Shared Remote Simulation with qtVlm

 A unique feature of qtVlm that is a powerful training tool is its ability to share AIS positions amongst simultaneous users. We can have, for example, three users in different parts of the world agree on a time and place to meet up for a practice sail. To do this we need to have the three (or ten!) users set up the program in the same way. Each will then drive their own boat, but see on the screen the other boats taking part.  We will turn on collision avoidance setting in AIS to see how that works as well.

We can do this with a canned environment by forcing a fixed wind and fixed current, or we can share the same model forecasts for wind and current. The Training Mode install includes wind and current for a section of the Eastern Strait of Juan de Fuca, between WA state and Canada, so we will use those for the example.

These are text notes for the process, followed by a video demo of the operation. Here is what we need to have the same:

1) Same polar loaded. We can use the default classic 40 version.  These could be different but if we want to race it would be more interesting if all the same.

2) Same GRIBS loaded. In the Training mode, you can load the HRRR wind in Slot 1 and the OFS current forecasts in Slot 2.

3) These environmental data overlap starting at 4/11/2025 at 15:00:00.So after loading the GRIBS, use the clock icon to set the the Grib time to that value.  (If needed, go to Config/ General / Units,  and choose UTC.) 

4) Since we are not using live data, we also need to tell qtVlm when we want to start the simulation.  This is done under menu Boat / Boat Settings / Navigation Simulation Mode. Force the starting time to 4/11/2025 15:00:00

5) It might be helpful, but not crucial, to Display the Grib time slider in the Grib Config window.  If it does not look like this, then something is wrong. Do cmd+I or ctrl+I to see what is loaded. 

Check in the Grib config / Corrections window to be sure you have no forced wind or current. That will over ride the grib data.

6) For simulation and real sailing it is valuable to turn on the Microboard (this is not on in the initial Training Mode configuration). Menu / Config / Boat /Show microboard.

7) We might also share 3 marks to show where we should start our boats. If we are not mindful of this, we could run into each other quickly, which ends the simulations. Here is a sample. None of this has to be precise.

8) When you start the simulator, the boat will start head to wind, so it will not be moving. If you are moving, then the engine is on.  Check Boat / Boat Settings / Engine and Tacks/Gybes.  Be sure both are set to 0.0.

9) We also need to set up the AIS configuration to make this work. menu /qtVlm Config / AIS to see this:

And we need to turn on the AIS from the tool bar:

AIS icon must be green.

Then we should be ready to go, and each participant can start the simulation and it should look something like this, keeping in mind that it takes a minute or two for the actual AIS vessel name to show up.

Note in the Seattle screen above, he has a COG predictor cone turned on and we are sitting head to wind in a current, so he is drifting backwards.  All the boats are doing this, but only this one has that turned on.

In principle we could agree to a boat set up and a chart, etc. but once this is running there is much to learn and practice with.

Here is a look at the three computers running the simulation. 

Later we will discuss the various program controls we can use to optimize sailing routes.

Steering in a following current

There is a video of this article linked at the end. 

A few years ago we analyzed the grounding of the Ever Given in the Suez Canal just a few days after it happened. The results are presented in a 5-min video, followed by three support videos, one of which had to do with the effect of a following current on steerage, which was likely a factor in the event.  See discussion of tides and currents about 6 min into that video. There presented and discussed this diagram:

It shows what we mean when we say "steering is inhibited in a downbound vessel with following current."

When we turn left 30º relative to the water, we can see it on the  compass, but it is not apparent by looking at the water alone. Where we would see any change of course would be relative to, say, a rock ahead that we were turning away from. We will see that we have turned the heading away from the rock, but we will not be making good the full extent of our turn. The current causes us to lose some of the turn we made. In other words, the heading HDG (also called course through water, CTW) changes, but the course over ground (COG) changes less, which is the issue at hand. Conversely, when sailing into the current the maneuvers are enhanced relative to the ground.

Yesterday we got a call asking if this reckoning could be generalized into a formula which motivated some work on the topic.  The above diagram we got from qtVlm app using its simulator function plus a forces current function.

Here is the geometry

Vessel moving due west at 6 kts in a 3 kt following current. Then turns left 40º but only achieves a 27º turn in COG. 

Here is the trig and formula for the angle loss (beta) as a function of the turn (alpha) and the ratio of current to speed, r = C/S

We can also make a plot of the values:

For example, for r= 0.4, a 35º turn will lose 10º and you only make good 25º

Here are results as a table. Here speed over ground (SOG) is a ratio of the boat speed S.

Note in our figure above BA = SOG and we get SOG from:

Here are a couple examples from Excel:

Video presentation of the above discussion.

Symmedian point in a triangle

Some years ago we presented a procedure for finding the most likely position based on a plot of 3 lines of position (LOP).  We focused on finding that position when the random uncertainties in each of the LOPs was different, and we also added the crucial factor that there could be a fixed systematic error that applied to all triangles. We will return to that subject in the near future, with more details on the mathematics behind that solution.

In the meantime, we step back to the simpler case where the random uncertainties in all three of the lines are the same, and there is no systematic error to the set of sights—which could be cel nav sextant sights, or compass bearings, or any means of piloting that puts 3 LOPs on the chart. In this case, the most likely position is located at a very special point in the triangle called the symmedian point.

The concept of the symmedian was first reported in 1803, but  the name "symmedian" did not come about until 1883. It comes from an abbreviation of the original French name  "symétrique de la médiane," which was meant to convey "symmetrical counterpart of the median" — in that the symmedian line of a triangle is obtained by reflecting the median over the bisector. 

The discovery that this point reflects the minimum of the sum of the squares of the distances to the sides of a triangle, and thus could represent the most likely position from three LOPs, was presented in 1877 by another French mathematician, but the point had not been named at that time. 

Looking at the picture below, for any point P, there is a distance to each side of the triangle (dashed lines), and the argument is that the most likely position is where it is the closest it can be to all three sides, but since a distance can be negative outside of the triangle, the most likely position is the location where the sum of the squares of all the distances is minimum. 

One can solve for that location several ways, and always end up with the answer being the symmedian point K.

Here is an interactive online app that you can use to show that K is the minimum of the sums.

Least squares demo (to do it yourself)

Or watch this video on the process.

Navigating underway, we want to know how to find the point in any triangle of LOPs we run across. There are several ways to do this, maybe dozens! We go over here the basic method of reflecting the median over the bisector, which is all plotting, and then we combine a computation with plotting for a faster solution.

The traditional way reflecting the medians over the bisectors, but with some short cuts.

A hybrid approach of doing a couple computations first and then plot the results. 

Download the free online app that computes the distances needed.

Symmedian proportional distances computer

To get it on your phone, open that link in your phone and then share it to your home screen.

I will be back with more details on this topic as we proceed with our 3-LOPs rejuvenation program.

The app computations are based on this discovery from a German article in 1827:

Here is a form for solving the above rule by calculator:

Download a copy of the form.

Here we apply the plotting to a real cel nav sight session.

Role of qtVlm in the Starpath Coastal Nav Course

Starpath School of Navigation offers several online courses that use qtVlm for electronic chart work. 

In our Electronic Chart Navigation Course we cover qtVlm from the basics on up to advanced techniques of route planning and route safety evaluation, along with an introduction to optimum sailboat routing based on a boat's polar performance diagram and a wind forecast. We also take advantage of of its sophisticated simulation mode.  

In our Marine Weather Course we emphasize weather analysis using its sophisticated features (load multiple GRIBS, overlay images and maps, view near-live ship reports and ASCAT winds, meteograms, and more), and work on optimum sailboat routing using full environmental forecasts (wind, seas, and currents.) We also carry out simulated races where students in various locations can meet and compete in sailboat races using the same boat polars and wind, each seeing the others as AIS targets.

Our Inland and Coastal Navigation Course (ASA105), on the other hand, has a more basic, but still crucial, introduction to qtVlm, where we focus on the equivalent to paper chart plotting. The course is oriented toward paper chart navigation, as mariners would carry out using back up NOAA custom paper charts (NCC), but at the same time providing an introduction to a truly powerful electronic nav app, qtVlm. Below is a sequence of video tutorials that are limited to those applications we cover in that course, where we show that all of the traditional paper chart plotting and piloting we are accustomed to with paper charts, can be carried out more quickly and more accurately using qtVlm.  A broader range of qtVlm support can be found at starpath.com/qtVlm/#support.

If you are taking our Inland and Coastal Navigation Course, this would be the sequence to follow:

1) Install qtVlm and the Training Mode  Mac (9:09); PC (12:19)

2) Mac v. PC, chart types, 18465tr, menus, units, zooming, saved views. (13:59)

3) Compass set up, using marks, landmark searching (15:47)

4) Measure range and bearing between points (10:29)

5) Update a DR position from logbook entries (15:44) — WorkBook 5-24

6) Compass bearing fix (10:31) — WorkBook 4-9

7) Range and bearing fix (8:01) — WorkBook  4-15

8) Fix from two ranges (4:58) — WorkBook 7-5

9) Running fix (11:03) — WorkBook 6-3

10) Danger bearings (10:11) — WorkBook  6-2

11) Read Tides and Currents (past, present, and future) (23:39) — WorkBook 8-12, 8-13, 8-14

12) Find current from two fixes and DR between them (8:43) — WorkBook 9-1

13) Find CMG and SMG in a known current (7:59) — WorkBook 9-11 (A)

14) Course to steer (CTS) for desired course in known current (12:14) — WorkBook 9-2 (E)

15) Running fix with current and course changes (12:15) — WorkBook 9-14 (Errata)

          ________ How to load NOAA charts for your area ________

16) Loading NOAA Charts into qtVlm on iOS devices (5:03) 

17) Loading NOAA Charts into qtVlm on Mac or PC (7:06) 

If you are not taking our online course (sorry to hear that!), but you still want to practice with these methods, then you can work through the exercises in our Navigation WorkBook 18465Tr. This is a large set of plotting exercises that can be worked with qtVlm. The Workbook's support page provides an RNC of the needed 18465Tr.