Читать книгу SELF-STEERING UNDER SAIL - Peter Foerthmann - Страница 19

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”.

Nandor Fa onboard K & H Bank

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).