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Old 28-02-2019, 23:43   #1
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Power draw on autopilot and general cruising

Hi,
Only in my 3rd year cruising (Chesapeake bay), wanted to run this by you guys. I was on a weeklong trip to Baltimore and annapolis and on the last day, my batteries were discharged quite a bit (showing 11.9V). It worried me. We only spent the first day sailing for about 8-10 hours, then 4 days at anchorage. Even at winter, my 100W solar array will output from 0.5 amps to 1.8 from sunup to sundown even in some overcast weather (I have silicone polycrystallaine panels). It turns out my alternator belt was broken at some point, so when I ran the engine at anchorage for about 30 mins, it might not have charged anything. (55amp alternator running off of an atomic 4).

I actually don't know how well my amp meter is reading. The panels should top out at something like 5 amps. With suboptimal angle to the sun and my boom/sail covering some of the panels, I figured I should see 2.5 amps in the summer. I use the Wattsup device hooked up between the MPPT and the chargers.

I want to map out my hypothetical daily usage on an open passage as I am planning on trying a delmarvar penninsula circumnavigation this year. Does the below sound right to you guys? The number for the CPT autopilot, I mainly pulled it out of my a** based on what I read on different forums on the CPT autopilot power draw.

Batteries- two group 27 80AH AGM batteries in parallel. I also have a 90AH deep cycle lead-acid starter bank.

Use:
CPT autopilot - 72 AH capacity needed
- assuming 3 amps on average, 24 hours

charging cell phones via microUSB - 6AH capacity needed
- 2 devices, 0.3 amps, 10 hours (my primary nav device)

VHF - 0.5 amps - 24 hours

nighttime running lights - 12 hours - 33 AH capacity needed
- 0.6amps x2 (bow lights)
- 0.1amps x1 (stern light LED)
- Spreader Lights 2 amps

Total: 133AH capacity needed

Charge
Solar - assuming spring/fall use - 2 amp output @ 7 hours - 14AH capacity provided
Engine - at low RPMs, assuming 20amp output at 1 hour a day - 20AH capacity provided

Areas to save - switch spreader lights to LED (+18a). change bow lights to LED (+12a). Change pulley ratios on alterator to bump up output (+10a)

I'm getting a new mainsail soon and adding a bunch of mainsail controls (vang, outhaul, possibly cunningham), and hoping that'll cut down on the power draw of the CPT unit as well.

I should get a load tester to get a better idea of my battery usage as well, even just for bay cruising.

Even with that I'll be drawing down a third of my batteries every day.
I guess I better add a windgen. I don't really have any room to add another solar charger - I had to construct a dodger frame to mount the panels.
I understand this is not enough for extended cruising, but wanted to see if you think my assumptions are correct. I'd appreciate any tips as well.
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Old 01-03-2019, 01:25   #2
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Re: Power draw on autopilot and general cruising

how big is this boat? no fridge? that is normally the biggest draw.

two 27's is tiny.

and 20a from an alternator is pretty useless.

if a pair of 27's lasted you 4 days you are doing pretty good for battery size. now you just need to fix ways to charge them.
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Old 01-03-2019, 01:49   #3
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Re: Power draw on autopilot and general cruising

Going for windvane self steering will reduce your power consumption significantly. A bigger battery bank is not necessarily a good solution, it adds weight and it will only extend but still limit your "off grid time". Either more charge or less consumption are the better ways forward to become really independent of the grid for longer cruises.


A wind generator might be a solution, but the windvane steering would help (and going LED willl also help).
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Old 01-03-2019, 01:57   #4
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Re: Power draw on autopilot and general cruising

Installing a battery monitor will remove the guesswork, Their much more useful in situations like yours where you need to juggle supply an demand I'm sure you would find it very handy.


IMO, the most important factor when working with limited supply is % SOC of the battery come morning. This is because you don't want to repetitively run your batteries down overnight to the point that their life cycle will be severely reduced. A battery monitor will provide a reasonable indication of this and will certainly provide an indication of current flow to or from your batteries.



When calculating amp hour usage, you need to factor in duty cycle. For example, the autopilot won't be running continuously. If you have any kind of reasonable sail balance, it should only run about half the time. Same with spreader lights. You wont have these running all night on average. Ditto for the VHF. It won't draw 12 a/h a day because most times it will be sitting there silent and you'll probably turn it off at bedtime at the latest.



Ditch the polycrystalline panels and get monocrystalline. These are more efficient. Consider the squarer shape 100 watt panels and you might be able to squeeze an extra panel on board. Remember too that even the slightest bit of shade will seriously affect a panels output.
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Old 01-03-2019, 03:27   #5
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Power draw on autopilot and general cruising

Add a decent battery monitor like a Victron and then youíll know exactly how much each device is drawing. Just shut everything off except one device and you have your draw.

Your AP is going to draw a variable amount depending on how hard itís working (sea state, point of sail, how well the sail plan is balanced). Adjusting its sensitivity settings, like gain, can make it more energy efficient as well as provide a smoother ride.

For your purposes I would simply run the engine first thing in the morning to bulk charge, even when underway. Consider the cost of a wind generator and how many hours you would need to run your engine to make it an actually economical solution. Also, panel output varies dramatically and can be difficult to estimate and plan with unless you have an extended period of experience with them.

You need more precise info on your usage and generation (ergo the meter) before you start designing solutions or youíll end up wasting money.

And why do you use your spreader light so much? I live aboard, always on the move, and I canít even remember the last time I turned mine on. It was probably to test them after reconnecting after having the stick down three years ago lol.
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Old 01-03-2019, 04:20   #6
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Re: Power draw on autopilot and general cruising

I used just a 65 watt solar panel for years without any problems draining my two 12 volt batteries that are in parallel while running , VHF, Depth, and GPS during the day.

At night, I'd have my 120 volt/AC fan (power by 400 watt inverter) on if in Summer and a battery powered anchor light at night while anchored plus interior lights for an hour or two.

I also charged my laptop from the inverter

You added in your running lights. Were you sailing at lot at night?

I have no alternator on my 5 hp outboard. The 65 watt panel was all and it did fine. You must have had some cloudy days and maybe had on too many lights
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Old 01-03-2019, 04:57   #7
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Re: Power draw on autopilot and general cruising

I think your power estimate is low for the loads you list. But they are close enough for planning. Since you are going to run your engine it makes little difference in the big picture if you numbers are any closer.

Bet you can run the engine over 1000 hours for cost of a wind generator and twice that for cost a wind vane
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Old 01-03-2019, 05:21   #8
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Re: Power draw on autopilot and general cruising

If your CPT draws 3A on average, something is not correct. Either the autopilot settings are wrong or the boat is extremely unbalanced. My average draw is about 11Ah/day, a little less than 0.5A.

As sailor boy writes, you can run your engine many hours for the cost of a wind vane or wind generator. Some more solar might help, and maybe a bigger battery bank.
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Old 01-03-2019, 06:04   #9
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Power draw on autopilot and general cruising

I have to always throw this in, but if money is an issue, a little suitcase generator is tough to beat, they can charge at 100 amps for a Honda 2000 and donít cost much to purchase or run.
Of course you do need a shore power charger, but most do.
A windvane will actually save very little power over a CPT and only saves when your sailing.
Big alternators, big Solar, good wind gen etc are usually big $$$

My CPTís power consumption I havenít been able to measure as it comes in pulses that if your sails are well balanced and depending on how sensitive your settings are may be quite spaced apart, and I have sensitive instrumentation.
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Old 01-03-2019, 06:42   #10
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Re: Power draw on autopilot and general cruising

For less than $250, you can add another 100 watt solar panel and a Victron 75/15 MPPT Controller which allows you to monitor your system charge current and battery voltage from your phone

https://www.victronenergy.com/solar-...llers/mppt7510

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Old 01-03-2019, 07:19   #11
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Re: Power draw on autopilot and general cruising

Quote:
Originally Posted by MartinR View Post
If your CPT draws 3A on average, something is not correct. Either the autopilot settings are wrong or the boat is extremely unbalanced. My average draw is about 11Ah/day, a little less than 0.5A.
11 amp hours/day for AP? That seems quite remarkable. Are you saying you can achieve power draw that low even off the wind? If I recall, just the draw for a typical AP clutch will draw several amp hours/day by itself.

On a previous boat, 34' with a modified fin keel and semi-balanced rudder, a boat that I could trim, lock the helm and hold a course without AP for 10-15 minutes I could achieve power draw in that range. Sailing downwind I have never been able to come anywhere close, even in a 63' ketch with a 9', full length keel that as you could imagine, was quite stable even down wind.

Might I ask what brand and model AP you have?
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Old 01-03-2019, 07:39   #12
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Power draw on autopilot and general cruising

He has I believe a CPT, clutch is manual so no power draw.
Itís a toothed belt driven wheel pilot, but much more powerful that the average wheel pilot.
The drive motor I have been told is actually a 24V Bosch OTR truck windshield wiper motor driven at 12V.
It only consumes power when it makes a course correction, so power consumption is actually in pulses.
One setting control is sensitivity, if you set it to allow 3 or 4 degrees of wander, if the sails are balanced then it doesnít have to correct very often at all with wind from ahead or on the beam.
However a following Sea will have it often correcting so power consumption will be much higher then.

But the thing is absolutely silent and uses very little power.
Bad thing is itís only a heading hold, it wonít slave to anything.
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Old 01-03-2019, 08:25   #13
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Re: Power draw on autopilot and general cruising

Off the cuff, I believe you have 5 easy fixes for your problem. But before that, I'm of the belief, if you are that close on your power consumption, what happens if you suddenly need more power?

1. I agree with Reefmagnet; you need monocrystaline solar panels. If you don't have them, the investment is well worth it.

2. Do you have one solar controller for all panels, or one for each? If just one, are the panels set up in series or parallel? If you want to stay with just one controller, hook your panels up in parallel and make sure you are using an MPPT controller; they are far more efficient. Personally, I would go with one MPPT controller for each panel, so if one panel has partial shade, the others are not effected.

3. Get yourself a decent sized alternator. (Most car alternators are bigger than yours.) Go for 80-118A. To just put a larger pulley on it won't gain much, because once you reach a certain speed, the alternator tops out. Fuel saving? An 80A runs for 1 hour to create the same power your current alternator creates in 4 hours.

4. LED is the only way to go. Even a 1.5A reading/nav table light goes to .18A, ...a 90% power saving.

5. I would add another battery. That way your batteries will all last longer because you will be reducing their duty cycle. This would also assist in your concern about dropping to 11.5V because you will have more stored, so it would take 50% longer to get to that point.
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Old 01-03-2019, 09:35   #14
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Re: Power draw on autopilot and general cruising

One way to reduce power draw on an autopilot (AP) is to reduce the frequency of course correction. The Raymarine I used allowed one to adjust how often the AP makes a course correction. It has been a few years since I used this one but I vaguely recall quite a range; perhaps as often as every few seconds and as low as every couple of minutes. By lengthening the time between corrections the battery draw went way down.

Again, this was a while ago so my times may be off and I am not sure what the APs put out in the past year or two do.
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Old 01-03-2019, 09:43   #15
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Re: Power draw on autopilot and general cruising

Quote:
Originally Posted by a64pilot View Post
He has I believe a CPT, clutch is manual so no power draw.
Itís a toothed belt driven wheel pilot, but much more powerful that the average wheel pilot.
The drive motor I have been told is actually a 24V Bosch OTR truck windshield wiper motor driven at 12V.
It only consumes power when it makes a course correction, so power consumption is actually in pulses.
One setting control is sensitivity, if you set it to allow 3 or 4 degrees of wander, if the sails are balanced then it doesnít have to correct very often at all with wind from ahead or on the beam.
However a following Sea will have it often correcting so power consumption will be much higher then.

But the thing is absolutely silent and uses very little power.
Bad thing is itís only a heading hold, it wonít slave to anything.
Never had a wheel pilot, always below decks units but at the end of the day, I think the same amount of work would have to be done so power consumption similar based on similar performance.

Off the wind is where I see the problem of power use. It's been years so I don't remember the exact specs but at least a few amps draw by the motors when working. More when farther off the set course to move the wheel faster, less when making small correction.

Depending on the boat and conditions and exact course I recall at least 30% duty cycle. One trip, downwind in 30-35 kts and 8-10' seas I think well over 50%. That was after playing with boat trim and AP settings for days, adjusting dead band width, correction speed, delay times, etc. Assuming bad but not worst case scenario of 33% duty cycle, and 5 amps motor draw that's 40 amp hours a day. In a built in unit add another 0.5 amps for the clutch so now that's over 50. Based on tests done by Practical Sailer 30-40 years ago that is not unusual. So, would really love to learn how to get AP power draw down to 11 ah/day, especially since my clutch would draw that much.
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