Been playing with a pretty cool tool called GPS Action Replay.

Here's a screen shot of a track recorded during my single-handed test sail yesterday:



The winds were picking up and I chickened out from doing a spinnaker run on this downwind leg (though I do have a track from an earlier run in lighter breeze I'm analyzing).

Note that the polar seems a bit lopsided. This is likely due to the moderate swell being nearly head on during port tacks to weather and side on during starboard weather tacks. Downwind I was able to do quite a lot of surfing on port, but not on starboard. The polar reflects this. Also note the tool has no way of knowing the true wind at any given moment, so I basically just estimated it at 270 deg. Wind was ranging around 10-13 kts and rising as time went on.


Mike.

That looks very cool Mike. I'll have to test it out, especially the auto-polars function, so thanks for the link.

About your plot, were you going upwind against the current? Everybody say you should get 45deg angles upwind depending on conditions, but your plot show about the same angles as we got when we measured last year.
Nice lift at the top of the screen.

45º is an "ideal" place to be. When the seas are pounding at a 45º (or worse) against your bows, then you may or may not be able to obtain a 45º angle to the wind.

Add to that, the prevailing current and you have a very challenging sport of "sailing to windward."

I would very much much enjoy comparing (solo) upwind tracks with other cat sailors. I have some tracks saved in my hard drive. On a good (high wind) day, I have been able to sustain upwind sailing tracks of approximately 90º off of the opposite tacks; 45º to the wind.

This 45º was usually done on my Mystere 6.0, with the main traveler some several feet down from center, and no jib flying.

GARY

That software estimates wind direction by finding a symmetrical polar diagram, assuming you get the same course to windward/downwind on starboard or port tack. it's quite nice but a change in wind direction during your trip will mess up your polar. see the attached polar, it appears as 35 degrees would be fastest upwind, but this was probably caused by a 10 degrees windshift...

I was going upwind on the red colored trace (indicating lower speeds) from right to left in the picture. You can see a lift on the red port tack lay line (wind coming from left side at ~270 deg). Breeze was building slightly and shifty during this tack, swell basically head on. My two first tacks show about a 90 degree angle, so I was doing about 45 degree off TW. The third/last tack I footed off and prepped for beam reaching, so the tack angle looks bad. The sharp turn is the full bear away & gybe. I'm then beam reaching, jib & main alone. I'm able to get some surface action sustainng for 15+ seconds regularly and it seems to impact the polar significantly. On the next leg following a gybe to starboard, I can no longer surf and the wind lifts...exaggerated no doubt by the lower speed since not surfing. Gybe angles look pretty shabby...but I think that's an effect of the small jib...can't sail well anymore with the AW off the beam...got to go higher. Gybe angles with spin look more like 90 degrees, but this is in only 5-7 kts true...we do less than 90 in better air.

Mike.

I suspect you're right...your polar is much cleaner than mine. I will be doing some more tests in the coming weeks.

Mike.

Other uses for this software...race review & comparison.
Here's a screen grab of two tracks from the Frenchy Rum Run 6/30/07:


We had a 5 km lead upon entering the "zone of death" near the island:



By the time we got out some 40 minutes later, we'd lost 10+ minutes to the trailing boats.

At the end of the race, we were only 2.5 km ahead of Smoke 606
:


See the race between Full Tilt (Tornado) & Smoke 606 (Nacra 6.0) unfold using the GPSActionReplay applet here:

GPSAR of Frenchy's Rum Run 2007

You will need to install JRE on your browser to make this work...installer link provided on the page.

Smoke 6 oh 6 is a NACRA 5.8 NA

Some reflections on this after last year's Nationals at SDYC. 45 degrees is an over-simplification:

The smooth polars I've seen for the Tornado http://tinyurl.com/2ltfj4 put max VMG at 50 degrees, but anything between 45 and 55 is very close. I assume that these polars are from a velocity prediction program and assume flat water. IMHO, these numbers are consistent with flat water reality, but Mike was sailing in swell off shore. Other reasons the 45 degree number is unrealistic follow:

At competitive race starts people sail high, attempting to keep others from sailing over top of them and costing them several boat lengths. Once the better teams have given us bad air :-) they can foot for best VMG, and do so. [Confirmed by a former Tornado campaigner.] Tactics here demand winners be able to sail high fast.

In the Nationals' procession to the first windward mark, Mike and I struggled to point as high as the leaders. To do so, we had to over trim the jib (confirmed by Jay Glaser) and derotate the mast.. and fell back relative to the rest of the fleet. (I think weight was a contributing factor to this, as was our complete inexperience with the new carbon mast, which we had just installed.) Weight in chop requires footing for power, but we felt that would have put us in a whole lot of bad air from the leaders. Tactics here demanded that we sail as high as the lighter crews on this leg to avoid a continuous blanket of bad air, or take two additional tacks.

Similarly, when we sailed the first leg with local Nacra 20's, we had a similar problem if we did't have a clear start: we would be forced to sail higher than best VMG on the polars to stay clear of their bad air. The committee end of the start line can be a bad place for a Tornado in a Nacra 20 fleet.

So when do you want to sail a few degrees lower for absolute best VMG? Pretty much whenever you don't have reason not to, IMHO. But I think one needs to be aware of the two mode and embrace them. Otherwise, you will be trying to nail 90 degree laylines and pinching up the laylines while the leaders walk away.

I was surprised at the Nationals that the top teams never missed their laylines at the windward mark. (We didn't either, but had some close calls.) I think I now understand why:
First: I think experience has taught us not to touch the laylines when far from the mark. We plan for a last tack near the mark and getting the layline call right there. Calling the last layline close to the mark helps a lot. Overstanding the layline by a degrees or two costs very little close to the mark, but costs a fortune far away.
Second: Since the entire range of 45 to 55 degrees off the wind is nearly equivalent, you don't *have* to bang a 45-degree layline. In fact, best performance is achieved at the 50 degree layline, and tactically you don't want to tack before that, giving the competition a chance to sail faster over you. If you tack on the 50 degree layline, there is very little chance you will not be able to make the mark because you tacked a bit early, since you can sail 5 degrees higher or lower without losing much VMG. Why shoot for the 45 degree layline and risk not making the mark or having to pinch all the way there?)
In otherwords: *don't* tack a Tornado as soon as it can make the mark. Tack later for better VMG and better odds of making the mark.

As for calling shifts reliably, Sailing World had a very interesting observation on this: In top regattas, the committee tries very hard to keep the windward mark to windward on each lap, and notifies the competition as they round the leeward mark (with green or red flags, indicating if the mark has moved to starboard or port). Effectively, the committee is telling you how the wind has shifted at the windward mark, via radio, possibly before the shift has reached the leeward mark. If the shift has not reached the leeward mark, you should sail first in the direction the marked moved, to get upwind in the new wind (before the new wind arrives).
With this sort of information about coming shifts, I think one should definitely sail in VMG mode instead of tactical mode. (Tactical mode helps hedge against unfavorable shifts.)

Of course, having two modes of sailing complicates calling laylines, as one must know which mode is being sailed and factor it into the layline call. Also, only the person holding the mainsheet knows which mode is being sailed, so communication is required if the other crew member must call the layline.

On top of all that, even this "bimodal" model is an oversimplification, since one can sail high in flat water but must foot to power though chop. 45 degrees is just a reference, and one should think in terms of how much lower one must sail for tactics or best VMG in the conditions.

If I'm wrong, please let me know, :-)
--Glenn

Since those plots show boat speed, not VMG, are you doing the calculation to arrive at 50 deg. for max VMG?
The GPSAR software can show both speed & VMG on one plot (solid lines for speed, dotted for VMG)...but to be accurate you need to put in the true wind direction & velocity.

The beauty of conventional polars is that they show both absolute boatspeed and VMG to windward, which is simply the vertical component. The best VMG to windward is at the highest point on the curve. The best VMG to leeward is at the lowest point on the curve.

Keep in mind, however, that only a single polar curve is shown, and it's for a constant wind and sea state. The curves can change dramatically with wind speed, especially the downwind part. See Bethwaite's High Performance Sailing for dramatic examples.

--Glenn

Take a look at this thing:
EveryTrail

map in full screen view mode

GARY

downloaded their old free version and after installing the required java stuff was happy to see i was able to open all the sailing tracks i've made the last few years with my garmin

esp. fun starting the animation of the track and having the little boat race around at the plotted speeds

but as gpsar will only open gpx files it was necessary to 1st open my old sail tracks with garmin's mapsource and then resave them as gpx files

I did some high speed runs a couple of month ago with a Hobie 16 and used GPS Action replay to analyze it.

http://www.webbo.dyndns.org/hobbe/speed_test/

/hakan

Hakan,

You are SO fast!

GARY