Autopilot Drives

[gardenshed]

What are the pros and cons of mechanical linear vs hydraulic drives for an autopilot? The boat has a mechanical steering system (chain/wires onto the quadrant).
Which requires more power?
Which has the least impact on the steering when disengaged?
thanks /forums/images/graemlins/confused.gif

 

[pvb]

Hydraulic linear drives tend to be more powerful than mechanical linear drives. With Raymarine drives, for instance, the mechanical linear drive is suitable for boats up to about 20 tons, whereas their hydraulic linear drives will suit boats up to about 35 tons displacement. The more powerful the drive, the more current it will consume. When disengaged, there's negligible effect on the steering effort with either type.

 

[swanny]

Hi, I have a navico Oceanpilot Linear drive LR1. The piston seems to be very stiff when switched off, affecting the steering considerably. Anyone know how I can solve this problem????

 

[AlanPound]

"The more powerful the drive, the more current it will consume. " ... This will be true to a degree, because of the increased inefficiencies/friction etc in larger systems.

However, the current consumed will be much more related to the load the motor works against, which will be the resistance of the steering system and rudder - no resistance, then very little current. Hence it would be good practice to 'over size' the linear drive a tad, mainly so that it has plenty in hand in heavy conditions, and plenty of power to move the rudder quickly (improve response - assuming the control computer is up to it).

One insiduous consumer of current in hydraulic systems is that many of these systems use a solonoid to close a valve on the hydraulic pump, to 'engage' the hydraulic drive (otherwise the helm would lock almost rock solid when the auto-pilot was not engaged - might be related to the other enquiry here). This solonoid is typically a failry heavy current device, maybe an Amp by itself.

Alan

 

[charles_reed]

Linear electric drives are less powerful than hydraulic and so are more common on smaller boats.
For a given effort on the rudder quadrant they will use less power, many hydraulic systems require the pump to be running the whole time when the autopilot is in use - great for a power vessel but electrically challenging for a sailboat.
Generally the hydraulic unit is several orders of magnitude greater in cost than a comparable linear unit BUT
Hydraulic systems are far more sophisticated allowing for various degrees of trim adjustment, are likely (unless you can disconnect the drive) to have less frictional resistance when steering manually and are generally reckoned to be more reliable.
Personally I've always used a linear motor, with two units, preventative maintenance being carried out every 2nd year (<700 hrs), and found them to be adequately reliable.