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Old 08-12-2007, 23:08   #1
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Dual Autopilots

I'm wondering how you would install dual autopilots and have them hooked up without them fighting each other. I think Gunboat claims to have a dual setup but perhaps you have to do something mechanically or electronically to engage the second one? Even if it was turned off but connected, it would be subject to unnecessary wear and tear.

On my last boat I bought a complete spare autopilot and had several instances where that came in handy. Is there an elegant design that would allow me to have the second autopilot be closer to "live" all the time. I like the redundancy of using both quadrants on a cat even if it's not live.

Design alternatives/suggestions?
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Old 08-12-2007, 23:38   #2
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Do you want dual autopilots for the redundancy or because one would be inadequate? I think redundancy is a great idea.

If it is the latter, then the problem is easily solved by installing an autopilot motor or a hydraulic pump that is large enough to do the job.

Two separate autopilots using two different heading sensor inputs are not going to synchronize with each other. At times, one will be trying to push while the other is trying to pull. Other than redundancy, I don't see an advantage.

I would imagine you could set up one heading sensor where the output is split which goes to the servomotors, one servomotor for each rudder. You would though be relatively disabled if the power failed.
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Old 09-12-2007, 00:10   #3
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Could you give us some more info. Like is it Tiller steering or Wheel. If wheel, is it hydraulic, mechanical. If it's mechanical, is it chain, shaft/gearbox, or cable? Do you have twin helms or the one?
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Old 09-12-2007, 02:26   #4
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The simplest way to manage two Electrical auto-pilots is to power them from a double throw switch. Only one auto-pilot will get power at a time, so the other will be on “standby”.

Hydraulic pilots might require a “normally open” solenoid bypass valve in the standby pilot’s pump line. The solenoid would relieve the pressure in the inactive line when that pilot is unpowered & in standby mode. Don’t want that rudder to have to backfeed the pump.
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Old 09-12-2007, 02:58   #5
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How about a wind vane and autopilot instead of 2 autopilots?
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Old 09-12-2007, 09:36   #6
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A little more info:Twin wheel steering with a cable with a primary goal of redundancy since we'll be shorthanded doing a circumnativation.

I've never been a big fan of wind vanes. The few folks I've talked with about them on a Cat have had problems and for some reason I didn't see as many on cats as monos. Doing trade wind sailing with a light fast cat could be unreliable with a vane?

Gord, so I think your saying that a hydralic pilot could have a solenoid bypass valve. Something like this wouldn't put much if any wear and tear on the ram but it's still a little complicated and would have some additional resistance on the helm.

Has anyone rigged two autopilots and just kept the ram arm disconnected so that it can be easily connected in case of a failure.
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Old 09-12-2007, 10:06   #7
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It is not uncommon at all to have two ap's installed . You simply run one at a time not both at the same time. If one fails you flick it off and turn on the other pilot. You have a cat on order if I remember, this should be very easy for you to have an ap installed on each rudder shaft. I would consider a selector switch though so you can only power one at a time.
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Old 09-12-2007, 11:32   #8
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Keep it simple!
It is very unlikely for a drive unit to fail. You could keep a spare if you wanted to go to that expense, but only one drive unit is all that is needed. It's the electronics that tend to fail. But even then, it is rare in most events. That is providing you have a reputable brand name. But once again, if you want to part with the cash, a spare CPU would be a benifit. So if you have a spare CPU and a spare drive unit, you may as well have just purchased a compleate system and have it sitting in a box as a back up. Most CPU's will take multiple control units, so dual helm station control is not an issue. You just place a control at each station and they daisy chain into the system. That also gives you the benifit of having at least one operation point if the second should happen to fail.
I am not sure if I would bother connecting two seperate Pilot systems up. It would be better to keep one tucked away in storage and bring out what ever component you need if you ever do. That way, you do have a backup fopr the very rare event of lightening strike, or an electrical failure that has allowed over voltage and fried your system.
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Old 09-12-2007, 11:53   #9
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Alan,

That was exactly my thinking last time when I kept a spare in the box. All-in it was cheaper to just buy the whole unit and have a swappable parts. I didn't think about the lightning strike issue which is a good point. What about a mechanical failure on the quadrant or RAM that would make a replacement difficult? I guess that it unlikely?

It's probably overkill but somehow I imaging a gale in the south pacific which results in a failure of the autopilot. Then me climbing into the aft compartment to replace parts, getting sea sick and banged around while my wife tried to keep the boat steady in breaking waves. Seems to me that the minimal complexity of attaching another RAM and wiring up the brain and head to the other quadrant is much less likely to fail and much easier to fail over?

Could you wire up a secondary autopilot and have it off the power grid and therefore be unlikely to be fried in the case of a direct lightening strike?
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Old 09-12-2007, 19:20   #10
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Points to ponder

As capricious as is lightning you can not expect electrical or electronics components to be safe from damage when not connected to your system unless they are entirely contained within mu-metal or at least ferrous containers.

Most quadrants are not rated to carry the point loading of a ram and one must mount a stout "tiller arm" designed for such loading. Keep in mind that cable quadrants distribute the load over a circumferential distance that is quite large compared to a single bolt attachment that rams use.

Rams are very reliable. If there is a problem with wear it is usually an internal seal that merely makes the autopilot work a little more. We're not talking about external fluid leaks here merely internal bypassing. You can detect such wear as it comes on gradually and you can replace the seal when convenient. They are inexpensive.

This subject is similar to that of wanting to install a good AGM battery and inverter/charger whereas for the battery to survive you also must have better alternator, solar, wind, shore charger regulation than normal etc. etc. In this case if you want a good autopilot it necessarily will be an electrohydraulic one. Gee, if that's the case then you would begrudge having the autopilot energy backdriving the cable system which multiplies the wheel friction by the mechanical advantage squared and, therefore, requires larger battery capacity than otherwise.

So, if you want a good autopilot system that is redundant you might remove the cables and install variable ratio helm pumps (can be located beneath the pedistal using a continuous S/S chain to a sprocket on the helm pump with little backlash resulting). You can possibly use the Teleflex fittings and hose with superior non-leaking attachments. You can mount a ram for each helm pump, if you like and have completely separate autopilots if you like. The deal with hydraulics is that you place a bypass valve around each ram to allow one to "idle" (almost no friction or power loss to the other operating ram) and the other to work. Each autopilot pumpset will have (like the helm pumps) built-in check valves so that they operate independently with no interaction. The helm pumps can have valves to bypass the check valves if you desire to have a complete analog to a cable system (without the friction losses and backlash of the cables); i.e. if you push on the rudder the wheel will turn. I think that if you experience a proper hydraulic system you will not opt for that addition because when you turn the wheel one of the check valves is open and you feel the pressure on the rudder anyway.
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Old 09-12-2007, 19:57   #11
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Rick,

Thanks, not only do your comments make a ton of sense, I really enjoy listening to folks who really know their stuff.

I've only sailed on the sister ship once, but the helms were feather light and very responsive. I assume due to the light displacement of the cat and the cable system instead of some gear mechanism. Replacing the cable systems seems expensive and potentially detrimental to steering performance and also weight.

I think I'm back to carrying one in the box to keep it simple.
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Old 09-12-2007, 20:55   #12
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cost and weight

Yes, you are correct in that the hydraulic system is expensive. After removing my cable system(s) (I've done this to 3 boats) and the upper end "toy autopilot" I can say that I've saved weight. The cables and turning blocks are heavy even offsetting the heaviest component the helm pump (in my case, I'm using a bronze pump..Kobelt.. that is wonderful), and the Autohelm 5000 ("upper end toy autopilot) is quite heavy.

So, the decision is an economic one, I believe.
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Old 10-12-2007, 02:34   #13
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After all that great advice from Rick. The only thing I can add is, you will not need to worry about changing a drive unit in a heavey sea. If the sea is that big, no pilot will steer in a big sea, so you will be manual steering anyway.
Plus, if you have cable steering, you are more likely to ahve a failure with the cable system than you are with the Pilot. I have never liked cable systems. An enormous load is placed on the end of that cable and they have been known to break because of wear. It's just like SST rigging wire in reality. Hydraulic is not hugely expensive to turn over to. If I were you, I would take the nmoney you were going to spend on the spare Pilot, and fit hydraulic steering. The electro/hydraulic pumps are so robust and long lasting, you won't need to worry about having a spare.
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Old 10-12-2007, 08:14   #14
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Quote:
Originally Posted by Alan Wheeler View Post
The only thing I can add is, you will not need to worry about changing a drive unit in a heavey sea. If the sea is that big, no pilot will steer in a big sea, so you will be manual steering anyway.
I just don't understand this. I've read this in numerous books and articles but on my Tayana I never had a problem with the Autopilot keeping up. Force 9 gales, 20 foot seas, close hauled on a beat, rounding up occasionally but never a problem.

Isn't this a just issue with sizing an appropriate autopilot?
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Old 10-12-2007, 10:40   #15
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If the boat is well mannered and is sailed in balance and the ap is sized appropriately,,, the ap should be able to drive the boat in most conditions.

We use a hyd auotpilot and have found it to be robust and able to keep up. Hyd leaks can be problematic though.

Quote:
Originally Posted by BlueRhapCity View Post
I just don't understand this. I've read this in numerous books and articles but on my Tayana I never had a problem with the Autopilot keeping up. Force 9 gales, 20 foot seas, close hauled on a beat, rounding up occasionally but never a problem.

Isn't this a just issue with sizing an appropriate autopilot?
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