Electronic self-steering is controlled by electronics operating according to one or more input sensors, invariably at least a magnetic compass and sometimes wind direction or GPS position versus a chosen waypoint. The electronics module calculates the required steering movement and a drive mechanism (usually electrical, though possibly hydraulic in larger systems) causes the rudder to move accordingly.
There are several possibilities for the interface between the drive mechanism and the conventional steering system. On yachts, the three most common systems are:
Depending on the sophistication of the control unit, electronic self-steering gear can be programmed to hold a certain compass course, to maintain a certain angle to the wind (so that sailing boats need not change their sail trim), to steer towards a certain position, or any other function which can reasonably be defined. However, the amount of power required by electrical actuators, especially if constantly in action because of sea and weather conditions, is a serious consideration. Long-distance cruisers, who have no external source of electricity and often do not run their engines for propulsion, typically have relatively strict power budgets and do not use electrical steering for any length of time. As the electronic autopilot systems require electricity to operate, many vessels also make use of PV solar panels or small wind turbines on the boat. This eliminates extra pollution and cut costs.
Mechanical or "wind vane" self-steering started out as a way to keep model sail boats on course. The first time that it was used to cross an ocean was on a motorboat. The most widespread form of self steering, the servo pendulum principle (introduced by Herbert "Blondie" Hasler) uses power derived from the motion of the boat through the water to hold a constant angle to the wind with the use of the boats main rudder. Offshore, the wind direction is relatively stable, so over a number of hours this results in a reasonably constant compass course, and also means that the sails need not be adjusted. At present, new wind vane self-steering systems are present which incorporate a compass, to remove the disadvantage of having the boat's current coarse changed when the wind has shifted. Mechanical self-steering can be complicated to set up, so it is typically used only for long-distance sailing where the same course is maintained for long periods. Many boats fitted with mechanical self-steering also carry an electrical autohelm for use over shorter periods where it is not worth setting up the wind vane.
Mechanical self-steering gear is made by a number of manufacturers, but most share the same principle. A narrow upright board, the wind vane, is rotated so that with the boat traveling in the desired direction it is edge-on to the wind. The wind vane is held upright by a small weight below the pivot, but if the boat turns so that the board is no longer edge-on to the wind it will be blown over to one side as the extra surface area is revealed. This movement is transmitted by a series of linkages to a second blade in the water. In the simplest devices for very small boats, this blade acts directly as a secondary rudder, and steers the boat back onto the proper course. The force provided by the wind vane, however, is not sufficient to make this system work with larger loads, and hence a so-called servo pendulum system is used:
As the blade described above turns, the pressure of water moving past it causes it to swing out sideways on the end of a pivoted rod. The length of this rod and the speed of the water means that a considerable force is available at the top end of it, sufficient to change the course of much larger boats. This is achieved either by a connection to the main wheel or tiller (typically involving a complex arrangement of lines and blocks rigged around the stern of the boat) or by fixing the main steering in place and equipping the self-steering gear with its own rudder. Once the boat has moved back to its correct course, the wind vane stands up again as it is no longer blown over by the wind.
If the sails are trimmed correctly and the device is properly set up, wind vane self-steering is very effective. Some experimentation and judgment is usually needed, however, to determine the proper settings for a given vessel and steering mechanism. In addition, wind vanes perform poorly in very light winds, as the forces needed to operate them are much reduced. The same applies when traveling downwind, as the apparent wind speed is reduced by the speed of the boat.
As well as their requirement for power, many long-distance cruisers observe that electronic self-steering machinery is complex and unlikely to be repairable without spare parts in remote areas. By contrast, the often agricultural-looking mechanism of a wind vane gear offers at least the possibility of an improvised repair at sea, and can usually be rebuilt on land using non-specific parts (sometimes plumbing parts) by a local welder or machinist.
The topic of single handed sailors sleeping and hence not maintaining a 'proper watch for safe navigation' continues to be much discussed. Although this practice appears to be in contravention of maritime law, there has so far been no attempt to prevent it. A pragmatic approach is that such individuals are at least endangering only themselves, since the large cargo vessels that may collide with them while they are asleep typically are not even aware that they have hit something. In addition, radar alarms and the sheer vastness of the ocean mean that the problem is more one of theory than of practice.
A related device has been used on some windmills, the fantail, a small windmill mounted at right angles to the main sails which automatically turns the heavy cap and main sails into the wind, (invented in England in 1745). (When the wind is already directly into the main vanes, the fantail remains essentially motionless.)
A popular source on contemporary windvane technology is [The Windvane Self-Steering Handbook] (International Marine/McGraw-Hill--2004), authored by circumnavigator and Somali piracy survivor Bill Morris. One particularly valuable contribution of Morris's book is his coverage of the variety of alloys used in vane gear manufacturing. Morris admits to his practice of setting a kitchen timer for a half hour at a time and sleeping while the windvane steering device controls the helm, even in head winds of 25 to 35 knots. In a recent interview, he said he once narrowly missed being hit by a huge freighter while sleeping on his sail up the Red Sea. Morris points out, "An autopilot wouldn't have made any difference in this case. If I had been using an electronic autopilot, that freighter still would have been there. I made a choice to sail two-thirds of my circumnavigation single-handed, and I accepted the risks that came with that decision. I guess fate was on my side."