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SELF-STEERING UNDER SAIL. Peter Foerthmann
Читать онлайн.Название SELF-STEERING UNDER SAIL
Год выпуска 0
isbn 9783844256406
Автор произведения Peter Foerthmann
Жанр Языкознание
Издательство Bookwire
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 systems can best be protected by ensuring that all power cables are well insulated.
Extreme Sailing
Autopilots are unable to steer in areas where North is uncertain. Extreme sailors in races like the BOC and Vendée Globe run into problems in the high latitudes of the South Pacific with the autopilot suddenly cutting out after losing its fix on North. Nandor Fa, skipper of the Hungarian yacht K & H BANK in one Vendée Globe (singlehanded non-stop around the world) received the following answer from the manufacturer of his Robertson system after faxing for help with his confused autopilot: “Please perform 3 complete circles in calm water within a few minutes - this will enable the compass to reorient itself”.
Given the chaotic sea conditions in the Southern Oceans this was not the most practical piece of advice. Only after several days of steering by hand did Fa come upon the idea of removing the compass and rotating it as gently as possible in his hand. Since then he has used Autohelm systems, which now have special GPS-supported software to help the compass maintain clear steering signals even when North is uncertain. The close collaboration between manufacturers and extreme sailors in events like the BOC and Vendée Globe ensures continued development of the systems. Virtually all the boats in these races are now steered by Autohelm.
One result of this collaboration has been the development of stronger drive components for blue water use. Autohelm introduced the ‘Grand-Prix’ upgrade package for its 4000/6000/7000 series in 1996. The standard Delrin (plastic) load-bearing components in the drive are replaced with metal equivalents. Plastic, as several long-distance skippers have had the misfortune to discover, sometimes fails to measure up to the high stresses placed on drive components. For holidays and day sailing however, when extreme loads are rare, plastic components are perfectly adequate. Hydraulic systems are immune to overloading problems of this nature as they have no mechanical drive components (Autohelm 6000/7000 with hydraulic or hydraulic/linear drive, B&G NETWORK, HYDRA 2, Robertson, VDO).
Autopilots for Different Purposes
Holiday and weekend sailing
Most sailors use their boats primarily at the weekend or for holidays, which partially explains the rapid spread of electric autopilots. Power consumption is not really an issue on one day trips and the quality of steering performance is also relatively unimportant since it is always possible to steer by hand if necessary. Sea conditions rarely impair steering quality as the majority of weekend sailors do not venture into exposed waters. Taking the helm in any case forms part of the fun for the average sailor, so the autopilot is really just a convenience. It sees to the tedious work (steering while under engine) and gives the crew the freedom to eat together, for example. Autopilots, at least the cockpit models, are also within the financial reach of the average sailor.
The significance of a yacht’s autopilot grows with the length of the voyage. There will generally be no problem finding volunteers to steer on a shorter trip, but on a longer trip manual steering becomes tedious and the autopilot will eventually be called into action. The average weekend and holiday sailor has a good autopilot but makes relatively little use of it.
Autohelm has devoted far more effort to the weekend and holiday sector than any other manufacturer and is the world-wide market leader; thanks