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Follow the breeze

Jan 1, 2003

Little more than a decade ago when a sailboat embarked on a singlehanded transatlantic race, autopilots and their various spare parts were often stacked below in wine racks awaiting the inevitable multiple failures of the working components. Linear drives seized up. Control heads and computer boxes broke down. Reliability was not an autopilot's hallmark.

Since that time, however, autopilot manufacturers have worked with a wide range of sailors and power-boaters, including those aboard industrial-strength fishing vessels, and have made steady gains in reliability, functionality, and ease of use. Today's breed of autopilots not only are more reliable but are capable of impressive feats of interfacing. One example of this that is of particular interest to sailors is the option of having the autopilot steer by the apparent wind angle.

The Brookes & Gatehouse Hydra Pilot, for example, can steer to a selected apparent wind angle (AWA). When "Wind" is selected with a Hydra instrument/Hydra Pilot system, the pilot will steer a course that maintains a set target apparent wind angle (target AWA). If the AWA shifts, the pilot will alter course so that the actual AWA remains the same. Robertson and Autohelm also realize that, in sailboats, maintaining a steady apparent wind angle is often a high priority. Each has windvane input options that automatically steer the boat using the AWA as a set course reference.

Working with a B&G 690 system or higher, the B&G Hercules pilot system has the additional feature of steer to optimum wind angle (OWA). Data from the Hercules system's polar tables, stored in the performance processor, allow the boat to sail at the OWA. This feature is particularly useful for achieving maximum VMG (velocity made good) to windward, saving miles and making a racing sailboat more competitive.

There are, of course, trade-offs in sailing to an AWA or OWA. On the one hand, the advantage to sailing to a given AWA is that sails are set to that wind angle, and they will continue to drive the boat at optimal speed as long as the boat maintains a constant wind angle. However, there some inherent risks that must be considered. A change in true wind direction means a change in course. Boats have been known to run aground as the vessel changed course due to a wind shift without the skipper's knowledge. So having this capability means the captain or navigator must pay attention to the vessel's position in relation to hazards and shoal water.

Steering to an optimal wind angle also makes several assumptions in the quest to reach the highest level of performance. First, the instruments need to be properly calibrated, and, second, the polar performance data need to be correct. As the wind speed changes, the OWA may also change, requiring that the sails be trimmed to the new wind angle. However, for the prepared skipper willing to trim his sails, the results will be optimal sailing performance.

While high-speed surfing, such as in the Southern Ocean during the Around Alone Race, sailboats can radically move their AWA forward while racing down a wave. When the boat gets to the bottom of the wave, there is a danger that the wave will shove the stern around, accidentally gybing the boat. During the entire process, the autopilot could keep bearing the boat away from the true wind direction as it relies solely on the AWA information and inadvertently increase the possibility of an unintended gybe. Numerous inputsIn order to further understand these situations and eliminate them, autopilot manufacturers are working closely with many of the world's top offshore sailors. Numerous instrument inputs are sent to the autopilot, including compass input, wind data, rudder feedback, boat speed, positioning data, and so forth. The compass data can be supplied by the system's dedicated fluxgate compass, a stabilized compass such as KVH's new three-axis gyro-stabilized Azimuth GyroTrac, or other multi-purpose electronic compasses. On sailing vessels the wind data is usually supplied by the masthead unit, although aboard several of the Around Alone boats, including Marc Thiercelin's and Josh Hall's vessels, backup windvanes are located at the stern. Better technical understanding of the problems encountered by these high-speed race boats will help in providing solutions to voyaging boats. An autopilot that has numerous sensory inputs may be able to prioritize a series of solutions or responses.

The advantage of a "well informed" autopilot is that, when things go wrong, appropriate defaults can be employed and the display can notify the skipper of the problem. As an example, when the masthead unit self-destructs and the boat is being piloted by AWA or OWA, there could be a danger of the autopilot shutting down at a critical moment. Instead, the B&G autopilot, for example, defaults to its last-known compass course and a disaster may be averted. The skipper can then fix the problem, employ a backup windvane, continue to steer by a compass heading, or sail to a selected waypoint.

High-latitude sailingespecially in parts of the Southern Ocean south of Australiarequires special attention to compass input. The shape and angle of the Earth's magnetic field makes non-stabilized compasses slow to react as the compass card revolves very slowly. The delay in compass correction can then lead to slow response time by the autopilot. Robertson and Furuno have been leaders in the field of gyro-stabilized compasses for some time. Many of both manufacturers' earlier units were targeted to large commercial ships or fishing vessels. The sizes were large, and power consumption was high. The gyros took some time to start, so they were often left running while not in actual use. Robertson was one of the first to come out with a smaller version of its gyro-stabilized compass. With the introduction of KVH's Azimuth Digital GyroCompass, an even more energy-efficient and smaller-sized unit was made available. The ADGC was successfully used aboard several of the Whitbread boats in the 1997-98 event. The Azimuth GyroTrac is KVH's latest bid to further reduce cost while improvingamong other thingsthe performance of the autopilot.

The type of compass used isn't the only way to improve the compass data going into the autopilot. If the autopilot is set up to accept compass data from an NMEA data sentence, the compass input may be available to the autopilot only about once every four seconds. These "snapshots" of the compass may or may not be the actual heading. The data could be dampened or averaged over time, but that would further reduce the accuracy and speed of response. Having a dedicated compass provide real-time input can allow the autopilot to get its heading information up to eight times per second.Power use depends on conditions

All of this increased accuracy may seem to also imply an increase in the amount of electricity consumed, but that's not necessarily the case. Aboard some vessels such as offshore fishing boats, power consumption may pose a minor consideration. Aboard long-distance sailing boats, however, it may play a major role in how or if equipment can be used. Robertson and B&G both have a variety of settings from which to choose that allow the skipper to determine the rate of response for the pilot and its corresponding power consumption. Sailing a boat in light to moderate conditions to weather may require a "normal," setting, or an economy setting may be possible. More averaging is done by the software. Response parameters are widened, and less rudder is applied. High-speed sailing with heavy downwind conditions and a quartering sea may require the response of the pilot to be quicker. The rudder is applied sooner and/or more often to avoid broaching.

Starting with accurate information input and setting the response time to the current conditions allows the autopilot to respond appropriately the first time. Rather than over-correcting and then bringing the boat back to the proper course, the "well informed" and properly adjusted pilot is more likely to get it right the first time, saving wasted energy in the process. Getting it right the first time applies to maneuvers as well as straight-line driving. When engaged in the Wind mode of B&G's Hercules Pilot, the pilot computer calculates the True Wind Angle (TWA), thus enabling the pilot to indicate when it is possible to execute a pilot-controlled tack. When the "Tack" key is pressed, the boat will be steered on the same Target AWA but on the opposite tack. The pilot software steers the boat through the wind, calculates the new target AWA, and adds 5° to force the boat to bear away to ensure a fast pick up of speed. When the boat approaches the modified AWA, the pilot picks up the original target AWA. When the "Gybe" key is pressed the boat will be steered on to exactly the same target AWA, but on the opposite tack. The pilot software controls the steering so that the wind slowly passes astern of the boat, ensuring that the boom and sails are safely transferred to the opposite tack.

The last decade has given us a variety of reliable autopilot solutions for virtually any offshore vessel. They now take us from San Francisco to Boston or around the world without failure. That's not to say they are flawless. They still can and do make mistakes. But as the engineers designing the equipment continue to work on a variety of inputs and settings, those mistakes continue to diminish and overall performance is optimized. Spare parts, of course, are often prudent to have in the ship's stores, but no longer is a wine rack required to carry a huge selection of spare linear drives. There is, after all, a better use for that wine rack.