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Autopilots Explained

This is the most complicated, hard working piece of electronics that most people have on their boat. It is also a very complicated subject.

An auto pilot consists of a control head at the helm which hooks to a brain box. The brain (or computer) is responsible for making decisions to steer port, steer starboard, or maintain rudder.

The autopilot brain is an attempt at artificial intelligence. The eyes and ears or the pilot are the compass, the rudder sensor, the GPS and possible pitch and yaw and wind.

The compass is the heart of the sensor system. Typically this is installed under the step into the cabin or under the dinette seat, but it varies greatly. It is installed at a point of minimum magnetic interference, and as near the center of pitch and roll as possible.

The rudder sensor tells the brain where the rudder is now and feeds back how much it moves while steering. Having an accurate rudder sensor greatly enhances the ability of the pilot to do its’ job, which is to steer the boat straight.

Auto pilots operate in several modes. First is AUTO mode. This means that you aim your boat where you want to go and the autopilot will hold the existing course. All autopilots have AUTO mode.

Next we have NAV mode. This is the mode that allows the autopilot to steer to a waypoint destination entered on your GPS chartplotter. This is a fairly complex process for the brain to accomplish and one of the things you should watch is your Cross Track Error or XTE.

XTE is the distance you are off of you original line of position. Some pilots display the cross-track error right on their screens, otherwise, it can be brought up on the chartplotter.

It is very important that you engage NAV mode with a low or zero cross-track error. Otherwise the pilot will be squirrelly and could surge suddenly, especially as you approach you destination, since the course to steer changes more rapidly when you are close to a waypoint.

Another mode that autopilots run in is WIND mode. in this mode the pilot steers based on wind angle and only God knows where you will end up in a shifting wind.

Newer pilots can now steer the depth contour. In this mode, the pilot attempts to keep the depth constant and is useful for cruising, but mostly it is used for fishing.

Simrad is and has always been the finest autopilot in the world. Previously know as Robertson, these Norwegian made units are both rugged and high performance. Furuno pilots are also rugged and HP, but no virtual feedback and no depth steering. The Raymarine X Series pilots are not yet a proven product. The Garmin pilot, introduced in 2009 is designed for outboard boats only.

Virtual feed back is the name for the new pilots that steer without using a rudder feed back unit (rudder position sensor). Simrad, RayMarine and Garmin are the only companies I know of that offer it. It simplifies the installation substantially on outboard boats.

Stern Drive boats have always been a problem until now. With the Simrad AP14R, you replace the steering system with the pilot and it’s done. An awesome piece of engineering, if you have a stern drive boat (or outboards) that is cable steered, this is the only good solution.

Auto pilots are a mix of science and black magic. Each must be specified for the boat it goes on. With hydraulic steering, a reversible pump is used, for mechanical steering, there are a number of drive solutions, including large below deck drives for sail boats and wheel drives for sail and sterndrive boats.

Auto pilots add very little safety to your boat. Basically it adds danger. When returning to the harbor, if you forget to disengage the pilot, the results can be, well, have you heard of the Exxon Valdeez? The pilot caused that accident, or was it the drunk guy operating the pilot? Suffice it to say, the autopilot is the most dangerous, complicated and desirable marine electronics item.



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