Zezo forum:

Post by zezo | 2014-11-12 | 15:33:50

I started work on a polar chart graph. Preview available here http://zezo.org/vor/polars.html
Will be adding VMG and sail data soon.

34 Comments

Post by HenrikBo | 2014-11-12 | 22:13:05

Cool... ;)

Post by Snowpetrel-2 | 2014-11-13 | 09:03:39

Another great tool! Very helpful to know TWA for sail crossover at particular speeds. Want to avoid headings causing multiple auto sail changes. I suppose downwind VMG would be negative (mouse aft of 90 degrees). Sp

Post by zezo | 2014-11-13 | 10:26:54

The natural result is negative, but there is an abs() call to take care of the minus sign.

Post by BlackDiamond06 | 2014-11-14 | 19:41:56

Thanks a lot Cvetan :)

Post by Patricio | 2014-11-14 | 21:56:46

Hi Cvetan,
Great help and thank you for your great support.
Princess Marina
Patricio Mangino
Skipper.

Post by Hobart | 2014-11-15 | 06:05:49

Great new tool!

Thank you

Post by Sir_Petrus | 2014-11-16 | 01:14:49

Hi cvetan
How do you calculate the max and min wind angle.
I know it it is the tangent of the polar curve, perpendicular to the heading of the boat but how do you do it in terms of calculation?
Thanks in advace.
And thanks to your zezo tool.

Post by zezo | 2014-11-16 | 12:15:11

VMG is the projection of boat speed in the forward direction, so It's the maximum/minimum point of the boat_speed*cos(twa) function.

Post by Sir_Petrus | 2014-11-16 | 13:48:45

Hi Cvetan
I think I didn't explain well my question.
I was asking about those angle marked in red on the polars. Angles that we should not go beyond.
minimum when going upwind and maximum when going downwind.

Best Regards
Pedro

Post by zezo | 2014-11-16 | 14:38:43

Boat speed is calculated for every integer degree in order to plot the graph. Then VMG is calculated, and the angles corresponding to minimum and maximum VMG ares saved and used to plot the red lines.

It's possible that for certain wind speeds there is more than one angle resulting in the same (or very close) VMG. You can check by watching the VMG numbers while moving the mouse around the red lines.

Post by NOR7034 | 2014-11-16 | 17:16:16

Thank You Cvetan, - this will be a very helpful tool!

I tried making a table showing the optimum TWA's for every wind speed and noticed what You mentioned that there are some wind speed areas where the optimal VMG can be reached with several TWA's and sail types. That makes me come up with a new question:
Does the Zezo tool take into consideration present sail selection and course (extra jybes) when calculating the route or is it just getting the present boat position from VR?

If the latter is the case using the tool too frequently in such wind areas could result in unneccesary sail changes, - or does Zezo have some kind of a preference sail in such cases?

Best regards
NOR7034

Post by zezo | 2014-11-16 | 17:36:31

The router accounts for sail changes, and tries to account for your present heading and avoid unnecessary tacks, but it does not always work, so it's a good idea to check the polars for critical spots now and then.

Post by Ivato | 2014-11-18 | 17:56:47

Once again, Cvetan have made a great job.
Sincerely thanks to you ...

Just a small idea : to put a color for the speed (or other font bold and so).
This will help to make a visual difference with the VMG speed.

Best regards.
Ivato

Post by zezo | 2014-11-18 | 18:14:59

Boat speed is bold now.

Post by manzana | 2014-11-20 | 21:30:53

Hi Cvetan !
Great Job.
Thanks a lot:)

Post by TomekN_zegluj_net | 2014-11-20 | 23:34:09

Hi Cvetan,
One again thanks a lot for your great tool!
Without it I would never win Sevenstar race. ;-))

I found one small thing in VOR routing...
Did you set "restriction" (exclusion zone) to sail below 50 degree (South) ?
When I set "destination" somewhere on 60 degree South isochronies are shown as straight line on 50 degree.

Could you check it?

Post by zezo | 2014-11-20 | 23:40:24

You can check "ignore gates" and disable all restrictions if you are sailing alternate race or want to see the routing down there.

I'm trying to keep feasible routes within bounds, and bounds ease the load on the servers at least two-fold.

Post by TheHC72 | 2014-11-21 | 10:41:09

Thanks a ton for the tool and keep up the good work!

Would it be possible to add a 24hrs function? related to the 24hrs record and regardless of direction towards goal :)

Thanks in advance,
J

Post by POR36 | 2014-12-01 | 02:28:14

Hi Cvetan,

I have been trying to understand how to work with the polar.

Sailing according to data provided below (boat position from zezo tool), am I on the best VMG?

2014-12-01 00:40 CET (T+0:00)
Wind: 222� 4.4Kt (TWA -100�)
Heading:323 Sail: LG
Boat speed: 7.94 Kts

Always grateful and looking forward to your comments.

Post by karriv | 2014-12-02 | 15:46:44

OK, maybe it would be good to make some terms clear:

VMG = Velocity Made Good = how fast are you making progress upwind/downwind

With TWA 100 it is quite certain that you are not doing best VMG.

Actually, looking at the polar, the twa for optimal VMG upwind is 47 and downwind 142.

I would guess that what you are thinking about is VMC (= Velocity Made on Course, how fast are you making progress towards the next mark). To know whether you are doing best possible VMC one would need to know the bearing to next mark. Once you know that you will get VMC with the formula VMC=SOG*Cos(BRG-COG).

Post by zezo | 2014-12-02 | 19:04:09

Yes, the VMC/VMG distinction is important.

But the important rule of thumb is that if you don't lay the mark (it's past the optimum VMG) you have to sail the optimum and tack/jibe. Unless it's close and within a degree or two where the tack penalty would be more than the loss of VMG.

Post by POR36 | 2014-12-02 | 21:00:28

Thank you for setting a few needed references. VMG got it! With this we have solved upwind\downwind.

Now for every other, either than up\downwind I will have to navigate the boat as fast as possible. This VMC. Correct?

For the VOR game:
SOG is the indicated boat speed, since there is no current to add to the calculation. The same rational will be applied to COG. Good?

Missing to the equation is the "BRG"
Taking into account one possible definition:
"A bearing is the direction from your location to any distant point given in degrees from north."

To get the BRG from my present position I would only have to take the mouse point it to my intended destination (mark) and read the number of degrees (red line zezo tool)?

Am I on the right track?

Thank you in advance for your time.

Post by Sir_Petrus | 2014-12-02 | 19:57:43

Since you are clarifying terms, what is the meaning of

SOG=???
BRG=???
COG=???

GW= good winds
Pedro

Post by POR36 | 2014-12-02 | 20:15:10

Hi, to my knowledge (used normally in GPS glossary)

SOG:Speed Over Ground (comparing with boat speed might give you a fair indication of current)
COG: Course Over Ground

Example: http://www.sailtrain.co.uk/navigation/sog.htm

BRG: ?? (Bearing?)
http://www.sailtrain.co.uk/navigation/bearings.htm

GW: ??? Not a clue! (Anyone?)

Carlos

Post by Sir_Petrus | 2014-12-02 | 21:53:57

Obrigado
GW = Good winds or Bons Ventos ;)

Pedro

Post by POR36 | 2014-12-04 | 11:16:08

Sometimes we just get to technical and hard to disconnect not seeing the obvious!
Abra�o
Carlos

Post by zezo | 2014-12-02 | 20:18:31

SOG = Speed over ground. In real life that is different from what the log show because of currents.

BRG = Bearing. Where your bow is pointing. Or direction to something.
COG = Course over ground

They are the same in the virtual world because there are no currents and drift/leeway.

Post by POR36 | 2014-12-02 | 21:34:19

I could do the calculation, but by experience I am sure that your reply will be way quicker.

From my position now indicates:

Wind:136 4.9 kt (TWA 95)
Heading: 37 Sail:LG
Boat speed: 8.51 kts

To build my knowledge from Leg to Leg - am I on VMC?

This is to say that while following the zezo tool course I should be on VMC? or the respectful calculation [VMC=SOG*Cos(BRG-COG)] is always required?

I have a boat just upwind of me heading more 16 degrees (21) but of course less speed (7.3Kt) and again my question is: I should have a rational over this!!! and is the main thing that I would like to learn.

Really appreciated the comments and insight.

Post by karriv | 2014-12-02 | 22:19:50

Unfortunately it is not quite that simple;) Or fortunately, depending on your perspective, as IMHO the complexity is what makes this so interesting;)

The first thing to keep in mind is what Cvetan already said. In 99 cases out of 100 it does not make sense to sail a TWA worse than the optimal VMG TWA, but rather tack and gybe. So in this case TWA<47 and TWA>142 would be no-go zones.

Now, asking whether you are on VMC would be like asking whether you are driving your car at a speed. Yes you are, the question is actually "What is my VMC?" and "Is that the max VMC possible?" The answer to the first question is simple, just apply the formula I gave. The answer to the second is a bit more complicated, you would need to do the calculation with multiple COG values and the pick the one with the highest VMC.

This would be the end of the story if the wind would be constant everywhere (well actually not, but quite close to the end;). But it's not. So again, when you have a long distance to cover to the next mark, very seldom the max VMC course is the one you want to sail. Which brings us to the next question "What is the optimal course I should be sailing?". And that, boys and girls, is the million dollar question this game is all about;)