SELF-STEERING UNDER SAIL. Peter Foerthmann

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СКАЧАТЬ shows a clear trend: in the last 10 years alone the average length of participating yachts has grown to around 44 feet while the number of sub-33 footers has dwindled to barely a handful. The boats are generally equipped to a very high standard as well, with most carrying navigation instruments such as GPS, plotters and radar, short wave, SSB and VHF radios, refrigerator, pumps, water maker and interior and exterior lights.

      Combining the 24 hour average power consumption of each of these appliances for a 44 foot boat in warmer latitudes gives a total of 120 ampere-hours (Ah) - even without an electric autopilot running. This example clearly illustrates the care needed in budgeting for energy aboard a sailing yacht. The impact of an autopilot on this energy budget is very substantial, particularly if the system has been chosen for its high performance rather than low power consumption. There are whole books devoted solely to the subject of energy management on board: pay too little heed to this complex issue before you cast off and you can count on a nasty reminder somewhere out at sea.

      

Navigating on deck with AUTOHELM

      The manufacturer’s recommended autopilot for our exemplary 44 foot boat draws between 2.7 and 6 A per hour, which means that if run continuously it will push up the vessel’s total power consumption by at least another 50% over a 24 hour period. We should also bear in mind here that some units in the vessel’s electrical system will fail if the voltage falls below 10.5 V. Against this background a seemingly large battery capacity of 600 Ah begins to look less impressive.

      Wind, water, wave and solar generators can help but depending on the conditions even these are no guarantee against a few hours of enforced motoring every day (as well-known circumnavigator and race organiser Jimmy Cornell confirmed after debriefing skippers at the end of his EUROPA 92 race). Whenever one of the additional generators malfunctions or fails, increased engine running time inevitably results. Without good sound-proofing, the iron sail can rapidly become an unwelcome intrusion into life aboard. The extra heat from the engine is often a bonus as well, just right to ease the afternoon chill in Bermuda...

      Energy matters are of course less critical on boats used mainly for weekends and holidays since this kind of sailing normally involves plenty of motoring and shore power is always at hand for recharging.

      Range of adjustment of an autopilot

      (Example: Autohelm 6000/7000)

      1. The Rudder Gain, which has 9 settings, specifies how much the rudder should be moved to return the boat to a desired course. Oversteering will result if the angle is set too high, understeering if it is set too low.

      2. The Rudder Damping function has 9 positions and serves to damp yawing motions.

      3. The rudder amidships position in the rudder reference transducer has an adjustment range of -7 to +7 degrees.

      4. The Rudder Limit function prevents the autopilot reaching maximum lock at full power, which could cause mechanical damage.

      5. Boat Turn Rate determines how quickly the boat turns when the autopilot makes course corrections.

      6. The autopilot can be set for an average Cruise Speed of anywhere between 4 and 60 knots (sailing boat or powerboat).

      7. The adjustable Off-Course Alarm sounds when the vessel’s course deviates from the desired course by more than a set maximum (in degrees) for longer than 20 seconds.

      8. There are four Trim settings. This function controls the additional rudder movement necessary to counter off-centre thrust (e.g. when operating a propeller mounted to one side, only used when motoring).

      9. The Joystick has two settings, but these are not particularly relevant for sailing boats.

      10. The control unit can be set for linear or hydraulic drive.

      11. The response angle function has 9 positions. It ensures that the response of the autopilot is appropriately delayed if there is slack or play in the steering system.

      12. Compass deviation taken from a chart can be input.

      13. The adjustable Northerly/Southerly Turning Error Compensation feature is used in areas where the orientation of North is uncertain to ensure the compass receives an accurate signal.

      14. There are three settings for the reaction speed of the autopilot; the higher the value set, the greater the steering precision and, consequently, the power consumption.

      All the functions mentioned are initially set at the factory. They can all be adjusted on board and it is essential that they are individually matched to the characteristics of the vessel.

      To summarise, each model of autopilot gives a certain level of steering performance which is dictated by its range of technical features and which cannot be improved. All that is left once the autopilot is correctly set up is to increase the time between steering corrections, and hence save power, by ensuring the boat is balanced and the sails are properly trimmed. It should be obvious that selecting a greater degree of steering accuracy will lead to more frequent rudder movements and increased power consumption.

      The Limits of Autopilots

      Even the very best autopilots struggle beating into a shifting wind. This is because they do not detect small changes in wind direction (the sails back). The only solution is to set a lower course which, unfortunately, means losing distance to windward. It is possible to connect a windvane to the course computer but, as we discussed above, this does not always produce satisfactory results.

      Blue water sailing though means winds from astern. The passage routes around the world are universally known; every long-distance sailor heads straight for the all but infallible trades, dreaming of pleasant sailing before the wind. It is therefore imperative that autopilots, and indeed any type of self-steering, can hold an off-the-wind course. No experienced sailor expects miracles of the autopilot: a steering accuracy of 5 degrees in the trades with a big following sea just is not realistic. Equally, it is no good if your autopilot follows the general course with occasional 100 degree excursions - you may still arrive, but probably not where you intended.

      The only way to be sure of good steering from an unassisted autopilot is to buy a fast and powerful system. While nothing else will be able to guarantee adequate steering performance in all wind and sea conditions, this solution does inevitably lead us back again to questions of power consumption. Ultimately each skipper has to decide, in light of energy budgets and daily power requirements, which answer best suits his or her particular needs.

      Issues of power consumption often tempt a skipper to risk a slightly undersized autopilot. There is no avoiding the loss of performance such a system will suffer as conditions deteriorate. With no reserves of speed or power to meet the increased demands it will eventually be overwhelmed, reacting too slowly and with too little force to keep the boat on course.

      The rated operating speeds and drive unit thrusts of the various cockpit autopilot systems are a good indicator of the steering performance you can expect.

      Electromagnetic Interference

      Electromagnetic interference originating from onboard high-frequency transmitters and receivers was once a common problem, causing autopilots to make sudden anomalous course changes. The European CE (Electromagnetic Compatibility) Standard should prevent this kind of disruption of the autopilot in future. Existing electronics СКАЧАТЬ