The Bench

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.). 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 busses 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.

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I resisted using the blog as an advertising tool as long as I could, but I figured this is as good a place as any to let people know about our deal for the Ockam U manual.

When you order the Ockam U manual through our web store, enter the coupon code BANDGSUX BANDGSTINX at checkout to receive free shipping within the US. For those customers outside the US, this coupon code will take off the equivalent charge from your order.

That’s it: just a short and sweet message for a special online-only deal!

EDIT: I changed the coupon code, as the original was not very nice. My grandparents would be appalled at such language!

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For those of you out there that use Expedition to fulfill your racing software needs, there is an update available that addresses a funny bug when communicating to the Ockam system (especially T1 processors). It seems that during the background housekeeping that goes on during a graceful software shutdown, Expedition can send commands that set all the calibrations to 0.00, causing some math headaches (multiplication by zero, for instance) when the system runs without Expedition. This usually manifests as ridiculously high windspeeds, or absurdly low boat speeds. A reset of the T1 processor returns the system to normal operation.

We believe that this problem only affected Expedition installs newer than v5.1, but if you are experiencing these symptoms, try updating your Expedition installation to resolve the issue, especially if you are using AutoCal tables in Expedition. I have personally only spoken with four separate boats having this issue, so I do not believe the problem to be widespread. As usual, only the earlier adopters felt the growing pains.

Many thanks to Nick White for his quick and capable resolution for Expedition!

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Alas, the plight of the ignoble load cell: doomed to ride for eternity at the point of a knife, dunked in salt water ever and anon, but expected to work flawlessly.

Load cells are a tremendous help when tuning the rig for different conditions. They are especially important on fractional rigs, where it can be hard to judge the headstay tension based on the backstay (unless you have done your math ahead of time). Load cells are also used frequently on larger yachts to judge rig tension all around, as it would be pretty difficult to use your typical Loos gauge on 3/4" PBO.

On land, load cells are pretty much indestructible. They are used widely in industrial and transportation applications with nary a problem. However, put them in the marine environment, and forget about that eternal reliability, especially when put on a yacht. Load cells, especially load pins, have some important considerations that are frequently ignored, leading to failure.

The load cell is basically a Wheatstone bridge. The actual configuration of the measured circuit can vary, but they are all essentially the same. Bending stress on the special printed circuit causes a change in resistance in one or more legs of the circuit. This change in resistance unbalances the Wheatstone bridge, and this is measured by the amplifier. The amplifier then converts the measurement into something usable by external devices, typically in RS-232 or NMEA 0183 format.

One of the most frequent failures I have seen involves the cabling. A well-sealed cable keeps out salt water and prevents infiltration into the electronics. Once this cable is damaged, ether through a nicked sheathing or a pulled-away seal, failure is sure to follow in short order. Salt water will wick throughout the wire by capillary action, and will also seep into the actual sensing portion of the load cell. As we all know, salt water and electronics don’t mix. I have received items back where the entire cable has corroded inside the sheathing due to one nick in the rubber.

Another common failure has to do with the way that the load pin is constructed. Many of them have a preferential strain direction due to the internal configuration of the individual strain gauge circuits. The preferential direction is usually indicated by an arrow etched on one end of the pin. If the pin is mounted in such a way that the strain is applied in a different direction than indicated, the printed circuit foil inside is either ripped off its mounting, or simply tears and breaks continuity. Either way, you end up with a non-functioning load pin.

Load cells are also susceptible to lightning damage. They are mounted to the mast and rigging, which are almost always made of a conductive material. This forms a large antenna on the boat, which can lead to damage to the load pin through induced current by a nearby "splash" strike - the lightning doesn’t even have to directly hit the boat! It’s usually pretty obvious when this happens, as there are a few other things on the boat that stop working as well. However, I have been on at least one boat that had a load pin fail due to lightning, but left everything else intact.

There are also the typical bug-a-boos of electronics: weak connections, bad power, et cetera. If the amplifier isn’t powered, or if wires aren’t connected, chances are that the load pin won’t work.

A load pin can be connected to the Ockam system through a variety of interfaces: the older 066, the newer 067, or the T2 (among others). Read the individual manual sections in these links for some
troubleshooting information specific to the interface; some of these
troubleshooting items are repeated below.

If a load pin is connected to an Ockam system, and it doesn’t seem to work, here are a few things to try:

• Check that the interface is sending data on the expected tag. It is possible to change the data tag on many Ockam load cell interfaces. If the display is set to look at the wrong data tag, then you will not see data. Also check that the data tag is not used by another interface on the system (for instance, a position interface).
• If connected to a T2, check that switch S3 is set to accept load cell data.
• On the T2 and e-Series (square box) interfaces, check that the data input light is blinking as though data is being periodically transmitted. If not, the signal cable or power cable for the amplifier box may be loose or disconnected.
• Check that the amplifier box is configured correctly. The Diverse Yacht Services load pin amplifier (the yachting world’s de facto standard for many years) has two internal rotary switches that need to be configured correctly. Refer to the documentation included with the amplifier for the correct settings.
• Check that the load pin is zeroed. With no tension on the backstay, the forestay load pin should read right around zero. If not, try re-zeroing the amplifier.
• Check the resistance on the unloaded pin. There are five wires from the load pin. One wire is connected to the shield and can be disregarded. The resistance between any two of the remaining wires should be 350 Ohms. The resistance should be the same for all possible pair combinations. If it is significantly different than 350 Ohms, then the pin is damaged and must be sent in for repair.

Beyond this, it can get complicated. Don’t hesitate to contact us if you have a load pin problem on your boat that you can’t resolve.

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For those of you who have raised children, you know how much a wet diaper can irritate a baby’s bottom. Changing the diaper is one of the "baby care triumvirate" when trying to soothe a little one (aside from feeding and nap time). Think of all those times that you have sat on the rail during a race and had a wet bum - it gets pretty uncomfortable after a while.

Likewise, wet electronics are unhappy. Marine electronics are surrounded by water, so it’s especially difficult to keep everything dry, but it’s worth the effort. It’s highly recommended to keep the CPU and interfaces in a sheltered location to prevent any contact with water. Special attention should be paid to the location of the interfaces by the mast. Many boats douse the spinnaker through the forward hatch, so it’s possible to get water on the interfaces if they aren’t sheltered properly. The CPU is usually safely located behind the nav station or in a similar location, so exposure to water isn’t usually a concern. All bus and sensor cables should be routed away from water where possible. This means that cables shouldn’t be run through the bilge (except possibly depth and speed transducers, because there’s not much choice). The displays are typically pretty immune to water, so they aren’t as much of a concern, although they also have some special considerations.

The mounting angle of some displays can cause water to accumulate along the bottom edge of the bezel near the glass. This isn’t a problem in the short term, but water shouldn’t be allowed to stand there for long periods. If you have an older display with a weak gasket seal, you may find yourself with a failed display once the water seeps in and corrodes the electronics.

Another issue that typically crops up in the spring and fall is the presence of condensation or misting inside the unit. This usually happens when there is a small amount of moisture trapped inside the display. The more extreme temperature cycles found in the spring and fall (warm days followed by cold nights) will really show the tiniest amount of trapped moisture. This moisture is easily removed from the Ockam displays. All displays (005, 007, and 044) have a blue plug on the rear of the unit. This plug contains desiccant that removes moisture from the inside of the display. The desiccant does need to be recharged every once in a while, but this can be very easily accomplished.

If there is a lot of condensation, you may also want to remove the display from the boat and bring it indoors. After removing the desiccator plug, place the display on the windowsill with the glass facing out towards the sun. This will help drive the moisture out of the enclosure. Then take the desiccator plug and place it in a WARM, not hot, oven. I find that a toaster oven at the lowest setting works pretty well. Leave the desiccator in the warm oven for 1-2 hours. Let the desiccator cool to room temperature before replacing it in the display, and then put the display back on the boat. DO NOT UNDER ANY CIRCUMSTANCES PUT THE DISPLAY IN AN OVEN - IT WILL MELT!!!

If you are doing this on one display, you might as well do it on as many displays as you can. This way, you know that you have dried out the interior of the display, and recharged the desiccator plug. It’s just one less thing to worry about. You may even consider adding it to your commissioning/decommissioning routine.

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