Zezo forum:

Post by toxcct | 2017-03-13 | 11:32:51

Hi Cvetan,

Here some little improvements I'd find useful if you could implement them.

It would be nice to have a polar chart page, unrelated to a given/selected race, with the a dropdown field to select a boat type charts (Imoca, Type40, etc...).
That way, we could use the polars even though you didn't make a full race setup (that particularly useful for the VMG, which I don't have on my VR dashbord because I sail without options).

Beside that, it would be nice to have the following improvements on the existing polar graph :
- draw a section line showing the exact angle we're inspecting with the hovering mouse (like VR polars for instance)
- below the optimums, could we have a display of the higher speed angle ?

Small question as an end. How do you calculate the VMG (math expected ^^) ? How comes that it equals almost 0 kts on higher boat speed ?

Thanks already to read and show interest for my post :)
And thanks obviously for your so nice work on your router.

14 Comments

Post by Alexandria | 2017-03-13 | 22:22:48

Hi!

I have just read your post and I think I can help.

We have VMG = V x cos (TWA).

The sign of the VMG will depend on the cos (TWA) sign.

If the TWA is equal or greater than zero, but not equal or greater than 90 degrees, cos (TWA) will be positive -- you are going into the wind.

If the TWA is greater than 90 degrees and not greater than 180 degrees, cos (TWA) will be negative -- you are sailing away from the wind.

If TWA = 90, cos (TWA) = zero.

Post by toxcct | 2017-03-18 | 16:16:54

Post by toxcct | 2017-03-25 | 17:29:10

Post by toxcct | 2017-05-22 | 20:00:45

Post by zezo | 2017-06-04 | 15:01:30

Hi,

A have a real life, day job and family, and limited resources to support this site.

So I prefer to spend my time on improvements that matter the most - like get the winds in sync with VR and optimize the router.

The polar data is available in the game itself. I added it to the site mostly because the in-game polars were badly broken at the time. Well, they are still somehow broken because they don't show the VMG. Having a line moving on the screen just won't add any new information - I'm not saying it wold not look better.

About the catalog of available races - I sure can do that. But the polars change now and then, sometimes change back, so in reality a polar graph called Imoca 60 does not match all Imoca 60 games. And it will get out of sync sooner or later and will have to be maintained.

With a polar chart for each race it's obvious if the polars don't match because the track tool uses the same data file as the polar chart, so when you superimpose the tracks on the isochrones you see right away if they match.

Post by toxcct | 2017-06-06 | 11:35:09

Hi Cvetan,

Thank you very much for your reply.

I absolutely understand that you have few time to spend on the aesthetical aspect of the polar chart ;)

I was only proposing them if you would consider adding some graphical enhancements, but anyway, the router is a great helper.

Post by toxcct | 2017-06-23 | 18:28:33

Post by toxcct | 2017-06-26 | 11:26:50

I'd like to clarify that I already know how to retrieve VR polar files (.CSV), but these files don't provide a boat speed for every wind angle.
On your polars however, the "data array" used to draw the chart contain a value for every TWA.

My concern was more regarding the interpolation you use to build that dataset...

Post by zezo | 2017-06-27 | 11:35:08

Hi,

The reason behind the png file is bandwidth reduction. Modern browsers support native binary arrays that serve the same purpose, but at the time I wrote the page that was not so widely available.

The interpolation is bi-linear and has data at every integer degree and km/h wind speed (VR originally used wind data in km/h) so the speed calculation is a basic table lookup with additional bi-linear interpolation between the four data points.

All this saves code and time for the client program(s). You know that the CSV files come with different set of angles and wind speed, I just convert them to a standard format.

There is a slight down side to this - the ideal.png also has information about the proper sail at that wind speed/angle, but that could be wrong by half km/h because of its static nature. The data at the sail crossover points could also be slightly off, but that's not a real problem because you want to avoid sailing at the crossover point anyway if there is a big dip in the performance there.

Post by toxcct | 2017-06-27 | 12:26:10

Thank you very much.

I'll have a look at how to handle bi-linear interpolation which I'm not yet familiar with.

All this is quite interesting anyway (and I didn't know VR polar were still in km/h !! :O )

Thanks

Post by zezo | 2017-06-28 | 15:58:46

That's just a fancy name for a double application of linear interpolation.

Quick example if you have the points for 10 and 12 knots and 45 and 50 degrees you can interpolate the boat speed for 47 degrees for both wind speeds and then from those speeds get the boats speed at 11.5 knots / 47 degrees

Or you can solve the formula analytically and come to something like

(A[1]+A[2]-A[0]-A[3])*x*y + (A[3]-A[2])*x + (A[0]-A[2]) * y + A[2]

where A is an array holding the 4 values from above in the proper order (here x/y are offsets from those points, not absolute values)

Post by toxcct | 2017-07-17 | 11:55:58

Hi there,

Back on this topic.
As I said few weeks ago, I attempted to draw my own polars.
Actually, I mainly copied your code as a starting point, and I added the improvements I proposed to you back then.

I'd like to show it to you if you don't mind, and because I respect your intellectual ownership (I did not notice under which license your code was written anyway, so I'm asking).
If you're Ok to have a look, just tell me if I can contact you privately with the url...

Thanks very much.
toxcct

Post by toxcct | 2017-07-24 | 10:44:22

Hi Cvetan,

No reply from you on my comment above...
I guess that you are not interested into having a look, and that you won't mind if I publish it on my website then ?

Regards,
toxcct

Post by zezo | 2017-07-25 | 18:50:33

Hi,

I was on vacation and not really checking my email in the past two weeks.

My code is under "don't abuse" license ;-)

It would be nice to include a link back to this site if you publish something based on my work.

I actually haven't obfuscated/minimized the js on purpose - so other people can learn something from it (the VR specific parts, it's not a textbook level code)

You can always contact me privately at zezo@ this domain.