There are four alternatives. 1 Mechanical 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. The electric motor can be constant speed ( simple and cheap but power-hungry ) or variable speed ( more efficient). The mechanical linear drive is more energy efficient than its hydraulic linear sister but is also more susceptible to mechanical overload under extreme conditions. Wear and tear on this kind of mechanical drive also increase the operating noise of the unit under load, so it will get louder as it gets older and could eventually irritating. 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; Robertson and almost all other manufacturers fit metal transmission components as standard.
A hydraulic linear driving unit needs more installation space than a simple mechanical unit to accommodate the balancing ram which protrudes from the back. Mark Parkin of Simrad UK has observed that quite a number of naval architects 'forget about the bigger space required by hydraulic rams' and so end up having to fit a linear drive.
Autohelm mechanical linear drive unit aboard the 18m/ 60ft ULDB Budapest
The push rod is operated by a hydraulic pump. Hydraulic linear 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. Hydraulic drives are protected against mechanical overload by an overload valve, which opens above a certain oil pressure, and by the inherent 'oil cushion'. A hydraulic linear drive produces far less operating noise than a mechanical linear drive and will remain smoother and quieter, and hence more pleasant to have aboard, throughout its life. Hydraulic linear drives also last much longer, an important advantage for long distance cruising, and only a replacement set of seals needs to be carried as spares. As mentioned, hydraulic linear drives have a balancing ram which protrudes from the back of the unit. They therefore need to be mounted higher up to prevent the balancing ram striking inside of the hull.
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. The constantly high pressure introduces sudden high loads into the steering system with every rudder movement, and the resulting noise has earned this type of drive the name 'bang-bang pilot'.
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 on an older boat precludes the use of other drive units. Whitlock steering wheel drives offer the option of an installed mechanical motor which taps into the system's transmission below deck. Only the cpu and control module than remain to be fitted.
The drive unit has to be connected to the rudder with a comparatively short arm either via its own small tiller or at the quadrant itself. Both alternatives demand very strong mounting on the side of the hull, and structural reinforcements will often be required.
The existing wheel steering should be mechanically disconnected when the autopilot is in use to reduce inertia. This can be done using:
a) A mechanical pin clutch (Edson), b) A mechanical pin lock (Alpha), c) A solenoid activated mechanical clutch (Autohelm), or d) A solenoid activated hydraulic bypass
If the manual steering arrangement is not properly disconnected, the autopilot will operate with a delay and consume more power. Equally, when the boat is being steered manually, the drive unit should be disconnected or bypassed to afford better sensitivity on the helm and to allow the full range of rudder angle, which is normally limited under autopilot. Reducing interia for manual helming also means less work for the hand on the wheel.
When mechanically disconnected, the drive unit connecting arm should be fixed in position to prevent it from bouncing around. The end stops of the drive unit must be within the maximum limits of the rudder itself to prevent the autopilot from driving the hydraulic ram into the rudder stops.
It is absolutely essential that every autopilot has an emergency stop switch within easy reach of the helm in case the system runs into difficulties or manual steering suddenly becomes necessary. This switch should never be below deck. The distance from the helm to the nav-station or circuit breaker panel is simply too great in an emergency where the delay could result in damage to the autopilot or worse. Robertson autopilots all have such a switch included in every deck display unit.
It is extremely unwise to attempt DIY installation of an inboard autopilot. The procedure is very complex and there are far too many potential errors for the inexperienced yacht owner to make. Robertson, for one, refuses outright to provide any warranty for DIY systems.
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