Some of the greatest technologies ever developed became obsolete in our lifetime: moveable type, for example, and telephone wires and film navigation, an arcane science that once took decades to master. Today, anyone can navigate faster and better than the most skilled seamen of yore with the help of an inexpensive and readily available global positioning system (GPS) unit.
Mind you, contrary opinions still abound. Yes, the government can degrade the GPS system at will or even shut it off. Yes, the satellites might be shot down. And yes, it leaves you just one faulty transistor away from disaster. That said, prudent seamanship demands use of GPS as the primary means of navigation, nowadays, and no boat should be without it. For backup, use a second (and third or even fourth) GPS device or else use a sextant, your choice. If you choose a sextant, you will also need a chronometer, and good luck in high seas, storm, overcast skies, or fog, which in many areas is most of the time.
Navigation consists of two things: knowing precisely where you are and knowing in which direction you should be heading. Any GPS unit provides a continuous fix that tells where you are, instantly and always, with precision that Bowditch could only dream of. Most will also plot your course and speed. It is still important to know the basics of traditional navigation, primarily for the concepts and vocabulary, so that you can read about it and discuss it with others. A good way to gain this knowledge is by taking the navigation course offered evenings or weekends by your local USCG auxiliary.
USCG auxiliary. Many GPS units contain charts that are condensed and omit some of the data found on a printed chart. They GPS unit charts might lack on-shore detail, for example, or bottom contours.
Many GPS units contain charts that GPS unit are condensed and omit some of the charts data found on a printed chart. They bottom contours. Such units can be used for steering, but not for navigation, and should be used only with a full and complete chart at hand. Be even less trusting of GPS units that have not been updated with the latest charts, as these are often not merely incomplete but downright wrong.
GPS
errorGPS units occasionally show erroneous data. A good GPS device of recent manufacture is accurate to within 9 feet only 95% of the time. The errors can be caused by the satellites, atmospheric conditions, or most commonly by the device itself. The way to deal with such errors is by having two (or more) GPS devices and comparing their outputs. If they agree, and particularly if they are acquiring more than a few satellites, you can have high confidence in the fix.
Electronic charts in GPS devices are just as reliable as paper charts of the same vintage and equally susceptible to the same shortfalls as a paper chart. The important thing is to keep them up to date, including any pertinent notices to mariners. Expect the data to have errors occasionally. Always plot a course that has a margin of safely to allow for such errors. Stay alert to recognize errors when they occur.
entryPerhaps the single most important piece Date of of information on a chart will be its date last of issue. It is dangerous to use an out- of-date chart and foolish to purchase one. NOAA publishes on their Web site an online list of every current chart, identified by number, along with its printing date and date of last entry (DOLE). check the date printed on it or else look for an Info or About choice on the main menu. On a computer or handheld device running navigation software, look for an italic “I” icon in the upper or lower right corner of the main screen. All US charts with the same DOLE will have identical features, unless some data has been abridged, because they all derive from the same government agency.
Online
chartsCharts obtained from NOAA over the Internet are always up to date and contain all of the data found on a printed chart. These charts can be downloaded, at no cost, into inexpensive navigation software on a laptop computer, tablet computer, or even a smart cell phone. A tablet like the iPad, freshly loaded with charts at the start of a passage and kept in a heat-conducting waterproof pouch, can in some ways outperform a chart plotter costing many times more. You do, of course, need to recharge it daily. Chart marks and
Online
chartsCharts obtained from NOAA over the Internet are always up to date and charts contain all of the data found on a printed chart. These charts can be downloaded, at no cost, into inexpensive navigation software on a laptop computer, tablet computer, or even a smart cell phone. A tablet like the iPad, freshly loaded with charts at the start of a passage and kept in a heat-conducting waterproof pouch, can in some ways outperform a chart plotter costing many times more. You do, of course, need to recharge it daily.
and symbols Nautical charts contain a plethora of symbols, abbreviations, and terms that are equally arcane on paper
Chart
marks
and
symbolsNautical charts contain a plethora of symbols, abbreviations, and terms that are equally arcane on symbols paper and electronic versions. These are all explained on Chart 1, which is actually a book available online in PDF format and as a reprint from 3rd-party publishers. Include Chart 1 in every chart download to a navigation computer.
Fix by
sextantWhen you can see three charted objects and the middle one is the closest, a highly accurate method of plotting your
by sextant position uses the sextant to measure the angles between the two pairs of objects. Plot these on transparent paper, lay it over the chart, and position the sight lines so they cross over the charted features. Their line of intersection at the apex is your position, within a few feet, more accurate than a GPS, and without introducing any compass deviation. High and low tides Tide and Current Tide results from the gravitational effects of the moon and, to a lesser extent, the sun. As it orbits the earth, the moon pulls most strongly at the water directly below it, raising the water, and it pulls most weakly at the water on the opposite side of the earth, raising that water also. As the earth makes one daily turn beneath the moon, each place on earth goes from closest to farthest and back again, experiencing two high tides with two low tides in between. Since the moon orbits the earth every 28 days, and does so in the same direction that the earth rotates, the actual time for this complete daily cycle is 24 hours plus one- 28th of a day (about 50 minutes) and so the tides come about 50 minutes later each day. Spring and neap tides During a new moon or a full moon, when the moon aligns with the sun, gravity of the sun adds to the pull of the moon making high tides higher, or spring tides. At the time of a 1st or 3rd quarter moon, when the sun’s pull forms a right angle to that of the moon, gravity of the sun subtracts from the pull of the moon and tides are lower, or neap tides. Peak and slack current Along with tides come tidal currents that flow as the tide rises and ebb as it falls. Peak current occurs midway between the slack waters of high and low tide and can often be several knots. The height of tide ranges from a few inches in parts of Florida to more than 30 ft in parts of Nova Scotia. It can rise higher, and fall lower, than the mean high and low water soundings shown on charts. State and height of the tide is important when anchoring, tying up to a non-floating dock, entering a shallow channel, and passing under a bridge, for example. State of the tidal current can have a huge effect on boat speed. Data about tides and currents traditionally came from tables published for particular places but today is more easily obtained from chart plotter data. Symptoms of seasickness Seasickness Some people never get seasick. Some always get seasick. Most fall in between. At least half the crew will be seasick on a typical offshore passage. Women get seasick faster and more often than men.