointers" don't always keep a prudent offing, in my view, because so much of the navigation is left to the plotter and the AP.

Because of its greater "circle of uncertainty", by contrast, CN encourages caution and land other than the destination is ideally avoided altogether.

How do I know this? From the changing nature of boating accidents and groundings...people are very precisely hitting the same damn rocks four feet below the surface of the ocean! We've all heard about people piling into breakwalls, huge buoys, piers and the like, sometimes at speed. It's sheer carelessness, aided and abetted by precision instrumentation that itself doesn't have a stake in staying dry.

I have to wonder if some sort of cutting and pasting of lat/lons goes on instead of looking at the right charts, and manually plotting a course that gives a good offing that is clear of fringing reefs and yet close enough to get into a viable anchorage promptly.
02-18-2009 07:49 AM
sailingdog Also, I'd point out that CN techniques are primarily for use when in the open ocean, and that a mistake of even up to 5 NM is generally not a big problem given that you're hundreds of miles from land. If you're within sight of land, you should be using coastal pilotage skills, not celestial navigation.
02-18-2009 05:05 AM
AdamLein By the time GPS fails from old age and is inaccessible due to the LEO minefield, somebody will have produced a sufficiently realistic sailing video game, so that the electroboaters who relied solely on GPS can enjoy sailing without ever leaving their homes, while mariners who practiced CN will have our sextants to fall back on.
02-18-2009 04:49 AM
Valiente You can always stop, or nearly so, and confirming latitude with a noon sight is a matter of seconds. Confirming latitude and longitude is less than 20 minutes with current methods, and a lot less than that if you are in practice.

So say you are in a light current of about two knots, and you are drifting with it in a 10 knot wind toward the suspected reef. With all sail down (or better yet, sailing to windward with reduced sail to get you practically treading water), you might move at worst 1 NM.

A good sight will give you accuracy of 1-2 NM, so yes, some planning is involved, but as some charts (including electronic ones) have an error of 1 NM or greater (especially the ones last done by Capt. Cook (!) although he was surprisingly good), I would hope you would be keeping a dead reckoning and start trying to determine your location a lot farther away than that.

Seeing discoloured water or breaking waves from five miles off with binoculars from halfway up a mast shouldn't be that hard in daylight, especially if you have a bearing in mind down which to look.
02-18-2009 02:27 AM
JiffyLube If GPS was out in certain areas, how far would a person travel while they were taking a fix using a sextant and time ticks before they actually got their fix?

We know that a fix by sextant is not all that accurate, but it will get you in the ball park...but the ball park could be pretty big in rougher conditions.

Sailing in reefy areas using a sextant could land you on a reef by the time you figured out where you thought you should be, but the sextant is better than nothing.
02-17-2009 12:55 PM
Originally Posted by timebandit View Post
Maybe it is time to break out the Star Wars stuff for a little target pratice.

A nuke test in the desert along with a ICBM long range test might remind some countries of some things they may have forgoten.
Indeed it might; but be careful what you wish for.
02-17-2009 11:33 AM
timebandit Maybe it is time to break out the Star Wars stuff for a little target pratice.

A nuke test in the desert along with a ICBM long range test might remind some countries of some things they may have forgoten.
02-17-2009 10:57 AM
sailhog I would think that there is the real possibility that there will one day be so much debris in low earth orbit that high earth orbit becomes all but inaccessible. When a nav. satellites come to the end of its service life, there may not be a way to foist another one up to replace it.
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