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What are Polars

Everybody talks about them, but what are they?

Ockam37aWhat are Polars anyway? Ever hear someone talking about hitting their targets at the post-race party, and wonder why they were shooting at people? They’re not trying to shoot the competition, they’re just using polar plots of the boat’s performance to judge how they are performing against the boat’s potential in the conditions.

Many people are familiar with polars as the table and graph documents that detail the boat’s performance in a variety of wind speeds at wind angles from dead downwind all the way up to in irons (at least the good polars cover this range). It’s not very convenient to whip out some sheets of paper while racing, so the better instrument systems (Ockam included) can compute and display polar performance information on the fly. This may seem like overkill to win a race, but all the best racing programs use polars. To paraphrase a recent sailing forum post: “How do you recognize people who use polars? They’re standing up front with the trophy in their hand.

I’ll not go into the nitty-gritty details of plotting polars – that’s covered pretty well elsewhere. However, there are a few important details that are worth noting.

First, everyone should be aware that polars use true wind angles and speeds. This is what the boat “sees” as the basis for its speed, as true wind is independent of the boat’s motion through the water (unlike apparent).
Almost all polar sets, or at least the initial model runs, assume optimal conditions. This means that there is no accommodation for bad sails, bad trim, bad driving, bad weather, or bad luck. If you have old sails, it should be pretty obvious that you will not be reaching your polar targets. For those people inexperienced with the use of polars, it may not be as obvious that bad weather will also prevent you from reaching your polar targets. If you have to reef, or if you are pounding through waves, the boat will not be driving to its potential speed for the given wind speed.

A boat isn’t precluded from having more than one polar file. Many high-end programs will have the initial prediction file from the designer as a basis for starting measurement, and then also build a file from observed performance.

One aside:building a polar file from observed performance can be difficult since it’s hard to winnow out bad data. Performance analysis is typically done off the boat much after the race, so it can be hard to determine when the boat is responsible for a particular data point, or if an external factor is at work (e.g., bad helming, collision avoidance, weather, etc.).

Learn more about why you shouldn’t try doing your own polars.

A good alternative polar file built from observed data can provide a way to compensate for weather and sea state. It takes a lot of concerted effort by the person doing the analysis and a large data set in a wide range of environmental conditions to provide a good foundation for analysis. Some boats also have multiple rig or sail configurations that strongly affect performance, and require separate polar files for different configurations. The Ockam system has always allowed for the use of several polar files. On the 001 CPU with 037 Performance Index, there was a hardware switch to set the desired polar file. On the T1 CPU, the polar file can be selected with a software command.

Another detail that should be obvious, but really isn’t: you need good instruments to use polars effectively. Your instruments must measure the boat and its environment accurately and precisely to give you a good idea of the actual performance. This means that you must have instruments with reproducible results, and must have any measurement errors corrected (i.e, calibrate the instruments). The more astute reader will have realized that since true wind is the basis for polar performance, then good calibration of the instrument system is a must. Some instrument systems have no capability for calibration, and are completely unsuitable for using polars. Imagine driving a car with a speedometer that worked differently each time you drove – bad instruments are like that. It’s pretty impossible to know how well you’re doing from day to day if you don’t have reproducible results.

It may not be completely obvious, but GPS-based SOG should NOT be substituted for speed through the water! “Why?” you ask… SOG does not take into account any current. Those of you who have sailed in foul current (such as The Race in Long Island Sound) know how frustrating it is to trim the sails perfectly in good wind, only to make 1.0 knot headway over the ground. Now imagine if your polar performance was based off SOG. Assuming you have decent wind speed and a good point of sail, the polar performance would show you making some paltry low percentage of your expected performance! There would be much gnashing of teeth, since it seems like you’re doing everything correctly and not making any speed. However, if you use speed through the water, it will at least show that you are making the best speed through the water possible for the wind conditions. The Race is an extreme example, but it illustrates the point that current can significantly affect your speed over ground, thus rendering SOG a poor indicator of performance.

Resolving wind speed and angle to predicted performance can be a problem if you have a very coarsely granulated polar file. In the past, the Ockam system required very strict data ranges to provide polar information through the 037 Performance Index. Wind angles had to be provided from dead downwind (180 degrees), all the way up to extreme pinching (ideally around 15 degrees) in 2 degree increments. Winds speeds were provided from 0 to 25 knots in 0.5 knot increments. These rather strict requirements were due to the limited processing power of electronics back then. Remember when 33 MHz processors with 16-bit buses were the leading edge for PCs? More powerful processors have opened the door to better functionality; the more powerful processor in the Ockam T1 has loosened the requirements for the polar files. It can interpolate values with far less data points than before. However, the polar file shouldn’t be too sparse on data points if any sort of accuracy is desired. Data points every 10 degrees and 5 knots are a good minimum standard for the T1 processor, but higher data resolution is always better. Areas of the performance plot that have large changes in a small region should have data point higher resolution to capture the predictions accurately.

For most instances of simple performance comparisons, polar plots that cover the range from close-hauled to dead down wind will suffice. When using VMC sailing and Wally, having more information past close-hauled becomes important. VMC sailing becomes especially important when going to a mark that is not directly in line with the wind (typically some sort of distance race). It becomes even more important if the wind is shifting over time, such as is found in almost every distance race. The performance information for the region above close-hauled allows computation of the possible VMC benefit of sailing both above and below the rhumbline. This allows comparison of the distance advantages between sailing on a conventional rhumbline course and sailing off the rhumbline (either above or below) at the fastest VMC speed. Without the data above close-hauled, possible advantageous sailing is eliminated from the calculus of the fastest route! That would be like only allowing your trimmers to adjust sails while on only one tack, and hobble you from your possible best performance.

Sailing with polar performance comparison can induce a lot of headache, and has a pretty steep learning curve. Many people simply don’t have the time to fully comprehend all the nuances of using the performance analysis with their instrument systems. However, many of the more common functions can be easily incorporated into the tactician’s tool kit with a little study and practice.

What are polars

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