Article by Capt. Steve Katz
The Straight and Narrow
If you have an autopilot, you know how useful it can be for those long offshore runs. When it is not working properly it can be a distraction, requiring you to manually steer the boat.
An autopilot is one of the most complex areas of marine electronics on a boat. It consists of 4 major components - electric drive unit, heading sensor, course computer and display. If any of these four parts are not operating at 100%, the entire autopilot package will suffer. For those of you who are familiar with autopilots, you may have wondered why the rudder feedback is not mentioned as a critical component. Most modern autopilots are designed to operate without a rudder feedback, especially with outboards. However, they can also be used on inboards as well, although a rudder feedback on an inboard boat often helps the autopilot do a better job at holding a course.
The drive unit is most often a hydraulic pump (when used with hydraulic steering). Newer boats or engines that steer-by-wire will have a gateway module, allowing the autopilot to “talk” to the electronic steering system. Larger center consoles and sport fish boats usually have a traditional hydraulic steering system that can use a standard reversing hydraulic pump for the auto pilot. For other applications, a linear drive, rotary drive or other speciality drives are available from most autopilot manufacturers .
The typical hydraulic pump used with an autopilot is a 12 or 24 volt DC powered reversing pump. The pump is capable of running in two directions, allowing the boat to be steered to both port and starboard. This is accomplished by the course computer reversing the polarity of the electrical power going to the motor.
The typical drive unit used in a hydraulic steering system can be thought of as an additional helm pump (helm steering station). The autopilot drive unit serves the same function as the helm wheel, but is turned by a motor instead of a wheel. The maintenance and service of the drive unit is similar to the rest of the steering system when it comes to bleeding the system, checking for leaks and other maintenance. One important feature that should be added when installing a hydraulic drive unit are isolation valves that will allow you to close off the rest of the boats hydraulic steering system so that the autopilot drive unit can be removed and serviced without disrupting the rest of the steering system. Without these valves, the steering system will be unusable while the drive unit is removed and upon re-installation you will need to refill and bleeding the system after service – a lengthly process.
The course computer is the brains of the autopilot, receiving data from multiple sources and commanding the drive unit to steer the boat. This computer receives data from the heading sensor, rudder feedback and chartplotter at a minimum. The course computer then processes this information and determines when, how much and which direction to steer the boat.
The autopilot computer uses the heading sensor to determine the vessels heading, which is where the bow is pointed and not necessarily the direction of travel (course). Autopilots generally have two modes of operation, heading hold (auto) or navigation to a waypoint (nav). When engaged on heading hold, the autopilots job is to maintain the vessels heading, keeping the boat pointed in the same compass direction for the duration of operation. Depending on wind, waves and current this mode may not deliver you to your destination, since the direction of travel is not compensated.
When an autopilot is used in navigation mode, waypoint information, including cross track error, is sent from the GPS chartplotter to the autopilot and using that information, the autopilot will attempt to steer the vessel to the waypoint using any heading necessary to keep the vessel on the calculated path to the waypoint. This is most useful for long runs and when wind, waves and current affects your course. When engaged in navigation mode, the cross track error is the distance your vessel has varied to one side or the other from the straight line course to the destination waypoint. This information from the chartplotter is mandatory if you want to use your autopilot in navigation mode.
Of all of these components, the autopilot heading sensor is the most critical. This is often a stand-alone device, similar to a traditional compass enclosed a case with wires that connect to the course computer. This device is sensitive to magnetic fields, just like traditional compasses, it needs to be mounted in an area without magnetic disturbance. Be sure not to store metal objects in close proximity of the heading sensor after installation. There are some modern heading sensors that use multiple GPS receivers “satellite compass” to determine your heading. The newest heading sensors incorporate motion sensors, up to 9 axis in one device, that can help improve heading accuracy. They include accelerometers, which measure angular velocity in all three axes; gyroscopes, which monitor pitch, roll and yaw; and magnetometers, which measure azimuth information, these too need to be installed way from magnetic fields.
Whether new or existing, an autopilot system needs to be calibrated correctly for proper operation. Autopilots often have a calibration procedure that is partially completed at the dock and a second portion conducted on open water on a calm day. The sea trail portion often calibrates the heading sensor and also the course computer during the same sea trial. Once completed, the autopilot settings can be adjusted by the captain to fine tune the systems operation.
Keeping your autopilot system in top shape will allow you to concentrate on vessel navigation and operation functions instead of hanging on to the wheel for the entire trip.
Captain Steve Katz is the owner of Steve’s Marine Service and holds NMEA, AMEI and NMEA2000 certificates along with ABYC Master Technician certification and factory training from many manufacturers. To reach Steve, call (631) 264-1600.
