Electric or Hydraulic for Backup autopilot?

[Muaddib1116]

I'm thinking of installing a fully redundant backup autopilot. Since it won't be used that much, does one type of drive unit have an advantage in being inactive for long periods? It will be installed in a watertight lazarette that has its own bilge pump, so it will likely be relatively damp, but should not be in direct exposure to standing water.

[SY Harmony]

Will the back up always be connected?
The hydraulic could induce more drag and resistance your primary system may not like.

[Muaddib1116]

I'm still debating the actual setup, whether I will have it on the existing acu/control panel with a switch, or if it will be totally redundant with it's own acu/panel/compass/etc. I'm leaning towards the latter.

My understanding is that the hydraulic units have bypass valves that open when unpowered. I was shown that the piston can be moved by hand relatively easily in this state, which makes me think that the overall steering system on my boat won't really notice it.

I'm not sure how the electronic drives, linear or rotary, disconnect when not in use. A physical clutch? do they just push the unpowered drive motor? How much resistance do they add?

[SY Harmony]

I cant answer your questions. I just posted a thought I had when reading. Glad you’ve looked into it a bit.

[Muaddib1116]

Thanks, it's definitely something that should be considered. My suspicion though, is that they are designed to have a minimal impact on your steering drag when not in use, and that having two units adding inactive-drag won't be too bad.

I'm more concerned with potential issues arising from being inactive for long periods. I would probably exercise the unit periodically, but I don't know if I would give it equal use as my primary unit.

[SailFastTri]

What type of primary AP drive do you have? What type of steering?
Maybe you should use your current as backup after the second one is completed.

[Muaddib1116]

That is a thought.

My steering is the Whitlock/Lewmar Mamba gearbox/driveshaft system. The current autopilot is a drive unit bolted to one of the gearboxes, beneath my aft bunk. The ACU is Raymarine, a 400, I believe.

I've been told there's a way to hook up a second drive unit with a transfer switch to a single ACU. I don't know if this is how I want to go. I think ideally, I would have a full secondary system, independent of my current Raymarine system, so if the NMEA 2000 backbone went down, I could just flip on the other system and not have to crawl around swapping parts.

That being said, I would try to duplicate the ACU and compass unit, so I could use one as backup parts for the other if the situation called for that.

Then there's the idea of installing the new ACU and control head onto the existing primary, and using the used parts on the new secondary drive. Still working the pros and cons of that out.

But again, at this point, I'm just trying to decide on a drive unit type.

[SailFastTri]

I’m not familiar with the system you have, but for greater redundancy your best bet is to have the AP drive attached directly to the rudder shaft/quadrant independently of the existing AP drive and steering mechanism. That only gives you mechanical redundancy, however, so the second ACU, rudder position sensor, and heading sensor should also be duplicated for electrical redundancy.
Of course, all bets are off if lightning strikes. So it might be better to just carry spare electrical components stored in a metal box (Farraday cage) and pull them out if needed.
As for the transfer switch to the AP drive from the ACU - it’s very simple to switch motor power leads, but keep in mind a switchover would require the AP to be recalibrated.

[Muaddib1116]

Thanks for that point about needing to re-calibrate when switching drive units. I hadn't heard that before.

My Mamba system feeds into the lazarette to a gear reduction box that has a tiller and linkage that connect to a tiller on the rudder stock.

That's another thing to consider. Do I attach the back up drive unit to the same tiller or to a second tiller that I attach to the rudder stock above/below the primary. Again, I suppose the ideal thing to do would be to have a separate tiller.

Lightning strikes are something I haven't given much thought to, other than that it will suck big time. What is the chances of a lightning strike? What is the chance of one at sea, when the autopilot is needed? What is that chance versus the chance of an autopilot failure, where I would prefer to have the backup installed and ready to go?

[SailFastTri]

I wouldn’t concern myself with separate tiller arm if you have a way to attach both drives to the same one.
As for lightning - I owned my prior boat 12 years and it was struck twice. The autopilot drive motor never failed, but the AP electronic components all failed from each strike. That’s very limited experience on a database of 1 boat, but nobody knows how to predict when/where lightning strikes. It doesn’t even need to be a direct hit - the EMP from a near strike can fry the electronics.

[SailFastTri]

Also, if you have a common rail engine you should think about carrying backup electrical components for it. When underway in nearby lightning I start my engines and run at idle if sailing, in case of a nearby strike wiping out the start circuit wiring harnesses.

[Muaddib1116]

Originally Posted by SailFastTri

Also, if you have a common rail engine you should think about carrying backup electrical components for it. When underway in nearby lightning I start my engines and run at idle if sailing, in case of a nearby strike wiping out the start circuit wiring harnesses.

Yeah, I have thought about that. I do have a pretty new Yanmar common rail. My understanding is that even if it's running, it won't be able to continue running without the computer, since it controls the fuel injection.

[SailFastTri]

Isn’t technology wonderful?

[Windpilot]

And so I bring it up, windvane with little simrad tillerpilot if needed. So many pluses, save your main autopilot some hard work.

[Muaddib1116]

Originally Posted by Windpilot

And so I bring it up, windvane with little simrad tillerpilot if needed. So many pluses, save your main autopilot some hard work.

I have given this a lot of thought too. It would be nice to have a Hydrovane that is obviously a windvane, but also can act as a backup/emergency rudder, and the tillerpilot option is nice. The big problem is that there's a good chance that I'll add a large solar panel over my dinghy davits, so even if the dinghy is on the foredeck, it might not allow a Hydrovane.