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The Three Modules of an Inboard Autopilot

Control unit

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 possible to mount 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:

a. 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.

b. Fluxgate compass

An autopilot can only steer a good course if the steering impulse from the compass is accurate and clear. Fluxgate sensors, which supply the course computer with precise course data, are used by all manufacturers. Steering performance in testing conditions can be optimised by installing special additional fluxgate systems. Autohelm uses a ‘GyroPlus’ transducer while Robertson has a novel electronic compass, a modification of the conventional magnetic compass, which promises to help the autopilot steer more smoothly.

c. Rudder position indicator

The rudder position transducer is arranged on the rudder and informs the course computer of the position of the rudder.

d. Windvane (optional)

e. Peripherals

Signals from additional navigation equipment such as Decca, GPS, Loran, radar, log and depth sounder can also be fed to the course computer for consideration with regard to steering movements.

The modules of an inboard pilot; a Brookes & Gatehous example

Drive unit

There are four alternatives.

a. Linear drive unit:

An electric motor operates the push rod mechanically via a transmission.

These drives are similar in principle to cockpit autopilots, but are considerably more powerful. Depending on the particular use and the size of the system it may be advisable to use metal for the transmission components since plastic is not always able to withstand the heavy loading associated with extended operation. Autohelm offers the ‘Grand Prix’ package as an upgrade for its linear drive units and Robertson uses metal transmission components as standard.

Autohelm mechanical linear drive unit aboard the 18m/ 60ft ULDB Budapest

b. Linear/hydraulic drive unit:

The push rod is operated by a hydraulic pump. Linear/hydraulic drives appear on large yachts with particularly high rudder forces. The drives may be supplied either by separately installed hydraulic pumps (Autohelm, VDO) or by pumps directly incorporated into the push rod system (Brookes and Gatehouse, Robertson). Robertson also offers ‘dual drives’, in which two linear drives double the force applied.

c. Hydraulic drive units:

These electromechanical hydraulic pumps tap directly into the existing wheel steering hydraulic system. A constantly running pump may be used to supply the force required to steer boats of 25 tonnes or more.

Robertson hydraulic linear drive units

d. Chain drive unit:

An electric motor operates the main rudder via a chain. Chain drives are preferred where space is limited or where the rod-operated or geared wheel steering system on an older boat precludes the use of other drive units.

Blue Papillon, a 29m/ 95ft Jongert steered by a Segatron autopilot