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Old 22-06-2016, 17:45   #1
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Simrad Computer AC12 vs AC42

Replacing a failed Robertson AP300DLX. The hydraulic pump works fine so I'll continue to use it - it's RPU160 and it's always seemed sufficient for the boat, with my old Robertson computer having an output of 10A. The RPU160 is spec'd at 6 to 10A drive current.

The question arises since Simrad documents recommend, to drive an RPU160, an AC42 computer which has an output of 30 to 50A. This seems like extreme overkill. The Simrad AC12 would seem to be a better, and much cheaper match to the RPU160. The AC12 specs say output current 8A continuous, 16A for 1 sec.

Simrad docs don't imply any other difference between these two AP computers.

Especially like to hear from anyone using an AC12 to drive hydraulic steering and even better if they have the RPU160 pump and with experience in downwind steering.
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Old 15-07-2016, 22:52   #2
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Re: Simrad Computer AC12 vs AC42

Hi there. Just upgraded our autopilot from an old Furuno system to a new AC42 and we located the rate compass on our mizzen mast as we have a steel boat. Like you I used our existing hydraulic pump. Have just come back from three months cruising where I'm pleased to say it worked fantastically. The thing I dwelled on most other than the location of the rate compass was the current match from the computer to the autopilot pump and the breaker I used to protect the computer as simrad computer feeds the autopilot pump of course. I've a 25A breaker on the system. The big issue seems to be both the AC unit choice and the computer current capability. The spec came from the Simrad dealer we brought the gear from after I'd given him the autohelm current draw. One thing to consider is the plotter we upgraded to had all the capability of the AC42 which left the AC42 as redundant backup. More on this if you wish.

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Old 16-07-2016, 10:38   #3
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Re: Simrad Computer AC12 vs AC42

My previous boat (Whitby 42) had an AC12 driving a RPU160. I think the biggest reason to choose one over the other is the force required to drive your rudder in expected conditions, including rough and following seas. Our AP did not provide enough power during one scary 10 hour leg in rough conditions with wind and seas off the aft quarter. I had to hand steer to exhaustion. Scary is somewhat of an understatement.

I am going to put a Simrad (current thinking) on my new/old boat and I have to make the same decision. I can't say which I will choose at the moment but will go conservative if I think I need to.
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Old 16-07-2016, 21:30   #4
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Re: Simrad Computer AC12 vs AC42

I went ahead with the higher current unit AC42. After thinking about it, it doesn't cost that much more than the AC12 and even though as I said it's probably overkill, it is what's recommended for our boat size.
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Old 16-07-2016, 21:36   #5
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Re: Simrad Computer AC12 vs AC42

Know what I'd do if u had the coin (and this may not be immediately helpful) but I'd put a Hydrovane self steerer on the back of our boat and attach a tiller pilot. I'd pull the hydraulic self steerer out and just leave the low pressure steering pump/system. I had a very stressful event happen offshore where the head bolt of the autopilot pump vibrated loose and we were minutes away from losing all our steering as it shares the same reservoir as the low pressure system. Wad able to find a replacement bolt in time but I lost 5 years freaking out. I agree completely that the electr-hydraulic system is very power-hungry on a quartering sea even with the rudder response set quite low.

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Old 17-07-2016, 09:08   #6
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Re: Simrad Computer AC12 vs AC42

I can't overstate how much I regretted that my windvane was inop on that trip in to NZ in that gale. It would have handled the boat and the seas well just as it had done several times before. Alas, it had finally fallen apart after one too many rebuilds. I'd recommend adding a windvane as well but wouldn't faulty you for having a strong backup power AP. I will always have both if possible.

The other poster has good thoughts though for you to consider.
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