Self steering keeps a boat on a course with no body at the helm. On long voyages, self steering is not a luxury but a must. It protects from boredom and corresponding fatigue. Simply put, self-steering may save you and your crews’ lives. Once setup, the system will maintain a compass course. The system adjusts to wind and wake. It comes in three variations: wind vane, above deck wheel/tiller, and below deck pilots.

Wind Vane
The wind vane is a non electronic option. When sailing, freedom from electrical power can be essential. The wind vane is a green option powered by the very forces driving a boat off course: wind and water. The vane redirects the elements to maintain a compass course. When forced to port, the wind vane changes steering such that the vessel turns and squares back to starboard.
Wind is the driving force. Just like a weather cock, the wind vane rotates according to changes in wind direction. This rotation couples with a paddle in the water. The paddle rotated around a vertical axis amplifies the wind power. The, paddle rotation feeds into the rudder which changes the boat direction. The complex rotations keep the boat on course. The wind starts the process and through the paddle directs the energy to the rudder.
And the more powerful the elements, the better the wind vane performs. The stronger wind more forcefully directs the wind vane keeping the boat more closely on compass.
Models include Monitor (US), Norvane (UK), Fleming (AU). Peter Tiez at Voyager Windvanes (Ontario, CA) makes great vanes and is a knowledgeable, honest guy who I highly recommend. Prices are about $2,000 to $4,000 depending on boat size and extras.
Above Deck Pilot
Above deck pilots whether tiller or wheel need minimal installation and easily accessible. The pilot attaches to the tiller or wheel, a non moving part of the cockpit, and the compass. Then, actuated by an electrical motor or hydraulic piston, the pilot keeps a compass course. The boat will stay exactly at that degree heading. Prices are between $500 to $2,000 depending on boat size and autopilot function. Raymarine is a popular brand.
Below Deck Pilots
Below deck pilots connect directly to the rudder shaft maximizing power and function. Because the below deck option is directly connected to the rudder changing in motion are immediate and full force. Also, because of the below deck location, these autopilots come with more advanced computers that integrate GPS, course plotting. The below deck can not only stay at a compass direction but follow a course changing direction as necessary. Prices are $2,500 to $4,500 for systems depending on boat size and computer function. Raymarine (Autohelm) and Simrad are two leading manufacturers.
Conclusion
Three types of self steering are a wind vane, above deck, and below deck autopilots. The simplest is the above deck. The wind vane and below deck options involve more installation and upfront costs. The wind vane is independent of electricity while the below deck has more brains. Choosing one depends on your personal preferences and goals.