Article by Capt. Steve Katz
Autopilots
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.
A typical marine autopilot is one of the most complex areas of marine electronics on a boat. Its job is to steer the boat on a selected heading or course, giving the Captain more time to concentrate on other navigational and operational tasks. The autopilot system consists of 4 major components – rudder feedback, drive unit, heading sensor and course computer. If any of these four parts are not operating 100%, the entire autopilot package will suffer. For those of you who are familiar with autopilots, you may have wondered why the display is not mentioned as a critical component. Most modern autopilots are designed to operate with a dedicated display or connected to a network with a Multifunction display (MFD) or even a wristwatch! No longer do all autopilots require a dedicated display.
The rudder feedback is a sensor that mounts next to the rudder and moves in unison with the rudders. This motion is converted to an electrical signal that is sent to the course computer for processing.
The autopilot then knows how much it is turning the rudders and learns what is needed to keep the boat on course. The rudder feedback can be an optional component for some boats where it is not practical to install a sensor, such on an outboard powered boat. In this case the autopilot computer is equipped with an electronic motion sensor (sometimes called phantom feedback) that “senses” how the boat is turning when the autopilot is in operation. For most boats, if you have rudders you should include a rudder feedback in your autopilot system.
The drive unit must match the type of steering system on the boat.
Most sport fish boats have a hydraulic steering system and therefore use a dedicated hydraulic pump as the autopilot drive unit. Some new boats have a steer-by-wire system (Helmmaster, Optimus, Viper) that incorporate an electronic gateway module, allowing the autopilot to “talk” to the electronic steering system. Some larger boats have a power steering system operated by solenoids, the autopilot system needs to have the ability to connect to and operate the solenoids to steer the boat. There are also autopilot drive solutions for other types of steering systems, such as a linear drive and rotary drive.
The heading sensor is another important part of the autopilot system, its job is to provide a fast and accurate measurement in angular degrees (0-360) referenced to north. Traditional magnetic or fluxgate heading sensors operate similar to a traditional ship's compass, with sensors that convert the compass heading to an electronic format that the autopilot computer can process. Many modern heading sensors are all electronic, typically incorporating an attitude and heading reference system (AHRS). Since these systems do not need the traditional sphere, they can often be installed in a variety of orientations and subsequently calibrated after installation. These heading sensors still sense the north magnetic field and can be affected by interference from nearby metallic objects. Previously only available for larger vessels due to the size and cost, the Satellite compass is now becoming a popular option as a heading sensor for autopilots and the rest of the boat's navigation system. A satellite compass uses multiple satellite receivers that each receive a GPS signal and perform internal calculations to determine heading.
While these satellite compasses are not affected by a magnet field, they do need to be installed with a clear view of the sky for proper operation.
The final key component is the course computer or the brains behind the autopilot system. This computer receives inputs from the heading sensor, rudder feedback device, chart plotter and captain to then steer the boat to a waypoint or heading as programmed. Each brand of autopilot uses a different computer with different abilities and adjustments. At a minimum one sea trial is necessary to calibrate and tune an autopilot to meet the specific characteristics of your boat. Once calibrated and tuned, there is often the ability to fine tune the autopilot settings to make improvements or customize the operation to meet your needs. Many modern autopilot computers feature unique steering patterns in addition to the typical straight-line heading or waypoint holding ability. Some of the steering patterns include U-Turn, Circles, Zigzag, Williamson, Orbit and Cloverleaf. Check to see if your system includes special patterns, they may be useful while fishing.
Auto vs. Nav
The autopilot computer uses the heading sensor to determine the vessel's 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 autopilot's job is to maintain the vessel's heading, keeping the boat’s bow 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 overall 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 chart plotter 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 affect 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 theoretical straight-line course to the destination waypoint. This information from the chart plotter is mandatory if you want to use your autopilot in navigation mode.
Maintenance
The typical hydraulic drive unit can be thought of an additional helm pump (helm steering station). The autopilot drive unit serves the same function as the helm wheel but is turned by motor instead of a person. 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 is to add 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 bleed the system after service.
An autopilot system needs to be calibrated correctly for proper operation. Autopilots often have a calibration procedure that is partially completed at rest 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. This calibration, both at the dock and at sea should be repeated as often as needed after any work to the steering system is completed and if any component of the autopilot system has been worked on or replicated. Many captains re-calibrate their systems on a regular basis or after they move the boat a significant distance from the original calibration, since the earth magnetic field varies with latitude.
If your old system is in need of an upgrade, you may be able to re-use some of your good components, such as the hydraulic pump. Often the same brand or another brand of autopilot can be installed reusing the existing hydraulic pump, saving a significant cost portion of an autopilot system.
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.
Until next time...
Stay Grounded!
