Thursday, March 19, 2015

Marine Weather Services Chart — How to Make Your Own.

For many years the NWS published Marine Weather Services Charts (MSC) that listed crucial information for mariners using their services. There were fifteen charts that spanned all US Waters. The page size was 13”x 21”, printed both sides, with an annotated great-circle chart of the region on one side and all text on the other. The last printed versions of these are still to be found online from unofficial sources, but they are outdated. The NWS no longer supports them nor makes them available—the only exception is Alaska, MSC-15, which is still available from NWS, although they have trimmed down parts of the original content.

Nevertheless, the concept of the MSC remains crucial to good weather work underway. Environment Canada still offers their counterparts called Mariner’s Guide to Marine Weather Services, which are equally valuable for their waters. One approach to the missing MSC is just to print a copy of the last known version and then make pen and ink updates as needed on that copy. On some charts, the changes are few, or not relevant to your needs, and once updated their value remains high.

In these modern post-MSC days, the latest data are readily available online, but the challenge is finding it and putting it together into a useful format.  There is often too much data! We are faced with the same information in multiple formats, with some parts more convenient than others. Or some seemingly obvious thing we would want at hand underway turns out to be difficult to find online. Remember, the goal is not to provide actual resources, but to provide the information we need to use the resources we have access to underway.

So as a temporary solution—hoping the NWS eventually brings them back—we offer here a way to gather together the same data that were on the MSC charts, which you can then combine into some convenient format of your choosing. A sample section of one of the older charts is shown in Fig 1. Then in the following figures are examples of recent equivalent data found online.

Table 1 shows the data that were typically on an MSC along with links to where you can get this data to make your own compilation. For regions you plan to sail in, you can download, print and combine into a thin binder of what was in the MSC. Having this information at hand is fundamental to taking advantage of the wonderful resources we have available. The exercise will show clearly why we miss the MSC so much.

The latest word from the NWS is they do hope to re-issue some of the MSC as online pdfs, but they do not know when. This might be up to the local NWS Offices.


Fig 1. Section of now-defunct MSC-1, Eastport, ME to Montauk Point, NY, showing forecast zones and VHF NOAA Weather Radio transmitters. Blue-green is VHF weather coverage. Notice the indent in the coverage approaching Rhode Island Sound, in forecast zone ANZ235. You could get data in this region from the USCG broadcasts of Item (13). Some of these zones (Fig. 2) have changed.



Table 1. Make Your Own Marine Weather Services Chart
Item
       Historic MSC content
            Links to online sources
1
FORECAST ZONES labeled and outlined on the chart

Coastal zone maps (including Great Lakes):
Offshore zone maps:
2
NOAA Weather Radio BROADCAST STATIONS and reception ranges

Start with this index map:
then click to state, then click the station, then click the map for an excellent pdf.
3
OBSERVATION STATIONS (light houses, buoys, etc) used in NOAA Weather Radio reports.

This is the place we miss the MSC the most, as we have to recreate these plots on our own. The best approach we have found is start here:
then click a region, then zoom in for a plot of the stations to print, then click each one to get the name of the station to transfer to your print.
4
TERMINOLOGY used in weather reports and forecasts.
5

LOCAL NWS OFFICES responsible for each of the forecast zones.
6
NAVTEX broadcasts.
7
USCG HF VOICE high seas and coastal broadcasts.
8
WWV and WWVH Storm warnings
9
USCG HF RADIOFAX high seas broadcasts

10

NAUTICAL CHARTS, how to order.
11
LIVE BUOY REPORTS
by email
You can get not just live buoy reports you can get essentially every NWS product available by email request through their program called FTPmail. See:
12
LIVE BUOY REPORTS
by telephone

This is the NWS longstanding Dial-a-Buoy program, which remains a very slick system, although smart phones offer even more options. See:
http://www.ndbc.noaa.gov/dial.shtml
13

USCG VHF weather broadcasts
These are the repeats and relays of weather information on VHF 22a, which reach out farther than NOAA Weather Radio.
14
CANADIAN weather broadcasts when applicable.

Here is the overview of Canadian marine services:
http://www.weather.gc.ca/marine/index_e.html
And here is Canadian Weatheradio (note spelling):
https://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=792F2D20-1
15
NWS INTERNET LINKS

The main list of NOAA/NWS Internet sites is at:
A shortcut url to all marine weather resources (which we hope will not change) is:
16
WEATHER SAFETY TIPS
for mariners, unique to the region.

The closest we could find in the same spirit:
http://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=656C03FF-1
17
PORTS — Physical Oceanographic Real-Time System.

An amazing resource for quite a few places around the country:
18
NOAA Weather Radio by telephone

Covered well in AK on MSC-15, but this seems to be a service of the local NWS Offices, so you will need to check with your local NWS Office. See Item (5).
19
General and special information about the local forecast zones covered.

Start by finding the main link to the local NWS Office from here:
Then there is much information about specific regions. The unique channel winds map on the back of MSC-13 for HI is one good example. (See Fig. 6)
20
Definitions of VHF WX channels by frequency

Most resources define the VHF broadcast products by frequency, but on the boat we may only have channel names, wx1, wx2, wx3... so this can be useful data:
21
Unified Analysis Maps
Not cited on historic MSC, but new valuable resources




Fig. 2. Coastal forecast zone maps available online, from Item (1). Coastal forecasts in the outer coastal zones (we outlined in red) offer only warnings. They overlap the Offshore zones on the East Coast north of Charleston. Full forecasts in these outer zones come from the offshore forecasts.

Fig. 3. NOAA Weather Radio coverage (white areas). The online data shows the coverage gap as well. See Fig 4.
Fig 4. Detailed coverage map of WXJ39 Providence (WX2, 162.400 MHz) showing why there is a gap in the coastal coverage. It is an inland station and there are is no overlapping coastal coverage.
Fig 5. Locations of the observations stations reported on NOAA Weather Radio, from Item (3). We must then click each online to ID the station and make a list. These are the places we get recent observations from (updated every 3h) in the continuous NOAA Weather Radio broadcasts.

Fig. 6. Back of the out of print MSC-13 for Hawaii.
Fig. 7. Section of a Canadian Marine Weather Services Chart. Their offshore zones have names, ie "Explorer."


Monday, March 2, 2015

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 is adapted from our book Celestial Navigation: A Complete Home Study Course.


Friday, February 27, 2015

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?

Sunday, February 22, 2015

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.


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.

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 ModernMarine 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.

PS. When you are setting off on an ocean voyage, be sure that your echart program options has magnetic variation set to automatic. This may be something you never ever 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.


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.


Wednesday, December 10, 2014

Sydney-Hobart Climatic Winds

The Rolex Sydney-Hobart Race is notorious for not just strong wind, but strong wind that changes rapidly. We in the Northern Hemisphere are not used to fronts and Lows that move as fast as they do along this route.

You can see the variability in the climatic wind patters at the wonderful COGOW site, from which we have captured the samples below. The COGOW data replace everything in the past as the best source of climatic average winds. They are actual winds measured by the Quikscat satellite. There are 9 years of data, with a pass over a particular region about once a day.  Then the folks at Oregon State University compiled the data and made this super nice presentation.  They have ended up with about 150 observations at each grid point. You can compare this to similar data in the Pilot Charts (much from 19th century logbook data) or other Navy Wind Atlases from the past, but the COGOW data are the best available and the best way to plan a sailing route…  when you might have some option on when and where you sail.

When you must sail on a given date at a specific place, this does not help much. We can then only look at the 5 day forecast at the start compared with latest satellite winds, and then keep updating the knowledge every 6 hr or so.  We get new maps every 6 hr, but we only get new forecasts every 12h.

The Race is on Dec 26th, Boxing Day, and the COGOW winds provide averages for the first and last 2 weeks of each month. Below we show Jan 1-15 and Dec 16 to 31. There is not much difference in these two periods.


COGOW winds overzoomed to compare the change with time.

The compiled picture below showing wind roses are from the Jan 1-15, primarily because it showed  the shear line about 37S, but it turns out we do not see much of this in the individual statistics. At the COGOW site you have several options on how to present the wind statistics. These wind roses are just one option.

You can vary the views on COGOW and get somewhat more precise data by selecting different regions to zoom in on from the base map…. but the general behavior is more or less as indicated in the wind diagrams shown, which are of course just averages.

Look closely at some regions and we do see interesting statistical details, for example how the wind changes near the coast in some regions.  This race is much influenced by strong currents just off shore. I will try to find some good links to these. I believe there is HF radar measurements available.

There are two ends to the wind pole.  The climatic averages of what has taken place at one end (shown below) and the winds at the very moment at the other end (see Sydney-Hobart scatterometer data). In this latter link we have compiled a very convenient way to look at all of the satellite wind data that are available, which is updated each time you look at it. The satellite data are not forecasts or model predictions, they are actual measurements of the wind. With 3 satellites ascending and descending, we end up with 2 or 3 looks per day at the real winds.  These satellite data are used to seed the numerical models, which are consequently pretty good at their zeroth hour report, but we still very often see details in the satellite data that get averaged out by the modeling.


COGOW data. The numbers are the percentage for the direction. The length of the line segments is the percentage wind speeds.  You can get this data in tablular form at the COGOW site. Other than west wind in Bass Strait, there is a lot of variability.  Click this pic to get a better view.

NOTE: sometimes it is tricky at the COGOW site to get to the popup that lets you change between wind rose and table data.  In principle you just left click on the main map and you should see the option, but once you select a choice that window dives to the bottom of all open windows and after that click away and nothing happens.  So if you can't get it, move all the windows around and look underneath for the popup.

Monday, December 8, 2014

Don't Blame eCharts for Anything

The grounding of Vestas Wind reminds us of basic issues in modern navigation.

We have long taught at Starpath that we should never blame echarts for anything that goes wrong. This is a fundamentally important rule that we see violated over and over again. Granted, there is much not to love about vector echarts—the ones called ENC (electronic navigation charts), as opposed to RNC (raster navigation charts), which are identical to their paper counterparts. Unlike the ENC, the RNC are updated weekly in print and echart at no charge. The rule applies to RNC as well, but they are not as often the brunt of criticism as are the ENC.

Vector charts are easy targets for blame. First, for general use in what is called ECS (electronic charting systems) there are no real standards in functionality and chart symbols, as opposed to ECDIS use (electronic chart display and information system, pronounced ek-dis) the professional system sanctioned by the International Hydrographic Organization. Sailors and other recreational mariners, however, do not often use official ECDIS software or charts.  We use mostly ECS, which means simply any combination of echarts and software we might have.  Some ECS strives for the ECDIS standards; others do not. The latest edition of the  Chart No 1 booklet now includes many of the ECDIS symbols, which is a valuable free download to have. The full range of ECDIS standards, however, is immense, as it also includes how the various aids and landmarks should be described, coded, and indexed for display in various layers. There are also standards on the ECDIS software functionality, which is often not as convenient as some ECS programs, which is in part why manufacturers do not use it.

A big difference between RNC and ENC chart symbols is the legends explaining the symbols are all in print on the RNC, whereas in ENC we usually need to right click it or highlight it and press Info to learn what it means. This has lead to several incidents. I recall a racing yacht grounding on the West Coast that blamed echart symbols, as well as a tanker hitting a bridge in San Francisco Bay because light symbols were misunderstood.

And of course there is the appearance of the land that differs between these two chart formats, sometimes tremendously.  All ENC start out as RNC that someone (or some software) manually (or electronically) digitizes. Eventually these might all be tested by satellite images, which is done to some level now.  Certainly at some point the ENC will be better than the RNC, and I would venture that someday all charts will be ENC. It is the logical direction to go, just as print on demand was a logical direction for paper chart production. But for now, we will find more errors in the ENC charts than in the RNC, and more of them blamed for navigation shortcomings.  ENC include all errors that might be in the RNC (from incorrect or outdated surveys) plus any new errors that come from the digitizing.

On the other hand, like any working database, the ENC will just get better over time as the errors are found and removed and new meta data describing the features are added. The attraction of the ENC comes from this great potential of including so much information, plus the fact that they are individually small files. Entire sections of an ocean basin can be included in one large file, including every harbor chart as well. Furthermore, we can have ENC charts for remote parts of the world where there are not many alternatives.

This last point came to mind immediately upon learning of the grounding of the Volvo Race boat Vestas Wind on a reef at the SW tip of Cargados Carajos Shoals on the 29th November at 1510 UTC. All crew are safe, which is quite a blessing in that the GPS track of the vessel shows 17 to 19 kts of speed within seconds of 0 kts. The grounding was just off Coco Island, meaning they were 5 miles off course to miss the hazard and at least 10 off for a safe passing at night.

This reef is in the quintessential middle of nowhere, about halfway through the 5,000-nmi leg two of the race, in the middle of the Indian Ocean, about 216 nmi NNE of Mauritius. But even though this is a remote location and a small group of reefs, islets, and shoals, it is not at all a secret place. In fact, the entire middle segment of the Indian Ocean has the Cargados Carajos Shoal’s name written across it in large capital letters in the graphic index to the US International Sailing Directions, Pub 171. It is Section 9. The Sailing Directions are the ocean counterparts of the Coast Pilots, and they have the same goal, namely to provide crucial navigation information that is not on the charts. For any long voyage it is fundamental that we check the Coast Pilots and Sailing Directions. In this specific case, however, we do not learn enough from the two paragraphs about this reef in the latest edition of Pub 171. In fact, the only chart of the area it refers to is DMA No. 61551, which has been out of print for about 5 years. It also has the confusing information that the SW tip might be 3 mi SW of where it is charted, then later says you can pass the SW tip within 1 nmi—presumably meaning from wherever it is.

Thus when planning a trip near this region we have to work harder. The natural next place to look is the BA Pilots, which are the British equivalent of the US Sailing Directions, though famously more thorough and famously more expensive. Vol 39, South Indian Ocean, is only $60 but other volumes are twice that. They are in good libraries, however.  Here we learn that  BA chart No. 1881 has pretty good detail, and No. 4702 is a smaller scale showing the extensive gamut the yachts must face headed north across the Indian Ocean. The BA Pilot devotes two full pages to these Shoals.

We do learn from both the US and BA sailing directions that the Cargados Carajos Shoals should not be approached from the east (breaking waves obscure the location of the reef, which itself is only poorly known) and that the Shoals should not be approached from any direction at night. The four boats that passed the Shoals well to the west some 5 hours earlier did so in daylight, and at least one may have been close enough to see them, which are described as clearly visible with some hills to 70 feet with vegetation and a few low-lying buildings. This can be seen on Google Earth (GE). There are even some permanent residents there, including two Coast Guard personnel—very isolated, but in a very pretty place.  Vestas Wind unfortunately came in from the east at night.

One other yacht passed well below 1 mile of the reef tip in daylight (too close for a navigation instructor, but not a racing tactician), and could have been lucky in light of the NGA warning about the SW tip.  Figure 1 shows the boat tracks. In daylight the closest may have seen the reef and its limits as they passed.



Figure 1.  Boat positions just after the time of the Vestas Wind grounding (blue track). The white boat is 111 miles ahead of this Vestas Wind position.  The yellow boat and white boat are 4.5 miles apart on this scale.

BA charts are available from all BA outlets worldwide (there are a dozen in the US), and remote  charts are probably in stock at many of them as it is part of a BA distributor requirement. The chart 1881 is the one of most interest here, being the basis of all other charts of this area, and the navigation charting story begins with it.

This chart is essentially the same as it was in 1941, based on a survey taken in 1894, with periodic updates over the years, but no real surveying improvements. There is the discussion mentioned in the sailing directions and much online chatter about whether or not the shoals are where they are plotted, but we can compare with GE to see that it is indeed very close (Figure 2). This is not an issue.  There is always the issue of having your GPS set to the chart datum of the chart, but that should be standard knowledge these days, and could account for hundreds of yards and rarely a mile. The out of print US chart 61551 is essentially identical to the BA chart 1881 (upon which it was based), but there is one important difference.

Figure 2. A Google Earth image overlaid onto chart 61551, showing good agreement. The approximate location of the grounding is shown with a red X. The segment shown is about 5 nmi across. The light showing on this outdated chart does not now exist. Soundings in meters.

If you compare 61551 with a paper version of 1881 purchased from an authorized BA distributor, you will notice one main difference in the vicinity of the grounding. The BA chart will have the notable light on Coco Island crossed out by hand in ink (Figure 3). All BA distributors are obligated to update the charts with pen and ink by hand, and this light was no longer functional after a BA notice to mariners in 2012. This light (nominally visible for 12 miles) is just  a few miles from where Vestas Wind went aground.

Figure 3. Section of the latest printed edition of BA chart 1881, hand corrected by Captains Nautical in Seattle, an official BA chart distributor. The correction is dated in the bottom left with the notice number. The BA RNC of this chart, on the other hand, has the light removed, which shows the value of official RNC. Soundings in fathoms

Which brings us around to the echarts. Unless you are purchasing official BA ENC or RNC (called ARC), which are very expensive, then you are buying from companies that do not necessarily have the same BA standards for keeping their charts up to date—and I must add NOAA standards as well these days, as all NOAA charts are updated weekly now. You can’t buy US litho charts anymore that may have been sitting in drawers for years. They are print on demand, and the masters, print and electronic, are updated weekly. 

But we are considering foreign charts, or charts of foreign waters made by commercial companies of the US or elsewhere. In the case of these shoals some of the electronic charts in use at the time of this grounding were not up to date in that they did indeed still show this navigation light, which could in fact (if there) have saved the day in this case. Hopefully it did not mislead any navigator.

Even worse than that, some of the ECS chart packages did not layer the scales of this region very well, and in fact this point was brought up in the chatter about this region by several sailors on the race on several boats. Namely on one scale the Shoals did not appear at all and on the next scale up they filled the screen (see Figure 4).  Thus if you did not zoom in at very near the right place, you would not see them at all­—that is of course if you did not know ahead of time that they were there from your homework, and that this was indeed the primary issue of the navigation for at least a day or so, and as such have all sorts of back up contingency navigation in place, including radar and depth watch, besides GPS waypoints and tracks. In short, we are back to our rule.

Figure 4. Example of one echart defect. This is a full screen view.  The top view (A) alerts us with the white patches that there are large scale charts available in that region. (B) shows zoomed into that region with multiple zoom-in steps, (C) is the very next scale step in from the view above it, which does show the light. (D) is next zoom in, and now the light is gone from all further zooms. (E) next zoom in.  (F) shows slightly different zoom from a different start that does show the light. The presence of the light and which zooms show it depends on where you start from the white region and how you move the screen before the next zoom.  But there is no way to watch your vessel approach this reef on a view that actually shows the reef. This is from an iPad app with poor  chart presentation. It might look different in computer software or GPS units running these same echarts.

No matter how bad the echarts and echart display might be, we cannot and should not blame them for anything. It is frankly our job as navigator to know such limits and work around them. When all is going well, echarts are an invaluable aid to navigation and racing tactics, but when the navigation is crucial, we need to back these up with other info. Figure 5, for example, shows a section of another commercial echart brand used on one of the race boats that also shows this light—I think.



Figure 5. Another commercial brand of echart used on some of the boats. It does show the light—or something on Coco Is (called South Is); it is not clear what that symbol means without right clicking it! We always have two issues: did the company up date the chart, and then did the navigator update from the company. Often the update option is a paid subscription.  It appears that this navigator had defined a custom boundary around the reef, which often has the virtue of scaling more nicely so it stays in view, as well as offering danger bearings and ranges. This chart shows much better sounding data than the BA chart. We assume the navigator highlighted the depth contour IDs (meters), again good practice underway.  We do not know which boats used which brands of echarts. Some I would assume have several.

Another flaw of at least one echart program in use at this time was when right clicking and getting the data for this (non-existent) light it gave us all the information except how high it was. This height is crucial for predicting visible ranges of lights, so I would consider that a major charting error. Its official Light List number was also missing.

But suppose this is the situation: You have this echart on shore before the race as you plan your route, and you look at the light and find the height is missing. You know how important this light might be, so you must find this height somewhere. A first place to look is the NGA Pub 122, List of Lights. It covers all lights worldwide and it is free.  In the latest 2014 edition of that book on page 566, sure enough you find that light. It is US No. 32892, International No. D6681.6. It is listed as Fl(3) W, 30s, 27 ft above MHW, with a nominal range of 12 nmi. All this info is on the echart except the height 27 ft.  That would mean standing on deck at 9 ft height of eye, you would see this light from about SQRT(9) + SQRT(27), or about 8 or 9 miles off. The 12-mi nominal range means it would show bright at that distance when it first comes over the horizon, and you might even “bob the light” by standing on the boom.

So you are better prepared... or so you would think. But not at all. This official reference book happens to be wrong. That light is not there; gone since 2012 as best I can tell.  It is an error in the 2014 Pub 122. So we are back to our rule.  This false confidence came from overlooking the main issue. The echart itself.  When things are crucial, and you are using echarts from a commercial company that are not certified by the Hydrographic Office of that country, then the first step is check the echart with a printed chart or an official echart.

At this stage, just a few days after the incident, we have no idea what led to the error. I have not heard of anyone directly involved with the boat blaming any echart, or in fact blaming anything.  We are all human, and humans make mistakes. The extensive online chatter from distant observers, however, went straight to the charts. 

The discussion here has been just one scenario. As it turns out some commercial echarts of this region are based on charts from the Indian Hydrographic Office, which happens to have an excellent chart of this area No. 2503 (Figure 6). Though much the same as BA 1881, they do they the light removed so users of this brand of echart would not be confused by this issue. Chart 2503 also has improved soundings data from their own surveys. I have not been able to test any echarts based on 2503 and what meta data appear in these echarts. I am trying to get samples of other brands for comparison. Also we have to assume that whoever makes the echarts (regardless of source) do keep them up to date. Chart 2503 was new in Mar, 2014.  Print on demand nautical charts from all nations are available online from EastView Geospatial.

Figure 6. Section of Indian Chart 2503, showing improved soundings over BA 1881.
With special thanks to the Hydrographic Office of India

Remember too that of the half-dozen or so commercial companies that make worldwide echarts, some are known to be better for some regions than for others.  We cannot learn which is best where from advertising; we have to check with experienced users.

The Volvo boats had been in difficult conditions for a couple days negotiating a tropical disturbance with winds up to 30 kts. Approaching the Shoals the wind direction was changing rapidly along the fringe of this Low (with clockwise winds in the Southern Hemisphere).  The grounding took place about 2 hours after sunset, which at this low latitude was well past nautical twilight, meaning dark enough that the horizon was most likely not visible.  Even in clear weather, you cannot see the reef downwind with waves breaking over it. There was a half moon about halfway up the sky to the east, which may have offered some light, but that near the Low this was likely hidden by clouds. I would have to guess that current was not a factor.  Both the OSCAR 5-day average and the latest RTOFS models gave current flowing to the west at about 0.5 kts at that time.  They were off to the east so this current would have helped, if anything.

I want to stress that vector echarts certainly have much virtue and not just limits. Besides the unique values mentioned, they offer custom displays and features that can indeed enhance safety not threaten it. Furthermore, all of the limits can and will eventually be removed.

For now we have the issues above in at least some systems, but these might not appear the same in all platforms—that is, in a computer versus in a tablet app, or integrated into the GPS display. On the other hand, it could be that even the best echart navigation software has limits on this that are inherent to the specific echart package. There is also the question of how this might vary between ECDIS standard ENC and commercial vector echart products. Though in use now for quite a long time, there is still a lot to learn and a long way to go.

PS. There is some discussion online, in print and audio, regarding work on the boat at high and low tide etc. But we must remember we are in the middle of the ocean here, so tide is not much of a factor. See Figure 7.

Figure 7. Tidal data near the grounding site. S=spring, N=neap.



A version of this note will appear in the Jan, 2015 issue of Blue Water Sailing Magazine, an issue that includes other topics of electronic navigation.