Tuesday, May 15, 2018

Paper Charts vs. Electronic Charts — Some Thoughts

This is an outline of a talk given at Captain's Nautical Supply on May 15, 2018 in support of the Coho HoHo program. 

Those reading online will need to do some reading between the lines... or better still, get a copy of our book on the subject! 

First a survey

• Who uses what?  PC, Mac, or Tablet...

 • What nav programs are used: OpenCPN, Coastal Explorer, Time Zero, Navionics, iNavx...

This is a short overview followed by details in questions, with a
reminder that Starpath offers courses in all aspects of marine navigation

_________________

• These days there is no reason not to have every chart we might ever want stored on our computers.

• Tablets are handy, but we can still do very much more with a computer.

• Generally PCs are still more functional than Macs for navigation and weather... the only exception is LuckGrib.com for Mac, which also has an iOS version for PC users with an iPad. This is the state of the art GRIB source and viewer without parallel, on any platform. Mac or iOS costs $20. (I have no financial ties to this product, but am a strong proponent... if you care about weather, it will change your life!)

• Main reference: Introduction to Electronic Chart Navigation. This is a unique resource with information not found in other sources. Crucial to successful use of ENC.


Main value of electronic chart systems (ECS) over paper charts 
demo in OpenCPN )

• Boat position tracks on the chart, as well as AIS targets

• CPA can be displayed on most ECS (electronic charting systems)

• Depth and other alarms can be set (easier on vector charts)

• Layout routes and compute route table (leg distances, headings, times, names)

• Shows tides and currents

• Quick range and bearing lines for piloting

• Easy to keep up-to-date

• In US waters and other places as well, they are free whereas paper charts always expensive

• Usually easier to see in various conditions (bright light on the deck is an exception)

• Vector charts have more detail than paper charts


Main value of paper charts over echarts

• They work when they are wet.

• They don't require power.

• The view is always the same and cannot be screwed up by wrong device settings, or a mis-click at a crucial time.  (Generally tricky situations are best navigated with echarts, with the best paper chart laid out on the chart table.)

• As such, a minimum number of paper charts are required for any voyage.


Overview of our resource starpath.com/getcharts

• For echarts and paper charts

• Check out the interactive viewer for chart selection

• Check out seamless ENC viewer to see how ENC charts work... i.e., click an object.

• Check out pdf charts... they can be stored in your phone or tablet.



Using ENC (electronic navigational charts) 

compared to 

RNC (raster navigational charts)

• See Fig 1.2-1  [ Figure references are to our electronic charts book above ]


• See also Fig 2.6-2




• ENC require an all new approach to chart reading.

• User controls what is shown on the chart (Base, Standard, Custom).

• User controls the scale of the chart, which is effectively done with RNC as well but this is more consequential with ENC.

• All objects have an attribute SCAMIN, which is the smallest scale the object will be displayed upon. This is a subtly to be reckoned with. A light with SCAMIN 21999, for example, will show when the display is zoomed to 1:20,000, but will not show if you zoom out to 1:23,000.

• OpenCPN has an excellent presentation of the S-57 format.

• Descriptions of symbols are not printed on the chart, but included in object and attribute descriptions accessed by a "cursor pick."

• Everything on the chart is an object, which has attributes, one of which is often a category, which in turn has a list of options.  See sample in Table 1.7-4. Check links below to see more.




• All objects and attributes are assigned a 6-letter "acronym,"  i.e., UWTROC

• These descriptions are in our Appendix, but also online at  www.caris.com/s-57   Another source online with a different layout is www.s-57.com  This one is based in Russia and we are told may not always be up to date, but sometimes its layout makes it easier to find something... also these values do not change very often.

• Where have all the towers gone?  There are no towers on ENC. They are now an object LNDMRK, with attribute CATLMK = tower. See other attributes of LNDMRK at caris.

• Chart names of ENC are different and convey more info that those of RNC.

• Regions covered by specific ENC are irregular and do not correspond directly with RNC.  See Fig. 1.5-1 and Table 1.5-2.






• Safety depth zones can be defined to match the vessel. See Figures 2.4-1a and 2.4-1b.





Special Value of ENC

• More info on the chart. Objects have more specifications, and this will just improve with time, plus they effectively include all Light List data.

• Alarms on obstructions and depths are automatic,  whereas if needed on RNC we have to define boundaries.

• Cleaner view of some areas, with options to "over zoom" in productive ways.

• US ENC one of the few that do the vertical datum contour correctly. Can use it to read MHW. See Figure 2.8-2.  The renowned UKHO ENC do not do this properly!  Canadian ENC are even worse on this detail, though both are otherwise fine sources of ENC.



• File size for storage and update delivery is much smaller than RNC. (Once activated in your nav program this is less of a factor, because the ENC size gets doubled when creating the necessary SENC files, but even doubled they are notably smaller.)

• Looking ahead, the new S-412 weather overlays planned by the NWS will be so powerful that we will be forced to use ENC just to access that program.  See discussion in our new book Modern Marine Weather, 3rd ed.






Wednesday, May 9, 2018

Ocean Rowboat Polar Diagram

Ocean rowing is a growing sport. This year the Great Pacific Rowing Race will be sharing the race course to Hawaii with both the Pacific Cup and Vic-Maui sailboat fleets. The dynamics of rowing are of course very different from sailing, but it occurs to me that we might do some form of optimum routing for the rowboats as we do routinely for sailboats.

Figure 1. Start of the 2014 Great Pacific Rowing Race. Photo by Ellen Hoke Photography. This shows the type of boats we are discussing, sometimes called "classic fours." Crew of four, rowing two at a time, about 30 ft long, about 3,000 lbs fully loaded for a crossing.

As it turns out, wind is as important to ocean rowboats as it is to sailboats, and ocean currents are proportionally more important. We have all the tools in place from the sailboat technology to compute an optimum route, providing we can come up with some reasonable set of rowboat polar diagrams.

I have been working with my friend Jordan Hanson and his OAR Northwest rowing projects for many years, starting with assisting them on their record-setting 2006 transatlantic victory, NYC to Falmouth, England. They also rowed around Vancouver Is, which is frankly as challenging on the outside leg as being in the North Atlantic—actually more so, because there is a rocky coast on one side of them. In 2013, he and his team rowed from Dakar, Senegal to just north of Puerto Rico, where an unusual back to back sequence of big waves flipped the boat during a watch change when a hatch was open. All were rescued, and in another feat of seamanship and perseverance Jordan followed up with an air search and subsequent tugboat rescue of the boat itself.

He and his team have also rowed around the Olympic Peninsula, a complete circumnavigation by row
boat. Yes, there is indeed water on all sides of this large section of our state... something that we at Starpath confirmed years ago.

What we have below is a compilation of his thoughts on this matter for a classic fours as we discussed it. We propose this as a starting point to build from. These days with all the sophisticated data logging possible, we should be able to home in on this fairly well. The upcoming row race that starts on Jun 2 will be online, tracked by YellowBrick. We also have Jordan's Logbook from the 2006 trip and the training for it. That includes wind speed and direction, discussion of waves, along with COG and SOG, although actual numbers from that data were not used here—that is on the list to do.

We make these assumptions:

(1) We consider two rowing speed limits. A boat that can row steadily, over long periods in flat water and no wind, with an average speed of 1.5 kts and a boat that can average 2.5 kts in these same conditions. Most boats will be somewhere between these two limits.

(2) We are predicting the average speed made good (SMG) over a long period, several hours or more. These boats can surf down big waves at up to 10 kts or so, but these are just very short bursts.

(3) We assume here that the wind has been blowing long enough at the given speeds that the seas have built to their typical potential for each wind speed. This won't necessarily be fully developed seas, but just typical of what you might see. (This is a different approach from sailboat routing where we consider effect of wind and waves separately.)

(4) We assume the wind and waves are in the same direction. It is likely fair to assume that the SMG will be lower when these two differ by 45º or more.

(5) We assume for now that we can base the full range of wind angles on vector components of the estimates for head winds and tail winds alone.

And we come up with Figure 2.

Figure 2. Estimates of rowing speeds vs. wind speeds.

Sailors will have to pause a moment when looking at this!  It looks a lot different than what we are used to.  Boats that row 1.5 to 2.5 kts in flat water will be assisted by tail winds quite notably. The issue here is momentum. The boats weigh some 3,000 lbs, so it is work to get them going, but once moving, they can be kept moving with less effort, which is notably assisted with wind behind them. With a sustained wind behind them of just 15 kts, the average speed, with about the same effort, goes up some 40%. This gain increases with wind speed up till there is a physical limit on steering the boat. At some point around 20-25 kts, it can be more efficient to give up, set out a sea anchor, and just drift with this tail wind in the right direction.

The amount they slow down on the sea anchor depends on the diameter of the anchor. Jordan's experience is mostly with a 9-ft model, which he says really stops the boat, leaving it drifting at some 1 kt or so. A smaller sea anchor would let them drift faster, which we have estimated in the diagram above.  This is just one aspect of this preliminary analysis that we await more input on.  We assume that by 25 kts most boats would opt for the extra rest rather than the fight to steer the boat.

Rowing into the wind, they get stopped at lower wind speeds.  Here we guess that some boats might fight it up to 17-20 kts, but at some point, again, the effort is not worth the value of rest.

With wind on the nose, when you stop rowing and set the sea anchor, you start drifting backwards, so your SMG is negative. In this case the bigger the sea anchor the less you lose. (Do boats carry sea anchors of different sizes?)

With these starting point estimates, we then look at wind at other angles, using the logic of Figure 3.


Figure 3. Estimating effect of TWS at various TWA based on head-wind and tail-wind values. For example, with a TWS of 10 kts at a TWA = 45, we would read the effect on the boat from Figure 2 using a TWS of 7 kts. 

We break the wind up into either a head-wind or tail-wind component and a leeway component, which in this first analysis, we assume does not affect forward speed.  With this assumption, a beam wind does not slow down forward progress, but just makes it harder to row. On closer look, this might end up being a negative number near the beam, if the rowers can only keep one oar in the water at a time. Again, we wait for feedback on this.

Figure 4. Wind on the beam. Jordan's 2006 crossing.

This simple model implies that if with 15 kts of wind on the stern you make good 3.5 kts, then with 15 kts on the quarter (relative bearing 135º) you would enter Figure 2 with 0.7x15 = 10.5 kts and find a SMG of about 2.8 kts.

Likewise, if 15 kts on the nose has slowed you to 1.0 kts, then 15 kts of wind on the bow (relative bearing 45º), enter the table with 10.5 kts to get a SMG of about 1.5 kts.

If this logic makes sense, then we can create the polars for all TWA and TWS.  Each boat would then have a set of curves for SMG vs TWS and TWA depending on their starting point at TWS = 0. But before getting out the spread sheets, we wait for comments here to confirm or propose changes to this crude  model.

Ocean rowboat routes are largely downwind (Figure 5), which can be seen by looking at the Yellowbrick tracks of past races along with the prevailing weather maps at the time. But there will certainly be times in a race where one might want to work toward a desired route, and indeed if this analysis works out, we should be able to compute when that is best done.  If the polars are right, the routing programs will tell us that.

Figure 5. July COGOW scatterometer winds as presented by Pitufa, overlaid with the 2014 routes from the Great Pacific Row Race site. The 2016 routes are about the same. The goal at hand is to figure how much a rower should strive for the best route when it does not follow the winds around the Pacific High route.

Needless to say, we face the same challenge of sailors in that the forecasts are good for a few days but then get progressively less reliable.  We can, for example, get 16 days of GFS wind forecasts, which would make an interesting test for the rowboat routing computation.  The wind forecasts are not dependable after 4 or 5 days, but they will be something to start with, and the amount the forecasts drift off of correct is more important to sailboats than to rowboats. So this too adds some confidence to this approach.

The working procedure with these tactics is you run the optimization computation every 6 hr when the new forecasts come out. We cover the ways to evaluate these computations while doing them in our newly released Modern Marine Weather, 3rd ed. The routes compute in seconds, so we can try lots of variations.  There are several high quality routing programs for iPad or Android tablets, and the wind data can be downloaded by satphone while underway.

Standing by...

Tuesday, May 8, 2018

Ways to Get Accurate GMT (UTC)

Celestial navigation is one of the few human endeavors that requires us to know the time accurate to the second. In earlier days of celestial navigation—which for the purposes at hand we can say means more than thirty years ago—this was more or less easily accomplished by HF radio broadcasts, but in these modern days of the Internet, cell phone networks, and ubiquitous GPS, it is now very much easier.

That does not distract, by the way, from the high value of having a good old fashioned watch on board whose rate we monitor frequently. A modern justification for learning cel nav after all is to be independent of electronics for ocean navigation, and we need to know the time to do cel nav well... or at least efficiently. Put another way, you can sail around the world fairly efficiently from port to port with nothing at all but a watch (and some books and knowledge), but take away any time piece and it will be difficult to DR for 100 miles. Our textbook Celestial Navigation has extended sections on time keeping in navigation.

Thus if you have a watch for navigation, you will need some way to check it frequently so you can establish its rate, ie how many seconds it gains or loses every week or so.  A typical quartz watch is 15 to 20s/month and they are not as well temperature compensated. But we need some way to test that this is really true, so we show below here four independent ways to get accurate GMT.

In principle any one method would do, and one could just list what the methods are, but unless you see them side by side, then that would have to be taken on faith—a type of justification we try to avoid in navigation whenever possible.

Here are the methods

(1) Tune in an HF (SW) radio to one of the international frequencies that broadcast time tics. These are listed in Radio Aids to Navigation, the applicable chapter we have online at this link.  The best known and most often used of these is WWV and WWVH at 5, 10, 15, 20 MHz.

(2) Call this phone number to hear the WWV broadcast on your phone: (303) 499-7111. This is a great trick, and it would seem that navigators might want to have this number in their contacts list.

(3) Logon to www.time.gov and select UTC and see the time presented for you. You will see their note that the displayed time is "Corrected for network delay."

(4) Use any GPS that is connected to satellites and giving an active location to find the display that will also show the UTC.  Note that the GPS will turn on without satellite connections and indeed might even tell you the time, but this is not dependable without the actual connection.

(5) Read your cell phone time.  When you are connected to a network the phone should give you the correct time. Note that strangely enough, the iPhones do not have a native display of time accurate to the second, but there are numerous free apps that read it and then show the time to the second. I should also note that i have seen rare instances when the cell phone time was off a few seconds over a period of several minutes, but I do not know what might be the source of this.  The primary source of time in the phones is the network providers, which are in principle getting the time from GPS.

(6) Most modern computers are designed to show the network time whenever you are logged on to the Internet.  If you are some period of time off line, then the computer could drift, but if you have a wireless connection, your computer should be showing the right time.

Here is a video showing the whole band playing at once... in keeping with our totally non-professional standards of production.



Here is an example for finding watch rate by recording watch time and correct time.


Figure 11.5-2. Chronometer log plots. Top is an inexpensive quartz watch, which is slow, showing a rate -3.5s/10d. Bottom is a $600 watch with a guaranteed rate of <10s/year (dashed lines), but actual rate was +1.1s/10d, which shows we need to check these things. Bottom data compliments of Shawn Cook.

Excerpt From: David F. Burch. “Celestial Navigation: A Complete Home Study Course, Second Edition.” iBooks screen capture.



Friday, May 4, 2018

Hurricanes on the Route to Hawaii — Weather vs. Climate

This is an even year, which means many transpac crossings in small boats from the US West Coast to Hawaii, sailing and rowing. We have the Victoria to Maui yacht race and the Pacific Cup yacht race and the Great Pacific Rowing Race. One issue on this summertime route is the chance of encountering a tropical storm or hurricane, together referred to as tropical cyclones (TC).  The statistics for all TC in the central Pacific of HI are found at the Central Pacific Hurricane Center (CPHC), and shown in Figure 1. Data at that link ended in 2013, but we can find latest data at another link at CPHC, and I have used that data to extend the data in Figure 1.

Figure 1. TC in the Central Pacific

We see a total of 212 events over a period of 47 years, which is an average of 4.5 systems per year.  That is consistent with data in Bowditch, as well as updated versions of that we have in our new textbook Modern Marine Weather, 3rd ed.  The CPHC includes tropical depressions in their TC counts, so a couple of these each year may not have made it to tropical storm. In 2015, for example, 2 of the 16 are depressions, not storms.

"Per year" is the same as "per season," as these are pretty much limited to the period July, Aug, Sept, as shown in Figure 2, keeping in mind we try to avoid this crossing in later months due to increasing likelihood of bad weather from other sources.

Figure 2, Monthly distribution of probabilities of TC. August is peak; July is next; June is rare.

The statistics are not quite as bad (4 or 5 per season) as it might seem, because many of these get to HI from a southern route that we do not see on the way to HI from CA or WA, and these are small, well-defined systems that are very well forecasted, with standard ways to maneuver to avoid them.

With that overview of the statistics behind us, we get to the topic at hand—Can we fine tune these statistics by knowing something of the climatic behavior of the ocean? In short, does El Nino and La Nina tell us anything dependable about the likelihood that we will meet one of these systems on the way to Hawaii in a particular year?

We might think so, because warm water leads to warm moist air, which is the fuel of all storms. This is why conscientious navigators always measure and record the sea surface temperature (SST).  The SST has to be above 80º F (27º C) for hurricanes to form, so any time we find the water temperature that warm or warmer we know we are in volatile conditions.  These days, if we have lost our thermometer or forgot to bring one, we can get SST in GRIB format from the RTOFS model by a simple email request to saildocs.

I do not know much about the science of the El Nino analysis, but there is much online about it, and for the Western South Pacific, the topic is discussed clearly and in practical details by Bob McDavitt in his excellent MetBob weather blog on sailing in those waters.

But even without a technical background in the subject we can get a feeling of what is taking place by looking at average water temperatures in the ocean band that is used to define the el Nino (5S to 5N,  120W to 170W). The data are found at the  Climate Prediction Center; it is shown in Figure 3, where I have marked in yellow the season of interest here.

Figure 3. Deviations of average SST in the Nino region, presented as a sliding 3-month average. Values greater than ± 0.5º C mark an El Nino (red) or La Nina (blue)  season. The season JJA means June, July, August, which is the one we care about.

These temperature anomalies are called the Oceanic Nino Index (ONI), which are used to define these terms. Note that some years like 2010 start the year with one characteristic,  but by JJA have changed to another. In 2018 we started off as la Nina but it looks like the trend is toward normal.  These data are often plotted as shown in Figure 4 to identify the cycles.

Figure 5. A  plot of the ONI data from Figure 3 used to describe the seasons.

Within these cycles, we can look at the actual seasons we care about from the data of Figure 4, which are tabulated in Figure 5 below, along with the corresponding number of TC.

Figure 6. Summary of the number of tropical cyclones (TC) compared to the Nino index ONI. Red ONI (>0.5) means El Nino; blue ONI (<-0.5) means La Nina.  Red TC# means more than the average 4.5; blue TC# means less than the average.

Over these 47 years, the yellow bands mark the years when the Ocean Nino Index was a good predictor of TC likelihood, such as 2015 with ONI of 1.5 and TC# of 16, which is big El Nino and big TC count.  Similar observations occur in 1997. Likewise, in 1975 a big La Nina correlated with no TC at all for that season.  There are 8 out of 47 years when this worked.

On the other hand, the green bands mark the years where the ONI made the wrong prediction. In 1987, for example, a strong El Nino came with just less than average number of TC, and in 1985 a La Nina occurred with about twice the average number of TC, and so on.  There are 8 out of 48 years that the ONI was wrong in its forecast.

Over these 47 years, the Nino Index was useful 17% of the time and opposite of useful 17% of the time, and the other 66% of the years there was no correlation between ONI and TC count.

But the real statistical correlation is even harder to establish than what we have done. The above covers TC in all of the Central Pacific, but in fact most of these that approach HI do so from the south of the islands and do not threaten our typical sailing (or rowing) routes to them from CA or WA. Relatively few of these actually cross that route. Spot checking two big years we see the following, where it is the green systems that crossed the route, the others being well away from it—though possibly affecting sea state at a distance:

2015
July 8-10 Tropical Storm Ela
July 10-12 Tropical Storm Halola
July 10-13 Tropical Storm Iune
August 1-7 Hurricane Guillermo
August 8-13 Hurricane Hilda
August 20-31 Hurricane Kilo
August 20-26 Hurricane Loke
August 27-September 4 Hurricane Ignacio
September 1-9 Hurricane Jimena
September 18-22 Tropical Storm Malia
September 24-28 Tropical Storm Niala
October 3-8 Hurricane Oho
October 3-4 Tropical Depression Eight-C
October 11-15 Tropical Storm Nora
October 20-26 Hurricane Olaf
December 31-January 1 Tropical Depression Nine-C

2009
July 30-August 2 Tropical Storm Lana
August 3-11 Hurricane Felicia
August 11-18 Tropical Storm Maka
August 12-19 Hurricane Guillermo
August 22-28 Tropical Storm Hilda
August 28-30 Tropical Depression Two-C
October 18-27 Hurricane Neki

Figure 7. The regions of interest, showing possible encounter zone. The inset is from CPHC showing TC within 200 nmi of HI from 1950 to 2013. There have been notable new ones across the route zone since then, but this shows we are dealing with a fraction of the total.


The point here is this,  the likelihood for a TC reaching the HI routes is even more random than the above analysis shows, because the time and rate of the curvature of a TC path are notably random themselves. Even if we had good climatic forecasts of the likelihood of TC creation, we would sill have a big uncertainty in their getting in our way.

So we are left to stand by our standard advice. There are some 4 or 5 TC per year within 200 nmi of HI, spread over July, Aug, Sept, with each lasting maybe a week or so in these waters. There are  extensive records of past storms at the CPHC to check out the specifics. We see no evidence for climatic guidelines to fine tune those statistics when it comes to predicting if there will be one on the race course in our particular race.

Under sail or power, it is highly unlikely you will encounter a TC that you could not maneuver to avoid.  Even rowing without maneuverability it is very unlikely to have one cross your path, but it would be wrong to modify your estimates of that ahead of time based on the average SST in the Nino region.

We sail in the weather, not in the climate. Take a barometer and thermometer, and modify your estimates that way.

_________

I am standing by to hear from our climatologist friends on maybe other ways to interpret this data.





Wednesday, April 18, 2018

Effect of Leeway on Knotmeter Speed

This note is about a very small effect that we can normally ignore and usually do. But when it comes to optimum sailboat routing computations, small effects can matter, so we have to address all we can. There are so many unavoidable uncertainties in the process, we are obligated to do our best to correct the ones we know about.

One key thing we need in routing is accurate true wind so we can monitor our progress properly and also build proper polar diagrams from measured wind and performance data. With no current and no leeway, we can figure true wind (TW) from apparent wind (AW) in standard ways using either knotmeter boat speed (BSP) and true heading (HDG) or SOG and COG. As soon as we have current or leeway, things get more complex.

We address these issues in the new third edition of Modern Marine Weather, but one of these small effects is just stated without further illustration, and this note is intended to clarify that point. Namely, when a knotmeter paddlewheel is slipping through the water when we have leeway, it is not measuring the speed we want.  It displays BSP, but we want the actual speed through the water (STW), which is slightly different.

Figures 1 and 2 show a typical knotmeter assembly and the paddlewheel. Each blade of the paddle wheel has a small magnet inside, and each time it passes a closed wire loop it generates a small electric current pulse in that circuit.  The rate these pulses are received is converted to a boat speed, and the integrated sum of the pulses is converted into a log record  (odometer) of distance traveled through the water.

Figure 1. A knotmeter through-hull fitting.


Figure 2.  The paddlewheel (these may be from different models).

Figure 3 shows the geometry of the paddlewheel slipping through the water in the presence of leeway. The idea here is, since the axis of the paddlewheel is rigid, the only component of the motion that turns the wheel is that which is perpendicular to the paddlewheel, which is not a true measure of how fast we are actually moving through the water.  Recall that leeway, unlike current, is actual motion through the water. You are sailing, just not quite in the direction the boat is pointed.



Figure 3. Geometry of the paddle wheel moving through the water

A common way to estimate leeway digitally for routing is to compute it based on the measured heel angle. This means that you have to measure leeway directly a few times for your boat do determine the value of k, which typically varies from 9 to 13 or so and may vary with wind speed. Generally leeway is only a factor going to weather. For modern sailboats, it diminishes quickly as you fall off the wind. Note leeway is large for low boat speeds.

BSP is what is displayed on the knometer read out, but you are in fact going slightly faster than that at normal heel angles. Numerical values are shown in Table 1.


Some wind instruments and associated displays account for this if they include a heel sensor. Navigation programs that compute accurate true wind account for this as well, if they ask for leeway or heel inputs. The distinction between BSP and STW is noticeable in some displays; in other cases, the correction is used in true wind computations, but it does not show up in the displayed BSP.

Below is an example from the Tactics plugin to OpenCPN, which does show when it makes this correction.


Figure 4. OpenCPN with Tactics plugin, not correcting BSP for leeway

In this picture the right hand panel is the normal OpenCPN dashboard; the left two panels are from the Tactics plugin. With "Correct STW for leeway" shut off,  the two STW displays are the same—OpenCPN does not use the term BSP. Below, when we turn on the correction, the value shown in the Tactics display reflects the change.



Figure 5. OpenCPN with Tactics plugin, correcting BSP for leeway turned on. Compare results with Table 1.


To make these tests,  we simulated the NMEA input (GPS, wind, heel, current, HDG, BSP) using the NMEA Converter plugin to OpenCPN.



Sunday, April 15, 2018

OpenCPN: Quick Start, One Chart

We encourage the use of OpenCPN in our Inland and Coastal Navigation course and in the Marine Weather Course. To that end, we have several videos on the subject of loading charts (see below) and other maneuvers.

For now, however, the goal is to get our one training chart into the program as directly as possible. Later you can look over other options. Once the training chart is installed you can start working the practice exercises on an echart as you work them as well on a paper chart.

First download and install OpenCPN to your computer as shown in these video instructions. Then:

Step 1. Create a directory on your computer in the Documents folder called   .../mycharts

Step 2. Go to www.starpath.com/navbook and download the "RNC of 18465TR," and then move that zip file to /mycharts.  That file is called 18465TR.zip.

(The file size is 16 MB, instead of the normal chart size of 3 to 8 MB because it is higher resolution than regular charts.)

Step 3. Unzip that file so the contents remain in the /mycharts folder. That process will create a folder with the name 18465TR, and that folder will include 3 files.

That completes the download process

Step 4.  Open the program OpenCPN, and click the wrench, and then, charts icon on the top, then click far left tab Chart Files.  This may yield a blank field or there could be paths listed that you added earlier.

Step 5. Click Add directory, then navigate to the folder you have created called 18465TR, highlight it and press Apply.   You should now see something in the field called /Users/computer_name/Documents/mycharts/18465TR.  Then press OK. and we are done.

Check that it is there:

Step 6. Go to wrench, then Display icon, then General tab, and in Chart Display, uncheck Enable Chart Quilting, and in Dissplay Features check Show chart outlines.  Press OK.

Step 7. On the base map background pan around to the Pacific Northwest and you should see a small red square over the Eastern Strait of Juan de Fuca. Zoom in on that and when it is about 20 or 30% of the screen chart will show up in the chart bar as a blue line. Click that line and the chart should appear.

Once you have found the chart, you can return to the display settings and turn off the chart outline option as we do not need that with just one chart.

Below is a video that shows this process.  Then later on you can look over the full discussion of loading charts.

If you are not a Starpath student then you might prefer a different chart for this start up. You can choose one from NOAA interactive viewer.  They will be some 4 times smaller and look pretty much the same.

Quick start, one chart
________

Here are the two short follow ups which would be the next step to using the electronic training chart to practice echart navigation.

Basics 1: M-key, Units, and Variation

Basics 2: Help, Scales, Zoom and Pan




Saturday, April 14, 2018

Introduction to Charts in OpenCPN

Choosing, loading, storing, displaying and updating charts is an issue with all electronic charting systems (ECS). It is fundamental that we learn the details for the program we choose. Some programs have more convenient ways than others, but some of the most convenient interfaces do not offer as much versatility as others do.

In our Inland and Coastal Nav and Marine Weather courses we use OpenCPN. It has a remarkable versatility on chart options, but like any ECS there is a procedure to learn. To try to help with that, we have made a few videos on the topic, breaking it down into topics.  These are designed primarily for our students, but if others might find them useful, all the better.

Some ECS, if not most, refer to this process as "installing charts," but that simply means that we download the charts by some means, either with a tool or manually, and then copy them to a folder on our hard drive, or on a network drive. What is actually being installed and saved with the program are a few lines of text that tells the program where the charts are located on our computer.

For the quickest access to actual charts, you could go to #2, download all the charts for your state, and skip the rest.

OpenCPN is the only program I know of lets the user select the resolution of the background. They do this to accommodate computers of all types and levels—some ECS, for example, require the latest operating system and a fast computer with a lot of memory.  So the first topic addresses updating the stock base map with one of higher resolution. This is not strictly required, as the charts will overlay them, but it does offer a better echart experience.  Later we cover cases where it does matter.

1. Changing base maps

There are two basic types of echarts, RNC and ENC. For now we are looking only at the RNC, which are images of the paper charts.  There is much virtue to the vector charts (ENC), and we will add notes on these shortly. We have done a lot of work on these and have a textbook devoted to them.

There are two ways to load RNC into OpenCPN, use of the built in Chart Downloader plugin, which is super convenient, and second is loading charts manually which we do have to do periodically. (I am not 100% on this, but the cases I tried with WinXP did not bring the Chart Downloader with it.)

2. Loading RNC with the Chart Downloader plugin

This tool not only loads the charts but sets them up to automatically updated. The only issue, as noted in the video, is we must know the actual name of the chart, as it does use chart numbers. We can identify charts we need from the top link at www.starpath.com/getcharts.

We can also load charts manually, which comes up periodically. It takes an extra step or two, but for some special charts it is needed. 

To load charts manually is a matter of keeping track of which folder stores the charts, and then using the tools of OpenCPN to turn them on and off as needed. When loading charts manually that we want to be updated, it is important to load the catalog from the chartdownloader, else the auto updates will not work

3. Manual downloading charts

OpenCPN also has easy access to other charts, both RNC and ENC. The video below outlines a couple of these, official charts from Brazil, a custom made training chart that we use in the Nav Course, as well as automatically georeferenced  weather maps that we access via the weather fax plugin.  The Chart Sources section of the online manual lists other sources of charts as well.

4. Other charts


Finally we address more details of base maps, which will come into play when we start doing optimum weather routing with the weather routing plugin. This is definitely extracurricular, and not needed till we work with weather routing.

5. Base map details

I will try to group these together at YouTube for easier access, but for now this index might help pull this sequence out from the others we have on related topics, some of which are outdated.



Wednesday, January 17, 2018

Sadler Tropical Atlases

Before the internet, the US Navy and several universities were sources of climatic marine weather data that could be traced directly back to Matthew Fontaine Maury in the mid 1800s. It was his original idea to study old logbooks to extract and record weather and sea state observations and compile them into what evolved into modern Pilot Charts and now COGOW, which replaces even the Pilot Charts for climatic wind data. 

This work was extended in tropical waters worldwide in the 1980s by James C. Sadler at the University of Hawaii where he did the same thing with hundreds of thousands of ship observations from the mid 1800s to mid 1980s. 

Samples of Sadler’s Tropical Atlas for July are shown below. They remain an interesting depiction of average wind flow each month that could help in the planning of ocean voyages across the tropics—or at least help us understand why the traditional routes evolved as they have. 





Samples of wind and pressure data for July from James C. Sadler’s Tropical Altas. The wind lines are are called stream lines. They show direction without speed. Actual average speeds (in m/s) and the number of observations that led to the average are in the small numbers across the chart. The data are based on compilations of shipboard observations over many years. This wonderful work for its time has now been superseded by the COGOW program

Volume 2 covers twelve months for the tropical and sub-tropical Pacific Ocean. An electronic copy of Vol. 2 is available at the University of Hawaii meteorology web site  at https://www.soest.hawaii.edu/Library/Sadler_et_al.html, although you will not find any link to this location from their web site.

Volume 1 covers Atlantic and Indian Oceans, available in some libraries.

These plots are easier to use than Pilot Charts for seeing obvious sailing routes. They can be useful to investigating winning routes in ocean races and historic sailing routes of discovery. They would also indicate likely drift routes across the ocean.

LuckGrib is one viewer that can show true streamlines (example below).  OpenCPN has an option called "particle map" that simulates them on some level. Streamlines are a nice way to note forecasted convergence and divergence zones. Not many viewers show true streamlines as it is a difficult computation.




A streamline display of 10m winds from GFS shown in LuckGrib.




Monday, January 15, 2018

Shortcut to NGA Publications

The primary link to NGA publications is a long one that some browsers stumble on, so we created this custom link that goes directly to this important list of navigation publications. Use: 


Capitalization does not matter; you can use /ngapubs. 

Here are the pubs available at that link:

American Practical Navigator
Atlas of Pilot Charts
Chart No. 1
Distances Between Ports
International Code of Signals
NGA List of Lights
Radar Navigation and Maneuvering Board Manual
Radio Navigational Aids
Sailing Directions Enroute
Sailing Directions Planning Guides
Sight Reduction Tables for Air Navigation
Sight Reduction Tables for Marine Navigation
USCG Light List
World Port Index

Or, you can go directly with this one:

https://msi.nga.mil/NGAPortal/MSI.portal?_nfpb=true&_st=&_pageLabel=msi_portal_page_62

The actual link you see seems to depend on how you got there.  It is marked as a secure page (https), but it is not secure. This is the strange situation we see with many NGA and Navy pages.

LATEST and BEST update... I think.

We have found that 



will do the job. Don't use www, or anything else. Just type that in the url and hit enter.  Then select Publications link on the left.

Monday, January 1, 2018

Decision Making in Weather Routing

Lets look into decision making as it might apply to weather routing. 

I have a forecast that says the wind is going to veer by 30º overnight.

Suppose if I jibe now and the wind does veer 30º by tomorrow morning I will gain. But If I jibe now and the wind does not veer 30º by tomorrow morning then i will lose.

What do I need to know to decide if I should jibe?

We need numbers, or estimates of numbers, on all three factors involved. 

P = probability of the wind veering as forecasted.

Gain = how much you gain (in hours or miles) if you jibe and the wind shifts as forecasted

Risk = how much you lose (in the same units) if you jibe and the wind does not shift as forecasted.

We can figure the Risk and Gain numbers from our polars.

We are left with computing a take point probability P for the forecast. In other words, we calculate how big does P have to be so that the chances of gaining are higher than the chances of losing.

That means


P x Gain ≥ (1-P) x Risk.

In words, the probability of the veer (P) times the Gain from the veer must be greater than the probability of no veer (1-P) times the Risk.  If the probability of yes is 70% or 0.7, then the probability of no has to be 30% or 0.3.

Now you can rearrange the terms to get


P ≥ Risk / (Risk + Gain),

which is our working guideline, and we can make a table to solve it by inspection. The units can be anything, miles, minutes, hours.



Suppose you figure you would gain about 4 mi if the wind veers, but you would lose about 1 mi on the slightly slower jibe if it did not veer. Thus P = 1/5 = 20%.  The forecast only has to be right 20% of the time and you come out ahead. A clear call to go for it.

On the other hand, you face a more ambitious maneuver to catch up. You figure you will gain 2 mi but would lose 8 mi if there is no veer. Then you have P = 8/10 = 80%. The forecast has to have an 80% chance of being right, which makes you study the weather maps very carefully, look for ship reports, read the Forecast Discussions, etc. Do what ever you can to add confidence to the forecast.  

If the winds aloft were changing rapidly, and the present surface analysis did not match what you actually saw in pressure and wind, then you probably can't believe this is 80% likely, and have to pass, or maybe try 3 hr and then jibe back to wait another 3 hr to get the next map. In other words, do half of what you want.

Needless to say, one can take that formula off of the boat and think about it in various social or economic settings. The decoration on the table is in honor of our lunch and break table at work, where it rules about 12.5% of our working day.