Piloting

[HenryC]

Now that the magazine I wrote for has gone out of business, I guess its OK to share some of the articles I wrote for them with you


Henry Cordova
Small Boat Navigation I: Piloting

The capabilities of the Global Positioning System (GPS) are so obvious, beneficial and available at such a reasonable cost that I feel every boater should take advantage of this technology.  But having said that, boats still get lost, run out of fuel and run aground. Just as radar did not make collisions a thing of the past, GPS will not prevent errors in navigation. It is no guarantee you will not get into trouble or suffer the consequences of bad seamanship.  Like any other device, it is vulnerable to malfunction or power failure, especially in an environment as hostile to electronics as the sea. Remember, the infrastructure it relies on is highly complex, fragile and dependent on a massive government bureaucracy to maintain it.  And as with any other system, if you don't trust your instruments or if you get confused and lose confidence in your equipment, it doesn't matter how well it's operating.

There is a navigational instrument even more important, valuable (and dependable) than GPS,  the magnetic compass. It has only one moving part, requires no power source and depends solely on the earth for its operation. It is also extremely easy to use, it  has no control panel and no controls. It provides navigators with a knowledge of direction, and at sea, direction is even more important than position.  Not knowing where you are is not a problem as long as you know which way to go, and if you don't know what course to steer, a precise knowledge of your position does you no good anyway!  It is alarming how many boaters today know their way around a GPS keypad but only have a vague idea of how to use a compass.  Without some mechanical aid, only a knowledge of the sky can give you a true sense of direction and even an astronomer is lost when it is overcast. 

Most boaters understand this, but many do not realize that precise navigation is also possible with a compass.  Using one, along with a chart, is the basis of the ancient art of piloting, the science of navigating by means of landmarks.  You cannot fully learn piloting from an article like this, but there are books, and websites which can teach you, and boating clubs and the US Power Squadron offer courses.  Hopefully, this will inspire you to follow up on this vital safety skill. Piloting principles are straightforward and common sense and almost anyone who knows them will be delighted to share his knowledge with you. Old salts love to pass on this information to a new generation of boaters.   It isn't just a backup skill for when your GPS is down, it is an art in itself and can provide great personal satisfaction and add to the joy of boating. 

Compasses consist of a magnetized disk, or card, which pivots around a central balance point.  Along the edge of the card are printed numbers, from 0 to 360 degrees.  These numbers ( called collectively the compass rose because it is  traditionally decorated with fancy artwork) represent direction so that 0 (or 360) is north.  90, 180, and 270 are east, south and west, respectively.  All you need is one simple number to unambiguously specify a direction. So for example, north-east is the direction between N (0 degrees) and E (90 degrees), or 45 degrees.  When you give a command like "steer 272" it means that the boat should be steered so that the bow is aimed in the direction of 272 and the steering  compass (which is installed so that it reads what direction the boat is pointing) will read 272 (just a bit north of due west).  The compass is built in such a way that the card always points to earth's North Magnetic Pole  (0 degrees) and as the boat changes direction think of the card remaining still but the boat turning under it.  What the mariner sees while steering is the numbers on the card flashing by until the boat is pointing in the direction he wants.  Most steering compasses have the card immersed in a clear oil to damp out movement caused by the boat's motion and it is helpful to think of the card as being rigidly fixed to the earth's magnetic field with the boat freely swinging beneath it.

Compass roses are printed on nautical charts, too.  In fact, there are usually several of them placed in various blank places about a chart for the convenience of the navigator and they contain a rose for true directions on the earth's surface, as well as another with the magnetic directions.  They are both clearly labeled. Unfortunately, true geographic direction and magnetic direction do not always coincide and may differ by several degrees;  they also vary from point to point on the Earth's surface, as well as over time.  Since you will be using a magnetic compass, use the magnetic rose for piloting.  The "true" directions are useful for celestial navigation and for GPS work, but when using the magnetic compass for steering, you will have to use the magnetic rose. It is usually printed on the chart inside the "true" rose.  There are other issues with magnetic compass correction, such as variation, deviation, and adjustments which are outside the scope of this discussion and which must be understood before you're ready to solo; but the basic principles should be clear from what follows. 

The fundamental task in piloting is laying out a course, or determining what course to steer to go somewhere.  Here's the procedure:  first locate on the chart where you are (for example, the marina where your boat is docked) and the destination you wish to go to (say, a secluded cove across the bay where you will rendezvous with friends).  This is the equivalent of entering a destination or waypoint into your GPS.  Next, draw a straight line on the chart in pencil between the two points.  This will be your course, the invisible line on the water your boat must travel to reach its destination.  Using your parallel ruler, or drafting triangles, "walk" this line (making sure to keep it parallel to the original course line you drew) to the nearest compass rose on the chart and read off the magnetic course, or angle, denoted by that line. This is the course that your steering compass will read once the boat is pointed at your destination. As a helmsman, your task is to get the boat moving and steer until your compass reads that course, then go.  In theory, if you've done everything right, and if there are no external forces such as winds and currents acting on your boat, you will eventually reach your destination. Of course, if there are one or more obstructions between you and your goal, you will need to break up your course into several legs to get around them, with separate courses for each.

Incidentally, parallel rulers are a drafting instrument used to move a line around on a sheet of paper without changing its direction.  They consist of two rulers connected by a mechanical linkage which keeps them parallel to each other.  They are available at any marine store.  A bit puzzling at first, once you've seen somebody use one you will understand exactly how they work.  Some navigators prefer twin drafting triangles, a protractor, or a Breton plotter to do the same job.

Of course, your boat is not on rails; there are forces as well as steering errors which tend to push you off course.  But the initial course line is your first estimate, it allows you to identify the landmarks and aids to navigation in your path that will allow you to correct your course over time, and it will alert you to hazards along the course to watch out for.  Both hazards and landmarks, as well as much other useful information, is plotted on the chart. When using a GPS, the computer is constantly consulting the satellites to update your position, and continuously recommending a course to reach your waypoint.  As a pilot, you are going to have to do this job yourself.  Piloting provides a variety of techniques to help you correct your position as you travel, allowing you to alter your course as needed to ensure you arrive safely at your destination.

As your craft gets further from the familiar waters near your marina, compare the landmarks visible to you on shore or in the water (buoys, range markers, obstructions, lights, etc) with those on the chart.   Charts plot many prominent objects carefully, and label them, so that the seaman has a selection of features to help with his piloting.  These may be official aids to navigation such as buoys and lighthouses or perhaps prominent buildings, radio towers, smokestacks and so on.  They are marked for identification (for example, "easternmost of three water tanks", or "steeple") and their positions carefully plotted .  If you can identify several of these landmarks and locate them, it is possible to determine your position by taking bearings. 

A bearing, or Line of Position (LOP) is like a course; it is a line of constant direction from you to a distant object, although it is not necessarily going to be a path for your boat.  Let us say you are on the water and you visually identify several objects ashore which also can be located on your chart. If you were to aim your boat at these objects and read off your compass headings to them, it would give you a list of bearings to those landmarks.  There is no need to actually use your boat and steering compass as an aiming device, most boaters carry a smaller bearing compass which can be aimed like a gun at those landmarks to read off the angles directly.  You might determine that, say, the bearing to the "lighthouse" was 21 degrees, and the bearing to the "center span of the bridge" was 105 degrees.  By locating these landmarks on the chart and using the compass rose on the chart to draw the LOPs, then "walking" them with parallel rulers to the appropriate landmarks, it is possible to draw these LOPs on the chart.  Your boat is somewhere on each of these LOPs, so where they cross must be your position!  Of course, we are assuming that you took all bearings very close together in time, so your boat's motion between the observations was negligible. 

If you take three bearings on three objects and they cross at one spot on the chart, you are assured you have identified all three objects correctly and that your position is accurate.  In other words, you are triangulating your position, doing the same thing with your sightings that your GPS unit does with radio waves from satellites.  Two LOPs crossing will give you a position, but it may not be accurate if you misidentified a landmark or made a drafting error.  If three LOPs do not cross near the same spot you know at least one was in error, but you won't know which one.  If you shoot four bearings you can afford to make one blunder and still get a good position (or fix, to use the navigator's term).  The three good ones will cross close together, forming a small triangle, the bad one will miss the intersection point of the other three altogether. In general, the quality of a fix is the size of the little triangle formed by the three LOPs. Sailors call it the "three-cornered hat".  A small one tells you the fix is a good one.  A big one tells you either misplotted, misread your bearing compass or mistook a landmark.  Of course, two LOPs can give you a perfectly good fix, but you'll have no way of confirming that you didn't make a mistake.  In other words, you won't trust it.

Piloting is a skill which depends on practice and experience, judgement and skill; but in practice it is based on the simplest of geometrical principles.  The aids to navigation marked on a chart are identified and catalogued by number, and their characteristics are clearly written down to help you identify them, even in the dark.  Every Coast Guard-placed light, Aid to Navigation (ATON) on the water has it's own flashing characteristics written down on the chart so you can clearly identify and locate it, even against the clutter and confusion of background lights on shore.  This is what all those cryptic messages on the chart tell you ("7", flsh grn, 4 sec) .  Even in a crowded harbor in the dark, you will be able to find that 4 second flashing green,  number 7 buoy and be able to shoot your bearing.

The practice of navigation involves sailing along your course, taking frequent bearings and plotting your fixes on the chart as you go to ensure you are where you think you are and on schedule.  If your boat starts to drift off course and you are confident your new positions are correct, you may have to lay out a new course to your destination, starting from your last good fix.  The chart will show you if these new courses stand you into danger, and they will also suggest new landmarks for bearings up ahead, landmarks you may not even see yet and that you will need to know where to start looking for them.  Perhaps the easiest way to visualize this is to consider how lighthouses were once used for coastal navigation.  In the old days, before GPS and radar, dangerous coastlines were dotted with lighthouses placed far enough apart so that a ship offshore on a safe course parallel to the coast could shoot bearings and ensure it was still on course, neither too close to shore and standing into danger, nor too far offshore to have no lighthouses visible. The lights were spaced so that at least two were visible at any one time, sometimes three, so that navigators could keep up a running plot by triangulation to make sure they were on track.  As one lighthouse would fall below the horizon behind them, one would always be reassuringly visible abeam and the navigator could glance at his chart and know the next one would soon be poking up it's beacon over the edge of the world ahead, He knew approximately what its bearing would be because his position was well known and constantly being updated.  The height above sea level of each light was also printed on the chart, allowing the pilot to calculate how far away the light was across the curvature of the earth, as a check on his other calculations.  After the fix was plotted, the distance to the light could be scaled off the chart to verify it was the correct one. If the fixes clustered along the course line, the situation was under control, and even the ship's speed could be determined.  There is no feeling in the world like going up on deck and watching a distant lighthouse pop up over the horizon just where and when you expect it, right on schedule. Your crew will envy and admire this feat of mastery over arcane knowledge.  They're not likely to mutiny, either, if you're the only one aboard who knows how.   It's a feeling no soulless GPS screen can give you...

You owe it to your shipmates to master these skills before you go to sea and to avoid relying on one method of navigation alone.  The tools needed for piloting, are simple: a steering compass, a bearing compass, a chart, and a few simple drafting aids like pencils, parallel rulers and dividers.. The basic skills are easy to learn, you can pick them up in an afternoon. But getting really good at them can take a lifetime.

[HenryC]

The previous article ("Piloting")  that this is a reply to should be read first.

Henry Cordova
Small Boat Navigation II:  Dead Reckoning

Bay or coastal cruising should be like walking around your house in the dark, you should know where everything is and where not to step. After gaining a little experience, you will use your piloting skills to plan your voyages so that you maximize the navigational potential of the waters you are sailing.  For example, you will learn how to lay out your courses and waypoints so that if an unexpected problem occurs (like an Aid to Navigation (ATON) has been blown away by a storm), you will have alternatives ready and mapped out. Don't paint yourself into navigational corners and always have a Plan B..  Once this all becomes second nature, (and piloting will teach you this), you can really enjoy the convenience and reliability of a GPS because you will know that if it does fail, you don't really need it.  But what happens when you can see no landmarks, when distance from land, or darkness, or bad weather leave you sailing alone, isolated from the rest of the world? The answer is dead reckoning.

The term "dead reckoning"  is of uncertain origin, but probably derives from "ded" or "deduced" reckoning. It is the art and science of navigating by making judgements of a ship's position based on its estimated speed and course and informed guesswork about the forces, like winds and currents, acting on the craft.  It is used to come up with estimates of position so that the crew can anticipate hazards, or new landmarks which can be used to establish a new position or fix.  In conjunction with piloting, it is the primary non-electronic means of small craft navigation, and until quite recently, it also played a major role in the navigation of large vessels on the high seas.  In practice, piloting and dead reckoning are intertwined and inseparable, and both are learned and applied simultaneously by professional navigators.  The traditional methods used for determining a fix, either by piloting or by sextant observations of celestial bodies, provide the raw data and initial estimates upon which dead reckoning operates.  Fixes provided by GPS can also be used as a basis for dead reckoning, and the old and the new can be used as a check on each other.

For example, consider the classic case of a coastal cruiser, traveling from one harbor to another.  The navigator would lay out a compass course line from his starting point to his destination and having a rough idea of the speed of his vessel, would mark that line on the chart with ticks indicating where he expected to be at certain times during the voyage.  As the ship travels, these location estimates, or estimated positions (EPs), would suggest which landmarks, such as lighthouses, geographical features, or large buildings marked on the chart, would be visible and available for triangulation and determining precise fixes.  As these fixes are plotted on the chart, the navigator gets an idea if his vessel is being slowed down or speeded up by currents, or if it is being set to one side of the course line or the other.  A new course line, a dead reckoning or DR track, is layed out every time a good fix is achieved and plotted on the chart.  In this way, the navigator always has an up-to-date estimate of his location based on solid data. If at any time his source of external navigational information is cut off, say if his ship is too far offshore to see charted landmarks, or bad weather sets in, he has a recent fix and a DR track which will give him some idea of his position for some time into the near future.  The time spent dead reckoning may be a few minutes, as when a small boat is crossing a foggy harbor, or several weeks, as when a ship is crossing an ocean, but the principle is always the same.  The dead reckoning position will be all the navigator has until he gets the next fix, so it is essential that fixes be taken frequently when conditions are good so that when they deteriorate a solid, recent position will be available to base estimates.  Many navigational techniques, such as piloting via triangulation, radar ranging, LORAN, radio direction finding, and even celestial, all require a rough idea of one's location as a seed position, so this is essential.  Even a ship successfully navigating with GPS should maintain a good DR track as a check and backup in case the GPS should fail or be misinterpreted. 

After each fix is plotted on the chart, a pencil course DR track is drawn to the next waypoint (destination or turning point) and ticks on the course line drawn with the time the ship expects to get there.  These estimated positions are drawn with circles around them (proper fixes are marked with triangles), also with the time.   Using the familiar "DRT" equation, Distance equals Rate times Time, or D = RxT and the laws of algebra, we can also solve this expression for either Time (T = D/R) or Rate (R = D/T).  There are nautical slide rules that can do this calculation for you, or you can use a pocket calculator.  The point is if you know any two of the variables in the DRT equation, it is easy to solve for the third.  For example, if you have been traveling for 20 minutes since your last fix at a speed of 12 knots, you will be 4 nautical miles along the DR track.  D = RT where 4 = 12 x 1/3.  (Remember, 20 minutes is a third of an hour.) 

A knot is the nautical term meaning "1 nautical mile per hour".  Never say "knots per hour". It is lubberly and not to be countenanced. A nautical mile is roughly 2000 yards, or 6076 feet.  Nautical miles are convenient at sea because the nautical mile is equal to 1/60 degree, or one minute of latitude, and the latitude ticks are always drawn on the east and west sides of the chart.  Use the dividers drafting tool to measure off distances from the latitude scale and transfer them to your DR track, or conversely, to measure distances on the chart and scale off the distances using the latitude scale.  Nautical charts usually employ the Mercator map projection, so distances on the chart can be measured directly off the latitude scale with your dividers. You don't have to learn all this today, dead reckoning is something you'll need instruction and study for and plenty of practice.  Just keep in mind that the DR track on the chart will serve as a record of your voyage and a paper computer to help solve navigational problems.  Your sources of information will include your compasses and your log (the device which indicates your speed through the water).  Incidentally, it is important to keep in mind that the log measures speed through the water, you need to know speed over the ground, so a constant comparison with your speed as revealed by your navigation and that indicated by your log will give you an insight into the currents affecting your boat.

The practice of navigating a boat using piloting and dead reckoning all revolves about the DR plot drawn on the chart.  Here is the record of all  navigational observations and calculations, a projection of future plans, and all the information the makers of the chart were able to include, all drawn in a systematic manner any seaman will be able to understand at a glance.  Make sure that if the vessel has to maneuver, or deviate from its course,  that the DR track is updated to include this change of course and/or speed.  The DR track represents the best possible estimate of your position until you plot your next fix.  It must reflect every maneuver and the exact time when it occurred.

Dead reckoning also allows the skipper to develop estimated positions based on partial navigational information, in addition to calculations or extrapolations of course and speed.  For example, suppose only one feature on shore is available for a bearing.  The vessel will lie somewhere on that line of position (LOP), but exactly where will not be known for sure.  The navigator can use as an estimated position (or EP) the intersection of the LOP and the DR track.  This is not as trustworthy a position as a proper fix, but it is better than most estimates because some real observational  information contributes to the estimate.  Later on, after that landmark is no longer visible, a new landmark may come into view.  The pilot can then shoot a bearing on that, plot the LOP, and use his knowledge of his course and speed to "slide" the previous LOP along the DR track using his parallel rulers.  The course and speed are already roughly known, so the old LOP is moved along that DR course (keeping the same angle to the chart grid) for a distance corresponding to the time since the last bearing (using the D = RxT equation).  The new and old LOPs, along with the DR track, will provide a triangle on the chart and the boat will probably be somewhere near it.  Of course, this is not as good as a proper fix made up of two or more simultaneous LOPs, but it's better than a sheer guess.  This procedure is called a running fix, and very often it is all a navigator has available when sailing down a barren coast where charted features are few and far between. 

There are all sorts of tricks you can employ to come up with a serviceable EP, or to help stay on track.  For example, suppose there is a featureless island near your boat, no landmarks are visible on it but you can always shoot bearings to either end; the tips of the island, after all, are landmarks, they are plotted on the chart!  Of course, you will have to convince yourself you are actually shooting the ends of the island, not a cape or promontory which obscures your view of the extremities of the land.  These are called tangent bearings.  Another trick is to select your course so that if you have to make a turn to a new course as you approach your destination (perhaps to avoid a reef), the point where you make your turn will be one where some landmark bears a certain bearing.  As you approach the spot where you plan to make your turn, you shoot bearings until the turn bearing you calculated earlier turns up on your compass. A related technique is the danger bearing, where you calculate ahead of time some bearing which must be greater or less than some value to ensure you are in safe waters.  Another technique is the range. For example, if you know a light on shore can be seen from, say, 21 miles away, draw an arc of radius 21 miles on your chart, centered at the light and when the light comes into view you know you are somewhere on that arc.  Range arcs are LOPs too, and they can be crossed with others to give a position. Again, none of these techniques is as reliable as a proper triangulated fix, but they are far superior to nothing at all.  On the other hand, a range derived with your boat's radar can be extremely accurate. Piloting and dead reckoning can be done with radar too, it is not just a collision avoidance system, it is a very useful tool for the navigator although it sometimes is difficult to properly interpret radar images.

It can't be emphasized too much that coastal piloting and dead reckoning are not skills that can be picked up by reading a magazine article or two.  The actual mechanics of plotting fixes and taking bearings as well as laying out and maintaining a DR track are fairly straightforward, but integrating all this information into a navigational plan requires experience.  Let's look at a typical navigational problem.

Your task is to sail to an island offshore, well out of sight of land to the east.  The island is long and thin, and it lies across your path.  The harbor which is your destination is at the southern end, the rest of its coastline is bordered by dangerous reefs.  There are no aids to navigation on the island, or on your way there, available to guide you.  If you should miss the island in the dark, you could sail on past it out to sea.  The usual drill is to simply draw a line on the chart from your point of departure to your destination, determining the course and distance from the chart, and getting underway.  Your knowledge of your boat's speed, or your log, will give you a rough idea of your speed, and how long it will take you to get to your destination, so you will mark off your DR track where you expect to be at certain convenient time intervals (an hour, a day, whatever the length of the trip requires).  You try to plan your voyage so you will arrive at the vicinity of the island well after daybreak, not too early or you might stumble onto it in the dark, not too late because you want a full day's daylight to work with once you start getting near it's dangerous waters.

It's highly unlikely your vessel will remain on its DR track throughout the entire voyage, or that you will adhere to your schedule as planned.  Wind and current will constantly throw you off, so it is possible you will be quite off your original DR track by the time you get to the vicinity of the island.  If you are set to the north of your DR track, you will hit the island for sure, but recall the harbor is at the southern end of the island, if the current sets you to the south, you might miss the island altogether in the dark or in bad weather.  It is better to set your original course and speed to the center of the island, so if you're off a bit you are not as likely to miss it.

As you sail away from your home port, take as many fixes as possible on known landmarks and determine how your true course and speed match up to your DR.  This will give you some idea of how the set and drift induced by wind and current affect your boat.  Make sure any maneuvers you make are reflected in your DR plot.  Once you're out of sight of land, you're going to have to use this knowledge to help correct your course.  As you approach the island, take tangent bearings to try and develop a fix, and if any landmarks on the island are marked on the chart, try and locate them to verify your position.  Any information on the chart might help develop an estimated position.  For example, if the island is surrounded by a shallow shelf that is marked on the chart, keep an eye on your depth finder.  Suddenly crossing into shoal water is a rough, but useful LOP!  It might tell you how far offshore you are where distances might otherwise be difficult to judge.  If you also have a GPS aboard, navigating with it while you practice these traditional skills is a great way to learn.

Even if your GPS and other electronic equipment are functioning perfectly, the ancient arts of piloting and dead reckoning are a useful backup for emergencies that also will enhance your enjoyment on the water, as well as give you great confidence and peace of mind.  But most important, they will put you in spiritual connection with all the generations of mariners that went to sea before you.  It's the difference between being a weekend boater and being a seaman.

[Salty19]

Henry,

As always---a most valuable contribution!   Thank you for sharing your knowledge with us!