The control unit is used to call up all the functions of the autopilot and any other modules linked via the data bus. It is usually operated via push buttons (Autohelm) or turning knobs (Robertson). Display sizes vary and, not surprisingly, larger displays are generally easier to read. Modern high-contrast LCD displays will fade if exposed to excessive direct sunlight, so they should ideally be mounted vertically and never flat on the deck. It is usually possible to fit additional control units wherever they are needed, so the operator is not restricted to the main cockpit. A hand-held remote control unit provides even more freedom to move about the deck. Joysticks offering direct control of the autopilot drive unit are also available.
Central processing unit
The central processing unit consists of: course computer, compass, rudder position indicator, windvane transducer, and peripherals. Course computer
The course computer, installed below deck, is responsible for processing all commands and signals, for calculating the rudder movements necessary for course correction and for actuating the drive unit. In short, it links software and hardware and converts signals into actions. There are two kinds of course computers:
• The manual version which is adjusted and set up by the user and/ or installer;
• The auto-adaptive version which learns from recent operations and from recorded data.
Both have their advantages, but sailors may well prefer the ease of the auto-adaptive black box. Aside from seeing to a few basic decisions (mode of gain, auto tack, compass or windvane), the user has only to sit back and watch that the software carries on doing its job. The overriding aim is to combine high performance with reduced power consumption and neither option is perfect: factory programmed units are never properly set up for real conditions, and manually-adjusted units are also unlikely to deliver their full potential unless the user is a professional.
Compasses work best on land. Once afloat, the trouble starts: pitching, rolling, heeling, acceleration and deceleration all make things difficult for a compass. The course computer needs a clear, readable signal from the compass to control the drive properly - an autopilot course can only be as good as the steering impulse from the compass.
The position of the compass is very important. Consider the following points prior to installation:
• The further the compass is from the boat's centre, the greater the number of movements which will have to be filtered out.
• Any variations in local magnetic fields will prevent an accurate signal. The compass should be kept well away from electric motors, pumps, generators, radios, TVs, navigation instruments, power cabels and metal objects.
• Compasses prefer constant temperatures; avoid sites exposed to sunlight or heat from the engine, cooker or heater.
Below deck near the base of the mast is a good spot for most cruising designes, provided they do not have a steel hull. The most stable point on more extreme modern yachts is further aft, normally about one third of the way from the stern to the bow. On steel boats there are different ways to get proper steering signals. An arrangement in which a magnetic compass with course dectector is fitted under the compass bowl detects changes in magnetic fields and has been use most successfully by Robertson on commercial fishing vessels. Other manufacturers position their fluxgate compasses above deck or even in the mast, not always the ideal location because of its accentuated motion. Careful installation and thorough calibration of the compass are particularly important on steel boats ( a fluxgate compass cannot be used below deck on a steel boat ).
The distance from the compass to the course computer should be kept as short as possible to minimise the problem of voltage drops. The longer this distance, the thicker the cables that will be needed. One final point to bear in mind regarding installation: the compass should ideally be easily accessible in its final position.
There are three types of compass to choose from, the magnetic compass, the fluxgate compass and the gyrocompass. Fluxgate sensors which supply the course computer with electronic course data are standard with nearly all manufacturers. Steering performance in testing conditions can be optimised by installing a special fluxgate system. Autohelm uses a 'GyroPlus' transducer while Robertson has a novel type of compass in which fluxgate signals are translated into frequency signals whose variations can more easily be monitored. Further optimisation measures include fluid damping and electronic averaging. The quality of the final signal for actual steering actions is directly related to the price and quality of the sensor unit. You really do get what you pay for, and unfortunately the price range, which starts around £200 for an ordinarily fluxgate compass and £240 for a magnetic compass and course detector, extends the way up to £9000 for a high-tech gyrocompass unit.
The rudder position transducer is arranged on the rudder and informs the course computer of the position of the rudder. It can be fitted inside the drive unit ( protected from errant footsteps ) or externally at the rudder post (more vulnerable).
A transducer attached to a windvane or to the masthead passes information of the apparent wind angle to the course computer.
Signals from other navigation equipment such as Decca, GPS, Loran, radar, log and depth sounder can also be fed to the course computer to give additional data to aid precise steering.
The modules of an inboard pilot; a Brookes & Gatehous example
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Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.