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Old 26-04-2006, 09:19   #1
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Autopilot Power Consumption

I'm trying to do an energy budget so I can best spec out battery bank requirements, power generation, etc. While underway, the autopilot will obviously be a significant consumer. It will vary according to a number of factors (sea state, balance of boat, etc.) and I understand that. I've looked on the manufacturer's spec sheets (Raymarine ST6001) and not seeing much help. The figures they supply lead to a calculation of up to 360 amphrs/day. I've got to have that wrong, surely, or lots of boats wouldn't be getting very far with that sort of consumption and the bank required to feed it would sink the bloody boat!

So, any guidance on this? I understand that the range of daily consumption may be pretty wide, but what is a reasonable figure to use for the budget?

Thank you.

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Old 26-04-2006, 09:51   #2
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How many hours of operation per day are you assuming?
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Old 26-04-2006, 10:20   #3
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Amps

Should be amps used times length of time used, and that is the easy part. I like practical application. With the auto pilot engaged measure the amps being used, and time the operation. The amount of time the unit runs will depend on the type of unit, the balance of the boat and the sea conditions. Last first. If the conditions are bouncing the boat around and making it alter course the AP unit will run more often, if the boat is not balanced the AP will run more often, and if the AP is a seeker it will run more often. The last part might be critical and all the new units may be seekers. By that I mean they do not go to sleep, they are always checking where they are going to make sure they are on course. A non seeker goes to sleep while on course and only wakes up when the boat goes off course. A seeker may be consuming power all the time, and more power when it makes a course correction. So as long as the boat is on course and the boat is balanced and the sea state is steady, the non seeker AP may use zero amps. That is the case with Ethel my old Tiller Master. Ethel however is slow when she wakes up which is fine for a balanced boat, but some boats may require more immeadiate action. Ultimately I think I will use Ethel while under power, and a wind vane while sailing.
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Old 26-04-2006, 10:41   #4
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I have the feeling that I'm not asking a very good question, or perhaps don't know enough to ask the right question, so I ask your indulgence and I'll try again.

The boat is a Lagoon 420, the first one hasn't hit the water yet so we have no empirical data regarding how the boat will behave. I'm assuming, though, that with sufficient care, it will be possible to reasonably balance the weight distribution of the boat and sail trim will, of course, be dependent on conditions.

As part of specing out the boat, I'm being asked about the size of the battery bank to install. So, I've put together an Excel sheet to do some projections and estimations.

Assuming that under passage-making conditions, the autopilot will be running 24/7, how much battery capacity should be budgeted? I understand that the actual power used will be a range that will vary by conditions, but is there some sort of average, perhaps with a standard deviation, that could be used for the budget?

Thanks.

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Old 26-04-2006, 10:51   #5
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Raymarine pilot power consumption has two elements - continuous (milliamps) to power the brain, instruments and fluxgate and a Much higherconsumption for the bit that actually turns the rudder. I suspect your calculations have been based on the power requirements for the latter. The number quoted is the maximum needed to provide maximum turning capability, and is likely to be very different from what is actually required.

A Catamaran does not need anywhere near as much power to turn the rudders as a mono. Balance of the sails will make a huge difference in power consumption, the rudder is unlikely to be moving all the time (especially if you have balanced the sails properly). If you have purchased the rate gyro system, rudder movements will be made almost in anticipation of requirement, thus much less than if left later. If your rudders are balanced at all, this will also reduce the power requirement.

It is one of the largest power requirements (with the Fridge and freezers) but should be manageable.

The highest consumption rudder is the linear drive unit, designed for use with wire steering systems, whereas the hydraulic system should be less.
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Old 26-04-2006, 12:11   #6
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I think that's making much more sense, now. If I assume that everything is OK and the autopilot decides that it does not have to actually move the rudder at all, then the daily consumption will be equal to the milliamps consumed to power the brain, instruments and fluxgate over a 24 hr period. If that is .2, then over 24 hours that will equal 4.8.

When it decides it actually needs to move the rudder, then it will consume the higher figure -- say, 15 amps -- but only for the time actually moving the rudder. The sum of time actually spent moving the rudder with a balanced boat in steady winds may only be minutes in a day. For sake of illustration, let's say 60. Then, the total power consumed over the course of the day will be 4.8 + 15 = 19.8 amphrs.

Do I have that right? Still not a trivial amount, but a lot less than I what I originally was computing.

Thanks.

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Old 26-04-2006, 12:19   #7
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I think If I recall correctly, I estimated out a similar system at abt 50 amp/hrs/day.
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Old 26-04-2006, 13:07   #8
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What drive unit?

What drive unit do you have? From your figures you might have the largest hydraulic drive unit that they offer. If you use a non-hydraulic unit what type of steering do you have? The non-hydraulic units are not as efficient, especially if you have to back-drive the friction of cable steering.

I had a 300 Watt hydraulic drive unit which pushed 200 Amp-hours for 24 hours in a hurricane and only 20 Amp-hours in mild stuff. In this case it is easier to figure an average power consumption equal to half the peak power rating of the drive motor for the 24 hour calculation for a "bad case" of power consumption. You will then be well under 360 Amp-hours for 24 hours.
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Old 26-04-2006, 13:13   #9
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We have the ST6000 on our boat, which I'm not on at the moment.

But...

In looking at my notes I have the autopilot using 2 amps per hour. Not sure if that is 100% correct but I think very close.
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Old 26-04-2006, 13:14   #10
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Yes, Drifter, that is the right formula.

The hard part is estimating the actual time the servo will be under power (the second part of the formula). A few seconds each minute? In a seaway, up to 15-20% of the time?

You may have to live with a wide estimate, and figure conservatively, i.e., on the high side, when designing the house bank capacity, to make sure your "average" day is covered comfortably.

Any excess built-in capacity is sure to be tapped into on aytpical days, or will be available to use for other purposes, or extend intervals between chargng, if you use active (engine, generator) charging strategies, which of course, you already know.

The Devil is in the details…
Good luck.
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Old 26-04-2006, 17:46   #11
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Based on experience with our raymarine linear drive [type 2 L] which is driven by Simrad electronics -- I would estimate at least 65-70 AHrs/day based on 24x7. This assumes a relatively balanced boat in normal conditions - i.e not flat but not heavy weather. This might be a little high or low but will put you in the range I believe. We sized our bank assuming 100 /ahrs day for autopilot. Unfortunately can't measure as standalone as we have other systems running all the time so can't break it out.

But we avg around 225 Ahrs day total consumption including : refrigeration, autopilot, stereo, lights [house/nav], computer, instruments, radar as needed, etc.
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Old 26-04-2006, 19:44   #12
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Jemsea

The model ST6000 does not denote which drive unit you have (the thing that actually delivers power to turn the rudder)....could be linear drive or one of several hydraulic pumpsets or???

Not to be too critical it kinda grates to see someone trying to describe a form of energy use as Amps per hour....there is no such thing....use Amp-hours because Amps per hour would mean current divided by time whereas Amp-hours is current multiplied by time which, then, is only missing the voltage multiplier (often assumed to be a nominal 12V or whatever is the case) to get true energy consumption, what you pay for when you pay your electricity bill. No one has ever been billed for Amp-hour consumption.
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Old 26-04-2006, 20:54   #13
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Sorry Rick - Didn't mean to grate on anyone, even a Moderator

Actually I was trying to help, ...oh well back to lurk mode.

Let me attempt to restate this.. In about 12 hours of sailing with the autopilot engaged we attributed 24 amp-hours of energy consumed to the autopilot.

YMMV
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Old 27-04-2006, 10:36   #14
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Jemsea

You DO help. Your added information helps compise a better overal view of energy consumption for others to evaluate in terms of being able to make more intelligent decisions regarding their own boats.

My curmudgeon-like appearance in writing does not show a more understanding side not governed by my engineering discipline.

Thanks,
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Old 11-06-2006, 18:03   #15
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Rick - Just in writing? Anyway - I have an ST6000 series system. It includes a type two (2) linear drive, the BRAIN, fluxgate compass, rudder sensor, and two control heads. I also run an anemometer, Depth sounder, speed transducer, and four display heads (all RayMarine). The Radar is a VERY old Ratheon unit (25 plus years), and doesn't play with others.

I am VERY happy with the Autopilot system. It is difficult to judge the amount of current drawn by the autopilot system. In standby (which I think would be indicative of power consumption when not actively controling the rudder), my system (without radar or refrigerator, but with my VHF) drawns what appears to be less than one amp. When the autopilot is engaged and steering in light to moderate seas and winds (<6' and <20kn) with a reasonably ballanced sail plan it hardly seems to draw much more than 2 amps for a very short period of time (NOT continuous). And, damn it, it steers a much better course than I do. grrrrrrrr
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