The concept is the same as in airplanes and similar, albeit way more advanced, than the Cruze-feature in an automobile. Marine autopilots take the task of steering your boat allowing you to go grab lunch, a drink, or just have your back face the vessel wheel and chit-chat away with the others onboard.
Autopilots are pretty darn good at holding a steady course in light to moderate conditions with minimal helm movements. They don’t tire or get bored, and have lots of patience and plenty of focus. Autopilots are also smart and intuitive; they steer accurately enough to save fuel and get you to your destination faster, especially when interfaced with a GPS.
Operating an autopilot is the same as giving an easy task to a dedicated servant. Put the vessel on the desired heading, hold the course for a few seconds, press AUTO, and release the helm. That's all. The autopilot will eat this information, keep the details in its memory, and will respond in kind with helm corrections to keep your boat on course.
As the technology advances, sophisticated autopilots come with better and more accurate methods for maintaining the course. Some of those include auto-trim, auto sea-state, and integration with GPS, as well as an ability for learning your boat’s handling characteristics to improve steering over time.
Do keep in mind, that even though there are various Autopilot models available and they keep on getting better, these devices to some extent rely on your experience and steering capabilities. If you have a hard time holding a course, so will your autopilot. Unlike a wind-vane (a stern-mounted mechanical self-steerer used on cruising sailboats), autopilots work harder as seas build and wind gets stronger. Eventually, the limits of the pilot’s power output are reached, and the device gets overwhelmed.
An overworked autopilot can try as it may to do the job, but chances are for it to result in failure. One of the biggest causes for finding yourself in the middle of the sea with a tired device is that long-distance sailors frequently buy small, cockpit-mounted autopilots, which aren't meant to operate flawlessly over a multi-year cruise. Relying on your autopilot to be doing most of the steering on a long boat-ride might be setting yourself up for disappointment and frustration.
So, unless you absolutely enjoy steering by hand for hours, do prepare -- by getting the correct device and using it properly.
Options and Integrations
Interfaces can connect the autopilot to a GPS or GPS chartplotter, so for it to follow a course to a waypoint, or correct for set and drift. The correct interface will enable you to set up a trip on the chart plotter, with course changes at different points. The chartplotter will control the autopilot, so that it stays on the course, making course changes at the proper location. Compatibility of equipment and the proper interfaces are critical, and you should consult the manufacturers or qualified marine electronics professionals before you buy. Qualified professional help is often needed in the installations, but this will soon pay for itself in the pleasure and assistance you get from a well working system.
Displacement and Boat Length
Boat design, length, and displacement are critical factors in determining what type of device to get. Long, narrow boats, and boats with v-shaped forward sections and long-keeled sailboats tend to be more stable as far as the direction of its movements is concerned.
Another factor to consider is how easy or difficult steering your boat is going to be. The more difficult the ride, the more power steering will take, the more sophisticated a device you should get. An autopilot might not be the ideal solution for a hard-to-steer boat. Below-deck autopilots are far more powerful, more reliable, and better at steering, so they should be your first choice for long distance cruising.
The Right Size
The manufacturer-recommendations of both, boat and autopilot, serve as helpful pointers towards the right direction. They aren't perfect, but these are a good starting-point. Always try to err on the safe, conservative side, especially when it comes to displacement. When it comes to displacement, for example, manufacturers tend make a recommendation for your boat based on it being empty, while the actual displacement when loaded for cruising could increase by as much as 20%.