When your GPS fails, how do you navigate?

[Allan12210]

A professional Mariner will maintain and track his progress on a paper chart by plugging his position every hour even though he may be using GPS to navigate. If the GPS fails he will have a close indication of his location and then he can plot a dead reckoning course.

Good advice. When I am using the GPS on an extended passage, I always navigate either to or from a known waypoint. Every hour I take the distance (range) and bearing to that point (the GPS shows that counting either up or down) and transfer it to the paper chart in a little triangle with the time. It's the easiest way for me to accomplish. I set the dividers to the distance and using a plotting tool for the bearing to the object, then mark the chart. This is much simpler and faster than trying to interpolate latitude and longitude from the GPS on the chart and it gives the exact same answer. If the GPS conks out, I am within the hour of the last chart entry and take over with deduced reckoning. Sometimes I take bearings on objects ashore to confirm the readings, but with GPS I've become less disciplined with that method as the GPS is so accurate. I've been comfortable with the paper record of the GPS location. Its also easier when I can call my friend on the radio and ask where he is. If he gives me range and bearing to any point, I can locate him on the chart quickly as well.

 

[Eddie7045]

Carry one chart that encompasses the

entire route and all of the more detailed charts within that area. When crossing open water to an island, set a coarse to a point that is in the center of the island or group of islands and adjust your coarse as you come closer to the island. Commercial boats leaving San Pedro or Long Beach for Avalon, Catalina Island will head for the center of Catalina rather than directly at Avalon as Avalon is near the Southern most tip. They will then veer south as they approach Catalina. Similarly heading for Hawaii you would aim for the center of the island group which gives you a nearly 450 mile wide target to head for. That way you would likely sight one of the islands should there be a failure of your navigation equipment.

 

[Bryan9]

Situational awareness is really important... which is why I plot my course using paper charts and rely on the GPS/plotter only as a means of double-checking my position. As a refresher to navigation courses taken long ago, or as an introduction to the subject, I highly recommend Bill Bragdon's BOAT NAVIGATION FOR THE REST OF US: FINDING YOUR WAY BY EYE AND ELECTRONICS (Camden, Maine: International Marine, 1995). Bill emphasizes my point about situational awareness. The book is probably out of print, but well worth a search on eBay. Some of the material is dated -- LORAN is covered -- but this thread is about the timeless stuff, and Bill does a magnificent job of explaining this material in approachable, readable prose.

 

[Maine Sail]

I'd just go back to how I did it for 30+ years before GPS...

Here are the REAL QUESTIONS..? (based on real world experiences working on hundreds of different boats)

• How many of you have a professionally swung & calibrated compass with a deviation card?

• How many of you even have a professional compass adjuster left in your area? Do you know them? Have they been to your boat recently?

• How many are aware that a lightning strike can not only can take out electronics but can also throw the compass compensator rods way out of adjustment.

• When was the last time you had your compass checked or professionally swung?

• When was the last time you changed any electronics near your compass?

• How many keep a ferrous Leatherman etc. or a battery powered HH VHF next to or near your compass?

• How many are aware that the many of the new plotters use MAGNETS in them to keep the chart door closed. This can be compensated for in the compass but requires a professional to do it correctly.

I always get a grin out of some of the old schooners who's boats I work on. In 90% of the cases there is no deviation card to be found. How is that DR working if the compass is out by 15 degrees or more on both cardinal and inter-cardinals??? When I make any changes to cockpit electronics I always ask for the deviation card and then run the boat on the known cardinals and some known inter-cardinals to compare.

On nearly 85% of the lightning investigation/quotes I do the compass has been whacked hard yet most boaters and insurance companies never even check for this.

Things to think about.........

 

[Kings Gambit]

I'd just go back to how I did it for 30+ years before GPS...

Here are the REAL QUESTIONS..? (based on real world experiences working on hundreds of different boats)

• How many of you have a professionally swung & calibrated compass with a deviation card?

• How many of you even have a professional compass adjuster left in your area? Do you know them? Have they been to your boat recently?

• How many are aware that a lightning strike can not only can take out electronics but can also throw the compass compensator rods way out of adjustment.

• When was the last time you had your compass checked or professionally swung?

• When was the last time you changed any electronics near your compass?

• How many keep a ferrous Leatherman etc. or a battery powered HH VHF next to or near your compass?

• How many are aware that the many of the new plotters use MAGNETS in them to keep the chart door closed. This can be compensated for in the compass but requires a professional to do it correctly.

I always get a grin out of some of the old schooners who's boats I work on. In 90% of the cases there is no deviation card to be found. How is that DR working if the compass is out by 15 degrees or more on both cardinal and inter-cardinals??? When I make any changes to cockpit electronics I always ask for the deviation card and then run the boat on the known cardinals and some known inter-cardinals to compare.

On nearly 85% of the lightning investigation/quotes I do the compass has been whacked hard yet most boaters and insurance companies never even check for this.

Things to think about.........

Maine Sail: Is it a must that one needs a professional compass adjuster to make a compass deviation table or graph for it to be reliable? Suppose a boater wished to make his/her own using a set of true headings determined by GPS. For example, for a GPS true heading of 000 deg, compass should read 347 deg @ 13 deg E variation @ no deviation. However, if compass reads 350 deg then there is 3 deg W deviation attached to that compass (magnetic) heading.

Thus, coverting compass reading of 350 back to true course (CADET): 350 - 3 deg W deviation + 13 deg E variation = 000, etc. Wouldn't this work?

 

[Johnb]

Maine Sail: Is it a must that one needs a professional compass adjuster to make a compass deviation table or graph for it to be reliable?]

You can easily find out for yourself if your compass is grossly out. Just get some bearings off of Google Earth, know the compass deviation at your location, line up your boat in at least 4 directions at right angles to each other. You would know if something you have installed or is nearby is messing it up.

After that, it gets way complicated.

http://www.compassadjustment.com/

is an interesting read.

We have the binnacle compass, a handheld magnetic compass (Davis Instruments) and a handheld electronic (Autohelm) and take heart from the fact that they indicated pretty much the same in multiple directions in different locations on the boat.

 

[Kings Gambit]

What if they all agree?

Yeah-- altogether I have the binnacle (Suunto) at the helm, a Ritchie at the nav-station, fluxgate (Autohelm) forward, Davis hand-held, plus another one in the Fujinon Polaris; I think I'll start keeping records of periodic checks against one another.

 

[Maine Sail]

Maine Sail: Is it a must that one needs a professional compass adjuster to make a compass deviation table or graph for it to be reliable? Suppose a boater wished to make his/her own using a set of true headings determined by GPS. For example, for a GPS true heading of 000 deg, compass should read 347 deg @ 13 deg E variation @ no deviation. However, if compass reads 350 deg then there is 3 deg W deviation attached to that compass (magnetic) heading.

Thus, coverting compass reading of 350 back to true course (CADET): 350 - 3 deg W deviation + 13 deg E variation = 000, etc. Wouldn't this work?

Once you see the process done by a professional (they are sadly a dying breed) you will quickly realize you probably don't have the expertise to do this with any good level of accuracy. When I work with my compass adjuster it is sometimes 1.5 hours to get it right. He knows just how much he can tweak a compensator rod to get a desired effect. We run all the cardinals at least three times then the inter cardinals at least twice. This takes considerable time and patience and two guys, one to steer and one to adjust...

His reference compass is large and unaffected by anything on the boat so that is what we use as the known reference. After our lightning strike it took two adjustments over a few months because the compensator rods had been affected and were still changing. For me on the foggy coast of Maine it is the best $150.00 piece of mind I spend....

 

[Kings Gambit]

Forgive me if I seem dense here. But it sounds by your description as though the professionals are compensating the compass to erase deviation. What I was asking about was making one's own deviation card from data; not attempting to compensate the compass per se, but to manually convert the Compass course to True Course by reference to the deviation card.