Telling Time by the Stars

As is the case with tricks for finding directions from the stars, there is no exclusive way to tell time from the stars, so we are free to make up whatever method works. To make up generalized star clocks that work on any arbitrary day of the year, however, does require some background, to be reviewed here. It is much easier to make up specific clocks on the spot, using a correct watch to calibrate it for the present date, and then use it on following nights by applying a simple daily correction. This does not require special reference books and calculations.

To tell time from the Big Dipper, as one example of a generalized star clock, imagine its pointers as the end of clock hands whose pivot point is Polaris and imagine a 24-hour clock face printed backwards on the sky around Polaris as shown in Figure 1. Midnight (0000 hours or 2400 hours) is straight up from Polaris; 0600 hours is to the west of Polaris and 1800 hours to the east. In 24 hours, the pointers sweep counterclockwise once around this clock face.

When the clock hand points straight up from the horizon, the clock reads midnight; when the hands point east with the pointers lying parallel to the horizon the clock reads 1800, and so forth. To read the clock at any time of the night, estimate the hour and fraction of an hour from the relative orientation of the pointers on the imaginary clock face. That’s all there would be to it if the sun kept pace with the stars. But the sun does not keep pace with the stars, and our daily time keeping is based on the sun so we must make a correction for this.

 All star clocks are fast; they gain 4 minutes each day because we keep track of time relative to the location of the sun, and we are moving around the sun relative to the stars at a rate of about 1º per day (360º/365d). Thus when we make our daily 24h rotation from noon to noon (relative to the sun) we are then 1º farther along our orbit, so we have passed any stars overhead by 1º. At our daily rotation rate of 360º/24h this 1º is equivalent to 4 minutes.

If you look at the same star on successive nights at the same time, it will be 1º farther (more westward) along its path across the sky. Thus if you want to see it at the same place on successive nights, you have to look 4 min earlier. This is basically how new stars appear on the eastern horizon at sunset as the seasons progress—although that is a bit more complicated because the time of sunrise is also changing. (We learn star positions relative to Aries, so check out the value of GHA Aries on successive days at the same time and you will see it increases by about 1º.)

At a gain of 4 minutes per day, star clocks gain a whole day in one year, so all star clocks reset themselves on a particular date that depends on the particular star clock in use—and by star clock we mean any two stars with the same SHA so the line between them rotates around the pole. The Big Dipper star clock resets itself on March 8th so all corrections must be reckoned from that date. (Official scientific star time used by astronomers resets on the Vernal Equinox, March 21st; the shift to March 8th comes about because scientific star time does not use the Big Dipper pointers for a reference line.)

To tell time from the Big Dipper, we need to know how many days have passed since March 8th. The time we read directly from the star clock is then fast by 4 minutes for each of these days. As an example, suppose the date was September 22nd and the stars looked as they do in Figure 1, with the star clock reading 0830. September 22nd is 198 days past March 8th, so the clock is fast by 198 × 4 minutes, which equals 792 minutes, or 13 hours and 12 minutes. The first 12 hours of the correction just switches the time from AM to PM, so the correct time of night is 2030 - 0112, which equals 1918, or 7:18 local time.

Figuring the correction is a bit involved, but this preparation need only be done once, after which the results can be rearranged to be more convenient. On September 22nd, for example, you could make an equivalent new rule for reading this star clock: change the star clock time from AM to PM (or vice versa, later in the night) and then subtract 1 hour and 12 minutes. Each subsequent night, you would subtract an extra 4 minutes, because the clock is still gaining time each night.

The time you figure from the corrected star clock will be the proper standard time for your time zone to within, at worst, some 30 minutes. It would be exact only if you happened to be located right in the middle of a time zone, each of which is about 1 hour wide according to star time. Star clocks also do not know about daylight saving time, so when daylight saving time is in effect, you must add 1 hour to the final result. Corrections for both longitude (the time zone correction) and for daylight saving time can be made simultaneously if you calibrate the star clock with a known time. In the last example, if the uncorrected star clock read 0830 AM at a time you knew was 8:10 Pacific Daylight Time, the rule becomes much simpler: subtract 20 minutes tonight, and then 4 minutes less each subsequent night.

The final accuracy of the time obviously depends on how accurately the star clock itself is read, which requires an estimate of the angle between the clock hand and the horizon—similar to reading a stylish watch with no numbers on the dial. Sticks held in one line with the Pointers and one with the horizon can help with this. The angle found this way can then be transferred to a sketch of the clock or to the compass rose of a chart. Reading the clock by eye alone, however, is usually adequate. Note that in normal circumstances most people have an adequate sense of time even without a watch, but under a great deal of stress this is not the case at all. During long storms at sea, it is possible to even lose track of how many days have passed. This is not likely to happen in a routine cruising, but one could imagine getting caught in coastal waters at night without a safe harbor nearby. If the wind and seas began to build on top of this, one could easily muster enough stress to lose track of time. Without a watch, you could monitor the duration of the adventure with the stars.

(Note: A star clock resets when the common SHA of the two stars making up the clock hand leads to GHA = 0º 0' at 00 UTC. For the Big Dipper clock, Dubhe and Merak have SHA = 194º 4.2'±14.3', so we need the nearest date when  GHA Aries = 360º - 194º 4.2' = 165º 55.8' at 00 UTC. You can get rough estimate from the Planet Diagram, or interpolate the Almanac to find that this is March 8.)

Stargazing for orientation in time and space clearly requires some hands-on practice. It is not like learning the combination to a lock, that once memorized can be opened at will. It is more like learning to play a kazoo. You start by learning to play a few notes well, and pretty soon you are playing a fine tune. And the enjoyment to be had from exercising this skill can be just as rewarding. It is one way to get in a little more in tune with a dependable part of the environment.

The above notes are adapted from our book Celestial Navigation: A Complete Home Study Course.

Sample of a Nocturnal used to tell time based on the location of the Guards, the two stars indicated here at the tip of the horn that we call tip of the Little Dipper. The brighter one is Kochab, which together with Polaris makes the clock hand. This is held up to the sky with Polaris in the center and the horn aligned with Kochab and then the time is read from the pointer, presumably with some instructions on how to make the correction from what is shown.

This concept and some form of this device was used on land and sea for "hundreds of years" before Prince Henry started his Navigation School in Portugal (~1415) according to EGR Taylor, a prominent navigation scholar.

Boxing the Compass

This name for the process of listing or reciting the points of a compass card arose after 1851 and before 1911. In the 1851 Bowditch “boxing” was a verb meaning to back wind the jib. In 1911 edition it was used as is done today. On the other hand, the size and concept of a compass point (11.25º) dates to the earliest navigation records from the 16th century.

The call for this note came from reading the 1851 edition of Bowditch; in particular the log of his voyage from Boston to Maderia that he made in 1836. It is a fascinating document that reminds us of many of the fundamentals of marine navigation. One of which is the procedure of taking a departure on ocean voyages. Taking a departure means simply recording the bearing to the last land you see as it slips out of sight, and adding to this an estimate of its distance off. We bring this important concept back into practice in our textbook Celestial Navigation: A Complete Home-study Course.

Modern navigators have mostly forgotten about this step in their navigation routine, and to the extent that happens we lose one more of the good procedures established over many years by our seafaring forefathers. Even in the age of GPS, we should take and record our departure. As we sail out of sight of land, it is in a sense the last thing we know for sure!  

The first thing you run across in the Boston to Maderia log book is “At 8 PM, Cape Cod Light-house bore S by E 3/4 E, distant 14 miles; from which I take my departure.” 

To a modern reader, the first job is to figure out what bearing this really is. He is speaking in terms of compass points. There is a point on the compass called “South by East,” and from this point you turn three quarters of a point to the east, and you are facing the lighthouse. 

The general procedure of converting compass points to azimuthal degrees is called boxing the compass. There are 32 points in a circle, thus each point is 11.25°. Easy enough it would seem, but nevertheless, boxing the compass is no simple matter. And it was at this point I realized that this question comes up to modern navigators more often than we might guess—usually in the context of reading an older book, but sometimes part of navigation tests that choose to hang on to some older traditions. Not to mention that compass points are still marked on compass roses of most US charts and magnetic compass cards, so an instructor is obligated to give some level of explanation. Compass points are also referred to in the Navigation Rules in that, for example, sidelights show from straight ahead to two points abaft of the beam.

Compass points date from our earliest record of navigation. They are shown, for example, in the famous John Davis work from the late 1500’s. Figure 1 shows this and also gives a hint of where the term "compass rose" might have come from. Though not named as points, modern compasses often mark the cardinal and inter cardinal points in the same style as used on older compasses.

 

Figure 1. Compass rose from Seaman's Secrets by John Davis. Note the center has a rose in it!  Also note that East is marked with a cross, which in those days marked the direction to Jerusalem, where the crusaders were all headed. Even poor ole Columbus had the vision of making enough money from his ventures to finance his own crusade to the East. It seems modern charts might have to start using that symbol again.

But when it comes to looking up how to box a compass we quickly learned that this is not easy to find. It has long been dropped from modern textbooks, and if you go back to the days when it was commonly used for bearings and courses (1800’s) you find it was then presumed a known basic, and so not covered there as well. Thus the best source is a text from early 1900s.

Referring to the figure of the annotated Kelvin compass card (Figure 2), we see that each point is named relative to the nearest cardinal or inter-cardinal point. Thus the name of the third point to the right of north is NE by N and not NNE by S. The word “by” means the point next to the reference point. It is sometimes abbreviated with an “x” such as NE x N.

Figure 2. This compass rose is from a drawing submitted with American Patent No 4,923 in 1889 by William Thomson, known also as “Lord Kelvin.” In small print in the fleur-de-lys are the words “Sir W. Thomson’s Patent”. It is marked off in quarter points and degrees. We have added the numbering of the points and we added the markings outside of the azimuth ring of degrees, else it is as he presented it. The inside shows what is presumably his proposed design for the compass needles. A sample of a modern version is shown above. Thomson was one of the leading physicists of the 19th century, but also worked on many practical matters, which brought him great wealth. Besides fundamental physics he (and his large staff of assistants) also worked on such mundane maritime matters as optimizing the design of a compass card and the creation of mechanical machines for tide prediction.
The motivation for the dominant use of compass points for courses and headings throughout the 18th and 19th century in place of actual degrees is not clear to me. We see that degrees were on the compass roses back in the 16th century, and all the reasons we use them now rather than compass points would seem to be true then as well.

The finest divisions used are quarter points (11.25/4 = 2.8125°).  The labeling of the quarter points is where all the fun begins. Fractional points are referred to the nearest whole point, but which one do you use. For example, the bearing one quarter point N of NE could be called NE 1/4 N or NE x N 3/4 S. Only one is right, however. 

The convention used is to box from the North toward the East and West, and from the South toward the East and West, except that the points adjacent to the cardinal and inter-cardinal points are always referenced to these points. Thus in the example given, the right answer is NE 1/4 N. 

The full compass is shown the table below. There is some rough analogy here with the use of roman numerals, which proceed upward for a period then back one then upward again: i, ii, iii, iv, V, vi, vii, viii, ix, X, xi, xii etc. Thus we count up to a reference point and the adjacent points to it are referenced to it and not in an ongoing sequence. But we all recognize this as a convoluted way to count. Movie makers even put the date in this format so we can’t figure it out as it flashes by. Could it be the early mariners used this convoluted system to protect the captain from mutiny by untrained crew in the sense that it is said they did with the practice of celestial navigation?

For the above, and any other Blogger image, you can right click, choose open in another tab, then you can often expand for better view. Not always, but often. This one can be expanded.

The 2017 Bowditch has a table in Appendix C that lists the compass points. A sample is below.

Reading and writing on weather maps

We need to write on and measure things on weather maps for several reasons. One is related to  evaluating a weather map so we know how much we can rely on it for weather routing decisions. A basic application is simply plot our position as carefully as possible on the latest surface analysis map at the valid time of the map and compare what it says the pressure, wind speed and direction were. We know from our own instruments what they really were, so the extent the map agrees is the extent to which we might believe the forecasts based on it.  And it is the forecasts we must use to select our route.

Then we turn to the forecast maps and do the same thing, to see what we should see when we get there, and to that extent, we know the forecast was right or not so right. This however, could well be too late!  Thus we have to know what we see now, and what is forecasted on a specific route, and then watch our actual conditions to see if they are evolving in that direction or not... and at what rate are they evolving.

In other words we do not really have to wait to see that the 48h forecast was wrong, we will see things changing from careful frequent observations to know if the rate of change seems consistent or not consistent.

Then to keep the navigator out of trouble on deck, they get to do this all over again every 6 hours to update the route selection and forecast evaluation.

The following are a few videos that address some of the issues, starting with a few of the basics of  map symbols.

We cover these processes and philosophies in the book Modern Marine Weather, but we are frequently reminded that in these modern times, videos are more popular than books. It could well be, however, that when you see our videos, you might vote for us sticking to the books. These are truly live presentations, unedited (at least for now), which has pros and cons. The cons are obvious; the pros are that doing things live you might discover interesting things or common snags that would be edited out of polished work.

Note to maybe come back to

When we are determining the direction of wind from the lay of the isobars in the videos below, it is often relative to circulation around Highs and Lows, but I want to stress that the direction of the wind does not matter where the actual Highs and Lows are located, meaning the local peaks in the distribution marked with big Hs and Ls. The direction at any specific point on the map depends only on the two adjacent isobars on either side of the point you care about.

One of these isobars will be higher pressure than the other. In the Northern Hemisphere, the wind flows toward the direction that puts the lower pressure isobar on your left-hand side facing downwind. This is the Buys Ballot law (actually discovered by the American William. Ferrel).

The rule is put your back to the surface wind, left arm out and slightly forward (20-30º) and you are pointing to the low.... which actually means pointing to the direction of the local low pressure. (The isobars could be winding all around a deep low center in some rather different direction.)

We are in effect using this rule backwards when we find wind from isobars. Place yourself on the point you care about, and then turn around so that the lower isobar is on your left and then you are facing the direction the wind flows toward.

Then with estimates of tangents to the isobars, one can fine tune that general direction with the slight shift into the lower side.. same as out of the higher side

PLEASE NOTE: the OPC has changed their website from what is in the videos. It is now ocean.weather.gov

Part 1 (3 min)
https://www.youtube.com/watch?v=HQbCjowiOBk

Part 2 (10 min)
https://www.youtube.com/watch?v=cNixQqFTI4c

Part 3 (12 min)
https://www.youtube.com/watch?v=pIiLfz_DIB8

Part 4 (19 min)
https://www.youtube.com/watch?v=SQJo966O1to

Part 5 (2 min)
https://www.youtube.com/watch?v=jTODJxmEe00

Part 6 (16 min)
https://www.youtube.com/watch?v=iAmXqk1PQBI

Nuts and Bolts of Successful Ocean Navigation

There is a lot we learn from a first ocean passage that we wish we had known before we left. We will look at a few of these from the navigator’s perspective, and focus on those that might not be on the standard list of forethoughts. Some are personal preferences, with obvious options, others nuances. We raise the issues so you can think on your own solutions. The many declarative sentences are for the sake of brevity, not authority. Experienced sailors will have valid differences.

Navigation means knowing where you are on a chart and then choosing the best route to where you want to go. It is always the latter task that is the biggest challenge, meaning it requires the most knowledge and skill. This is especially true in the GPS age, but it was just as true when we had only celestial navigation to go by.

So we will talk about navigation and not even worry about where we are! We get that from the GPS, and if all the back-ups fail, we get out the sextant. We look instead at the broader picture of successful navigation of a sailboat in the typical environment we have at sea on an ocean passage. There are some differences racing and cruising, but the basics are the same if you choose to get there in the most efficient manner, which includes of course just getting there at all if many things go wrong at once.

Accurate time

Dealing with that last thought first, it is important to know the correct time (UTC) at sea, because we can navigate to any port in the world with accurate time alone—we don’t even need a sextant—so it pays to wear an old fashioned watch and navigate by the time on that watch. Then maintain a chronometer log of the watch error from which we can confirm the rate of the watch, meaning how many seconds it gains or loses per day or week, and from that we can figure the right time by applying the ever increasing watch error on any date in the future. A typical inexpensive quartz watch has a rate of about 15s/month. Even if it costs $600 and is guaranteed 10s/year we need to check it. These specs are not always met.

You can check the watch with GPS as long as that is working, but to get started on a check without GPS, log on to www.time.gov and at the same time call (303) 499-7111 to listen to the WWV time ticks to see if your computer and cell phone are correct. Also add that phone number to your address book and logbook. You can call it with a sat phone if that is all that is left. A good way to check your computer and phone is to dial in that number and also login to www.time.gov and watch the UTC tick off on the screen as you listen to the ticks on the phone. They should agree. Modern technology has learned how to account for signal delays over the Internet.  The importance of time for contingency navigation is covered in the book Emergency Navigation. [Added 4/1/15: video on these ways of checking UTC]

And most important, do not change time zones while underway. Choose the zone you want for ship’s time before you leave and stick with it till you arrive. Changing times underway, or changing anything on it, is just asking for trouble, even if everything is working properly.

Notebooks and logbooks

The more we rely on echarts and GPS, the higher the temptation to under-do good old fashioned written records. It is fundamental to good seamanship to keep a written record of your navigation. Use log readings if you have them, or speed and time, and course steered. Also while all is working properly, record COG and SOG and GPS position, as well as wind info needed for sailing. More entries discussed later. Believe it or not, it also pays to record what tack or jibe you are on, though in most cases we should be able to figure that out—it depends on the wind and how well are records are kept.

There is a simple rule: make a logbook entry whenever anything changes. If nothing changes, make an entry every couple hours. The on-watch crew should generally make the entries, but you may find in the logbook only the navigator’s handwriting for the first half of the passage... till the value of this sinks in.

Also maintain at least one other notebook for navigation notes. In this you record everything related to navigation that you compute or think about. Do not use scratch paper for any computation. A book with numbered cross-hatched pages is ideal, such as National Brand Computation Notebook, No. 43-648, because you can then plot various graphs right in the notebook.

You might even want a separate one for notes on weather and a place to record forecasts and related routing notes. This one should include a time table of weather reports and forecasts. We have data from many sources, and they are valid at different times and then only available at certain times after that, and we need these times in UTC and in watch time, and we need a note of where we get each one, which may include radio or fax channel information. This is a very important schedule, which takes some time to prepare, and is easier done before departure. In any event, you will have it made by the time you arrive, but may have missed a couple reports in the process. The times of GRIB file updates as well as latest weather map broadcasts and voice reports can be sorted out at home.

Example of a navigator's notebook and the reminder that you have to look after your stuff.  In this case all the tape is there because I forgot to say "Do not use for cutting board."

Chart table and plotting tools

The value of pencil and tools holders outside of the chart table cannot be over emphasized. If you can’t find a pencil you can’t draw a line that could be crucial. The chart table itself is essentially useless space as it is too convenient a place for everyone to stash things. Unless it is built in, we also need to devise a way to protect the laptop used for navigation. It has to be fixed so it cannot slide around or bounce off the table and include some quick way to cover it to protect it from water when you are not there. The most vulnerable parts might be the connectors to it: power cable, USB and serial connectors. I have seen a person fall across the cabin in rough conditions and reach out to brace the fall and hit just the right place to break off the only serial connector of an otherwise bulletproof laptop.

A laptop stand that is raised a couple inches from the chart table is handy, so you can lay out plotting sheets or charts underneath it.

Practice with your night lights. A hand-held (teeth-held) light or head lamp is often a good solution. We need to see, but we cannot let any light out of the nav station. There is no virtue to red light; it is the brightness that matters; a dim white light is better than red, and it does not distort chart colors. I would always keep one AA flashlight in the pencil holder as well, because just like the pencil, there are times you must have one. Depending on your eye sight, you might want a magnifying glass in there as well to read small print on instrument specs, dials of a barometer, or checking the shoreline route on a chart—it would be a rare ocean voyage that does not have shoreline issues either leaving or arriving.

And you will need a way to lock yourself in place. A well designed foot stool that lets you brace your knees under the table is one, or a seat belt could do it. Another useful trick is a tight bungee cord stretched across the chart table near where you lift the lid. This holds the lid down in a broach (a safety requirement) and it holds charts and books in place in a seaway. It is not at all hard to work around during chart plotting. You can just pull the cord down over the lip to get into the table—ie to hand someone their sunglasses.

Several highlight markers and colored sharpies are useful, as are a pack of large rubber bands for organizing things. Blue painter’s tape is an excellent way to label things and use as Post-its for reminders. Headphones for the radios let you communicate at night and listen to weather reports without waking folks up whose sleep could be crucial. If you are sailing in the tropics, try to rig a fan for the nav station. A pad of universal plotting sheets is helpful for weather routing, the old fashioned way.

Ocean-going nav station, showing: A custom seat; B foot rest; C night shade; D,E tools holders; F bungee cord. Adapted from Celestial Navigation (Starpath Publications).

Share the navigation and radio information

Teach the SSB radio and sat phone usage to all of the crew. SSB transceivers can be complex, so posting a cheat sheet on how to use it is valuable. Even modern VHF radios might call for a note or two.

In the ideal world, you would have at least one person on each watch who is in tune with the navigation. That would mean knowing how to use the echart program and be aware of latest goals, weather tactics, and possible hazards. They can also encourage logbook participation. On larger racing boats the crew can get departmentalized and important navigation information is not shared enough to be as safe and effective as it might be.

One way to help with this is to post a small scale chart showing the full ocean route that is readily in view to all crew—sections of tracking charts no. 5270 or 5274 would do the job. Then plot and date your position once a day. The crew will get more interested in the navigation and indeed know where you are along the course. Discussing at any common meal times the latest weather forecasts and tactics can help as well. On a tight watch system this is might not happen very often, so the navigator’s helpers can fill in.

Also in this same vein, use some modern version of a route box in sight of the helm and deck crew. This could be just several strips of blue tape on which you write in big letters with a Sharpie the present course to steer. Or you could make something more elegant. The main idea, though, is to have a list of these courses, not just a white board where you post only the active course. We want to see the old course crossed out, and the new course written below it. This keeps all in tune with what is going on with the course over time.

Having the active course in view gives the helmsman a quick reference on what to come back to when thrown off course for any reason. Memory could hurt us if we had been on 220 for two days but now the course is 200. Also we could get confused if the course was 200 then 210 but now it is back to 200.

Sail waypoints

If we are not sailing to specific waypoints we are not navigating; we are just out sailing. Even on an ocean passage we need waypoints. There is essentially no efficient ocean crossing that has just one waypoint at the destination. Needless to say we want one there, and we should always keep an eye on the VMG to that one, but there will be intermediate ones we set and change as we proceed, and the immediate navigation is to maximize VMG to that active waypoint.

Sailing around the corner of the Pacific High, for example, you might use some guideline to mark the corner such as two full isobars off the central high pressure. This choice depends on how far you are from that point. If you have a 3 or 4 day forecast of the winds that might let you cut it a bit closer, then you can try that. But the main job is to set one and optimize speed to it until you have good reason to move it. The forecast might change and call for heading more south toward the trades for a while, or let you sail a bit closer to the rhumb line.

Once around the corner, you might set another waypoint based on the forecast of the trade winds closer to your destination. In other words, with the present forecast of the trades out to 800 nmi you might choose the point that sets you up for your best wind angles if you were at that point and the trades did indeed stay as forecasted in speed and direction. Then you again watch that and adjust as needed. Both the speed and the direction of the trades could cause the waypoint to shift.

When sailing waypoints in this manner, sometimes the course is crucial—that is, if we do not make that waypoint we could lose a lot of efficiency, so we have to fight to make it. In this case the navigators job is to stress this point and also keep a more careful watch on what is actually being steered and recorded in the logbook. With all the electronics working, we have an exact trail on the echart of what we are making good, so if we are not making it, we need to study the situation to find out why and try to correct it. Not to sound too crass, but you may have one watch that just wants to go fast, so they are reaching a little extra all the time... not looking ahead to the consequences. Again, we are back to getting the crew involved with the navigation.

On the other hand, there can be circumstances when you have a lot more freedom and you can simply say go as fast as you can (with present sails set), always looking ahead to see if a crucial waypoint might be developing.

Selecting waypoints and approach cone from forecasted winds. Adapted from Modern Marine Weather (Starpath Publications)

Stay on the right jibe

This may sound obvious, but in a long ocean race it might slip by us, especially in wonderful sailing conditions. Thus the task of a continual monitoring of the VMG to the next waypoint is crucial. It is also crucial to monitor this progress on both jibes. It could be the time to jibe is affected by the sea state, because in the same wind, one jibe is much better than the other because of the direction of the waves. This could take some testing; the interaction of wind and waves can be unique.

Depending on the boat and crew and sailing conditions, the decision could also be affected by how you want to spend the night. It could also be a time to decide, depending on where you are relative to the waypoint you want, if it might be valuable to set a head sail over night. If your route calls for fairly close reaching at the moment, it could be that the extra progress to weather could balance out a slower speed and reduced risk of sail trouble for the overnight run.

One way to make a quick estimate of the consequences of steering the wrong course is what we call the Small Angle Rule. Namely a 6º right triangle has sides in proportion of 1:10. Thus if I sail the wrong course by 6º for 100 nmi, I will be 10 nmi off my intended track. The rule can be scaled up to 18º and down to 1º. It can also be used to estimate current set and other applications.

Every mile counts

Sometimes it is hard to keep this in mind when we are in the middle of the ocean with 1,000 miles to go, but it is a constantly crucial matter.  Just imagine what that one mile looks like if your competitor is one mile ahead at the finish line.  This gives us the burden to compute once a day who is ahead and by how much in a precise manner, which is not a trivial process. Sometimes it is difficult because it depends on what we think is going to happen ahead with the wind, but it must always start from the best geometric computation, which means you must use accurate great circle computations—in fact, we should probably not even use great circle, which assumes a round earth, but rather use ellipsoidal distance, which takes into account the best datum for the ocean we are in. You can test these things, ie great circle vs ellipsoidal, by computing long distances with your GPS, since most of these do in fact read the datum you have selected and use it, compared to standard round earth great circle, which you compute at www.starpath.com/calc.  On the other hand, most echart programs use only great circle, unless they specifically ask you for the datum.  Spread sheets can be set up to do this, or there are good old-fashioned great circle plotting tricks using universal plotting sheets if the computers fail.

Evaluating the forecasts

We set the waypoints based on the forecasts, so it is important to remember that there will always be a forecast, and they are not marked good or bad. (Eventually we will get more probability forecasting into marine weather, but for now this evaluation is up to us.)

One obvious way to evaluate the forecasts is to see if the present surface analysis agrees with our own observations. To do this, we need calibrated wind instruments (to compute true wind speed and direction) and a good barometer. Then we plot our position on the weather map, read off wind speed and direction and pressure and compare to what we have recorded for these at the valid map time. If they agree, we have more confidence in the forecast. To the extent they do not, we have less confidence.

There are also well known properties of the winds aloft at 500 mb that tell us if the surface forecast might be strong or weak. These depend on the flow pattern and speed of the winds, as well as the shape and location of the surface patterns below them. Guidelines for these procedures are in Modern Marine Weather (Starpath Publications).

If we have surface forecast conditions that are very enticing for making a bold move, but our evaluation of the forecast is weak, then we should be cautious. You might then do just half of what you want to do, or do it for just half as long, then wait till you get another map (6 hours) to see how things are panning out.

Another simple and important guideline is to not rely on just the ubiquitous GRIB formated GFS model output. The minimum to do is download the actual weather maps produced by the Ocean Prediction Center and use them as an important criteria in evaluating the GRIB data. Once the GRIB maps are confirmed, then you can have more confidence in using their extremely convenient format. The first map in a GRIB forecast sequence will usually coincide with the latest synoptic time of the OPC surface analysis. As noted earlier, making a weather services time table is crucial to putting this together.r do it for just half as long, then wait till you get another map (6 hours) to see how things are panning out.

A quick and easy way to see the NWS forecasts is to download the GRIB version of the NDFD data from SailDocs. For many parts of the world this will be the main check you need.

Glue and Screws to go with the Nuts and Bolts

Keep Magnetc Variation on Automatic

When you are setting off on an ocean voyage, be sure that your GPS and echart program options has magnetic variation set to automatic. This may be something you never looked at, so it could be on manual, which means it will not change till you change it. I know of two real cases where this caused serious issues to the navigation, and one other that was caught just before that. Put another way, we need to be comparing the COG and heading all day every day. It is how we spot current. 

Review Celestial Nav Methods

You may not be called upon to find a fix by cel nav, but it is in fsct highly likely that on a long voyage you will be called up to use cel nav to check a compass off shore. This is easy going for anyone familiar with cel nav. We have many books on cel nav, both learning it from the basics and then advanced techniques on mastering the skills when that is all there is to go by.  Here is a link to a quick overview.

Watch that GPS does not switch to DR mode

 If your GPS signal is lost for any reason, some ECS units with a knotmeter and heading sensor input will switch to a DR mode, and the position output will look the same, ie 3 decimal places on Lat and Lon, which is clearly no longer true. With any leeway or current, or input instruments not calibrated, you can wander off course and maybe miss it. Sometimes the warning is clear, other times way more subtle than it should be.
A classic case of this error causing problems is the grounding of the cruise ship Royal Majesty. This error was the primary cause, but it is also an example of doing a whole series of things wrong:

— did not recognize that the GPS signal was lost and the navigation control was under DR only which was being off set by current

— essentially relied on GPS alone… i.e. only one nav aid (only backed up by unsubstantiated observations)

— failed to make positive, visual ID of a crucial buoy because it seemed to be in the right place at the right time (from radar observation based on what turned out to be a wrong track).

— let observations pass that did not make sense. If we ever see things, lights, water, landmarks that do not make sense, we should stop to figure out why we see what we see.  Recall on the Exon Valdez a third mate (who happened to be a woman) told the fellow in charge of the watch on two occasions that a light was on the wrong side of the bow. At least three similar incongruous observations or lack of observations were made prior to Royal Majesty grounding that were ignored. For more guidelines of safe navigation see Inland and Coastal Navigation.

Summary

Although the subject at hand is "ocean navigation," it will always be true that the biggest navigation challenge will be getting into the ocean from coastal waters and back out of it again into coastal and inland waters. Again, we have several real examples of boats that got themselves into trouble by relaxing the navigation once the perceived hard part is over. Indeed once we see land, we should be even more diligent until the boat is tied up at the dock or anchored.

References

A unique new Kindle ebook by Will Oxley called Modern Racing Navigation discusses the latest technology available to the navigator. We have looked above at a few of the old-fashioned ideas; Oxley's book is the place to learn about the powerful new resources, including specific recommendations for software, hardware, and apps. He focuses on the popular Expedition software as the base for navigation, performance, and weather analysis.

(Added 9/17/22): We also cover optimum ocean routing in our text Modern Marine Weather and we practice it in our new course on Electronic Chart Navigation.