Zezo forum:

Post by RickW | 2015-01-22 | 00:32:38

Studying some details of polars and their relationship with TW, boat speed, helm pointing and TWA.

Example: TW is 12.3. Optimum TWA is 44 degrees and there is a red line on the plot. In the polar plot arc of the plot is in three colors. There is a red line on that arc between 44 and 49. Then it turns green

What does the little piece of red telling me about my performance there even though within the optimum?
Thanks, Rick.

12 Comments

Post by lotsemann | 2015-01-22 | 03:58:14

Dear RickW,

The colors of the arc refer to different sails. In the case you presented, red is for Jib3, green is for Code Zero and orange is for Light Gennaker.

Important for the performance analysis may be the bumps in the polar arc for a given wind. Consider a wind of 24 knots. In the TWA arc between 71 and 73 degrees, just before the transition to Code Zero, the speed actually decreases with reference to the speed at 70 degrees. Losses also happen between 100 and 108 degrees of TWA (transition from Code Zero and Light Gennaker).

Such bumps help explain why the router eventually suggests oscillating between TWA 70 and 75.

Post by RickW | 2015-01-22 | 19:35:24

I had no clue about the sails to the colors. That was new information. The bumps I have seen. I use the track tool to move through the clutter of the oscillations. That seems to smooth it out.

Thanks for the learning. Hope things are well and sorry you are not up here with the rest of us nearing for the Cambodian Coast.

Post by smallcraftwarning | 2015-01-22 | 20:39:38

Here's a question: as we are beating to Sanya, the polar chart tells me I will maximize my VMG at 41 degrees TWA; yet, the routing program tells me to sail at 49 degree TWA. I shortened the forecast period to 1 day to minimize the effect of positioning for future wind periods, and this seems to have made no difference in the recommended route. Anyone have any comments/explanation for this significant difference?

Post by zezo | 2015-01-23 | 00:17:51

In shifting winds the router may want to get you somewhere alse than strictly upwind, and the optimum VMG might not be the fastest.

There is supposed wind shift along the Vietnam coast and the router tries to make the best use of it.

Post by RickW | 2015-01-23 | 03:27:01

I KNEW IT!! Programs have a digital life. Software is the nervous system and algorithms the brain. :-)

Post by Almosphere | 2015-01-23 | 17:11:04

I'm not so sure about the accuracy of these polars. 200 miles ago I started side by side with verylastmohican and I sailed to the max VMG TWA of 41 (ignoring zezo's recommendation to sail lower) and he sailed to TWA of 45. We have had the same wind speed the whole time. I had a slight TWD advantage to begin with until we crossed paths and now he has it. He also has a 15 mile lead now (in distance to the finish) although he is about 5 miles to leeward of me so maybe only a 10 mile lead. FYI, the game's polars show the same VMG in every case I've checked.

Post by zezo | 2015-01-23 | 18:21:59

Wind is never exactly the same for two positions.

There may be up to .1 knot wind/boat speed difference before you notice because of the rounding of the numbers in the interface.

Also, verylastmohican currently has a 1 degree better wind angle.

Post by RickW | 2015-01-23 | 18:22:28

I think the VR servers are slow or not enough in number. It is not common to be sailing overlapped with another boat and see that boat move ahead or back during iterations. I have learned the boat is relative to me over time. It is also more to being correct if I reacquire the VR server (not the control panel refresh. I do this anyway at each isochrome.

I have also noticed that at the wind update that it takes about 3 iterations for the 20 boats tracking around me to settle down to what I expect them to be doing. This is another indication or VR machines catching up.

I find the polars can be a degree off but not a bother. I used the VRTool boat attitude panel for the 10 minute details. I also use that panel for MY DESTINATION tracking which is often closer than that used in Zezo because of routing.

Sorry to be long but there is a lot going on in what you have observed, because there is. Rick

Post by zezo | 2015-01-23 | 18:30:35

There are two things at play here:

1) There is delay for your friend's positions. It's best to reload just before the game cycle.

2) The other boat speeds are delayed by 10 minutes and this is by design. For your boat you see what the speed for your NEXT move will be. For other boats you see what their LAST speed was.

Post by Almosphere | 2015-01-23 | 21:41:56

I thought watching for 10 hours would even out the variations between cycles.

I did notice in the mobile app that the TWA is shown to 4 decimal places when you adjust your heading making me think they are using an interpolation of the GRIBs for both pressure and direction and at distances like this, a tenth of a tenth difference means many miles.

Post by karriv | 2015-01-23 | 23:55:13

Guys, even if you're beating, maxVMG isn't always the fastest way to get where you're going. It is if you're heading dead upwind and the wind is constant in direction and pressure, which is rarely the case.

One good idea to understand what is the fastest route is to see what Zezo suggests and try to understand why it suggests that. If the TWA suggested is something else than maxVMG, there are many possible reasons: e.g. by sailing lower you get faster to better angle/pressure, wind shifts etc.

Post by Almosphere | 2015-01-24 | 10:40:49

Understood. Eat a header to ride a lift later, etc... or find the lane with best pressure across the course, but in this specific case I thought those other variables were minimized and when we crossed just a few hours ago, his lead was drawn down to 5 miles. This could entirely be explained by the slightly better TWD over the last few hours that I was pacing him, it was the early side by side comparison where I felt his VMG was better, despite the tools at hand.