Peter Foerthmann - SELF-STEERING UNDER SAIL

The following systems are available:

AUTOHELM 800

AUTOHELM ST 1000,

AUTOHELM ST 2000,

AUTOHELM ST 4000 Tiller,

NAVICO TP 100,

NAVICO TP 300.

Wheel steering autopilot systems are similar to those described above, except that the course corrections are effected by a driving belt, toothed belt or toothed wheel acting on a pulley attached to the vessel’s wheel. Cockpit autopilots for wheel steering may be linked to a data network.

The following systems are available:

Navico WP 300 CX Wheel autopilot

The AUTOHELM ST 800 Tiller autopilot

Inboard autopilots use push rod or hydraulic systems with powerful motors which are connected to the rudder post or quadrant and turn the main rudder directly. It is also possible to replace the mechanical linkage and shaft with a hydraulic system in which a hydraulic pump provides oil pressure to drive a hydraulic cylinder which in turn moves the main rudder. This type of system is suitable for larger boats. Vessels over 60 feet in length with sizeable hydraulic rudder arrangements use constantly running pumps controlled by solenoid valves for the autopilot.

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.

The central processing unit consists of:

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.

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.

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

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

There are four alternatives.

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

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.

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

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

Until a few years ago it was generally the case that boat owners acquired their instruments one by one. Depth sounder, radar, compass, wind instrument, Decca, GPS, plotter, boat speed indicator and autopilot might easily be individually installed stand-alone units from several different manufacturers.

The situation today is very different, with a few major suppliers offering complete systems from which the sailor can choose as few or as many instruments as desired. Essential to this advance was the development of a specialised data protocol (data bus): functions such as the steering performance of an autopilot module can now be optimised in more demanding systems by connecting a dedicated course computer. An autopilot steering a boat between two waypoints obtained from a GPS interface can thus correct for cross-track error caused by currents running perpendicular to the boat’s course.

The changing role of companies within the industry from instrument manufacturers to system suppliers explains the current extreme concentration of the market on just three major players.

Autopilots may be divided into three groups:

a. Stand-alone systems which operate solely on the basis of a windvane or compass signal (e.g. AUTOHELM 800),

b. Systems which are linked to other modules via a data bus (e.g. SEATALK from Autohelm, NETWORK from B&G),

c. Intelligent systems in which the data of the individual modules is optimised by a relatively powerful computer (e.g. ROBERTSON AP 300, AUTOHELM 6000/7000, B&G HYDRA/HERCULES).