Tuesday, March 12, 2013

When Your Chart Plotter Fails, Part 2 of 2

New Bern, NC

The old fashioned skills of coastal piloting are still valid. You will need binoculars with a built-in compass, or a hand bearing compass. You also, need dividers, pencils, parallel rules and a triangle. You need a little practice taking bearings on two or more known objects on shore to estimate your position, and to lay a course from that position to where you need to go, and to calculate your distance from shore.

Don’t forget your depth sounder as a valuable adjunct to coastal piloting. In New England, the bottom is sufficiently featured that you can follow a line of constant depth to your harbor. In the south Pacific, I read that it was good practice to set your depth alarm at 100 fathoms to give you ample warning that you approach shallows.

I carry a placard on board called Emergency Navigation. It was designed to be used in a life raft, but it can be very useful on your primary vessel if you are way out in blue water and you lost all other means to navigate.

Use your VHF radio to query nearby vessels. In a genuine emergency you can ask the Coast Guard to triangulate your VHF position, or ask a ham radio net to triangulate your SSB position. Just make sure the emergency is real before you call.

Don’t panic. A rough position is much better than no position. Say you were returning to the USA from Bermuda when all your GPS died. Simply head due west toward Charleston. Your rough estimate of latitude only needs to be accurate enough to let you avoid Diamond Shoals off Cape Hatteras and Frying Pan Shoals off Cape Fear. If your uncertainty in latitude is two degrees or less, you can stay safely away from the shoals until you approach shore close enough to use your VHF and/or cell phone. It is probable that you will come within VHF range of another vessel along the way who can verify your fix.

Of course the classical backup is to carry a sextant, and to maintain the skills to use it proficiently. I’ve carried a sextant for more than 5 years, but I’m ashamed to admit that I never practiced enough to be good at it. Using a sextant and reducing the sights to lat/lon requires a lot of skill and practice to maintain. Although it is a recommended skill for all offshore sailors, I expect that many if not most of us fall short in proficiency even if we have the equipment on board.

Perhaps the most surprising, a new crop of smart phone and tablet apps are beginning to appear that let you use them to perform primitive sextant functions, to produce estimates of lat/lon without the Internet and without GPS. Some of them use internal storage of a sky map, and then compare the map with the image of stars seen by the built-in camera, add to that data from the gravity and magnetic sensors and the computer can calculate a sophisticated estimate of position. I’m dying to try out the app Star Struck Navigation that I downloaded to my Android phone. It has you tape a soda straw to the phone and sight stars through that. A major advantage these electronic methods have over a sextant is that they do not use a horizon at all. You can use them to sight stars late at night and far above the horizon when they are much easier to find. Expect major advances in such apps in the next few years.

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