What size boat? Which auto pilot? So many variables. I would think the manufacture would have the power consumption specified.

 

That would depend on how much work the AP is doing. Motoring in flat seas the rudder hardly moves and power consumption is quite low. Sailing in sloppy seas the AP works much harder... especially if it doing a lot of corrections.

 

The autopilot is intermittently on and off. So just looking at the power draw when on will grossly overestimate the average Ahr usage. As SanderO said above, when motoring in calm seas, it will be off most of the time. When sailing it will be on more, but how much more depends on the boat, how well the sails are trimmed, how strong the winds are, how big the waves are, etc.

 

My experience is more in line with Sanders and JimsCAL. When you are motoring, it's not an issue at all because your alternator will be putting out plenty of power. As Jim mentioned just above, sail trim is a too often ignored factor while on autopilot that can cause your autopilot to work much harder than necessary and that means lots of extra amps. So when sailing on autopilot, be sure to take a look at your rudder position indicator and make sure it's very close to centered and if you don't have one of those on your boat, just disconnect the autopilot momentarily and note whether the boat immediately starts to turn. If so, then you are asking the autopilot to constantly fight that tendency and if you trim your sails properly your autopilot will be much happier and use much less energy. Probably no need to modify your electrical system to accommodate your autopilot because it's not a really big load item, especially if used properly. On my boat I have a rather modest 375W solar and a KISS wind generator. If not motoring we usually run the genset for about an hour a day, mostly to power the water heater and watermaker, and during this time the battery charger is also turned on. I try to do it early in the day when batteries are at their lowest ( evening usage and refrig/freezer running all night) to get the max amps out of the charger, and let the solar/wind gen top them off during the day. Almost all our lights are LED's but we have both refrig and freezer operating constantly and this regimen is plenty to keep our batteries topped up unless we have several cloudy days with little wind. My 5.5kw genset only uses about 1/3 gallon per hour so energy costs are not a big deal because for us as this equates to about a dollar a day.

 

Thank you. I don't know anything about autopilots. I just was wondering how much of a larger power system a boat would need if it has an autopilot and is selling in oceans, on average, in general. For example, would one extra solar panel provide the power an autopilot needs if you sail back and forth across the Atlantic 10 times? I say that so that it averages out all kinds of conditions. That sort of naive, beginner level info is what I was looking for. Thanks again

 

More than anything, it really depends on how well you have trimmed the sails to maintain the course you want. A heavy lee or weather helm will of course make the AP work much harder (much more than most seas), so your most important job is to trim her up properly.
I can't think of any reason why a good vane gear wouldn't steer a cat. If you know of one, I'd be interested.

 

Well I'm not in a position to judge them, but when I searched I found at least six or seven different kinds in YouTube videos and I bookmarked them for later watching . The one I see mentioned most is the hydrovane and I think it's pretty well respected. Kevin in how to sell oceans and the guy in sailing wave Rover YouTube channels both designed and made their own. My favorite two YouTube channels.

 

I don't understand why a windvane is not an option on a catamaran. I know at least two that have them. Granted I'm not a catamaran sailor, so, grain of salt and all that.

 

What kind of catamarans are they using them with? Thanks.

 

I noted that my 110 watts of solar panels supplied all the power the AP needed.... ie there was no net drain on the batts sailing offshore in bright sun, Alpha AP, Contest 36s

 

My apologies Sandero. I just reread my post and realised the danger of late night posting. I was not intending to question your figure for an Alpha AP and your boat setup. The 30A pull in heavy seas is similarly just what I observe. It's about 2x your unit which seems about in line other the specs that I see on line.

My 100-200 Ah draw is for the boat systems as a whole including the AP and I was trying to say that I would be very surprised if I could run the whole boat on a 110W panel outside the middle of the day... It sounds like my draw is high.

 

Unfortunately a windvane or equivalent is not an option for our boat so we went with L&S linear drives on a Furuno AP. We found that the draw on a 12 volt system when the AP is actuating is about 30 Amps. In challenging seas and overnight, we use about 200 Ah through the 12 hours (1800-0600) which we cover with a hydrogenerator. In easier conditions, it is nearer 100 Ah

I would be surprised if a 110W panel will cover an autopilot except maybe during bright sunshine for 3 hours either side of noon in gentle seas and consistent winds...

However for a cat, a windvane system may well be a better option.

 

I reported what I found with my boat... once when I checked. I used the AP for 95% of steering and I never saw draws at the levels you mention.

 

Hello, I'm wondering how much power an autopilot uses?

Ours uses less than I think it would, and much less than its rated power usage. Like others said, sail trim is important. I have an ammeter on ours and it uses 3-5A when operating the drive, but there is a duty cycle so the average draw is probably 2-3A. I doubt the electronics themselves draw more than 0.5A, so perhaps a maximum draw for planning usage during most conditions might be an average of 4A - leading to ~100Ah/24hrs. An unbalanced boat or rougher conditions may be higher.

I can't think of any reason why a good vane gear wouldn't steer a cat. If you know of one, I'd be interested.

Most modern catamarans have so much cabin freeboard, arches, flying bridges, etc that a windvane simply can't be placed in free air to allow it to work. Even with a vane on each hull, the disturbed air can be a challenge. The bridgedeck saloon on most catamarans actually pull a suction from the stern, and if you mount it high enough to clear all of that, the flow off the mainsail can be a problem. Performance designs usually have less of this issue, but their problem with a vane is how much they accelerate and respond to changes. A vane can't react fast enough to accommodate the rapid 20-30* shifts in apparent wind as the boat accelerates/decelerates, and it ends up hunting around. A technical issue for many catamarans is just how to connect the steering system for a vane. An independent rudder is the only workable design for many.

Mark

 

Ours uses less than I think it would, and much less than its rated power usage. Like others said, sail trim is important. I have an ammeter on ours and it uses 3-5A when operating the drive, but there is a duty cycle so the average draw is probably 2-3A. I doubt the electronics themselves draw more than 0.5A, so perhaps a maximum draw for planning usage during most conditions might be an average of 4A - leading to ~100Ah/24hrs. An unbalanced boat or rougher conditions may be higher.

Most modern catamarans have so much cabin freeboard, arches, flying bridges, etc that a windvane simply can't be placed in free air to allow it to work. Even with a vane on each hull, the disturbed air can be a challenge. The bridgedeck saloon on most catamarans actually pull a suction from the stern, and if you mount it high enough to clear all of that, the flow off the mainsail can be a problem. Performance designs usually have less of this issue, but their problem with a vane is how much they accelerate and respond to changes. A vane can't react fast enough to accommodate the rapid 20-30* shifts in apparent wind as the boat accelerates/decelerates, and it ends up hunting around. A technical issue for many catamarans is just how to connect the steering system for a vane. An independent rudder is the only workable design for many.

Mark

Our Searunner's vane gear worked impeccably from about 6 knots of wind to 60, and probably would have worked quite well above that, but at that point I considered it my job to steer the boat. She was certainly no racer, but 10 to 12 knots in the trades was a pretty normal cruising speed, if I wanted. I've literally been thrown off my feet when a gust hit and the boat accelerated.
Most folks who haven't sailed good sailing multihulls don't understand that bringing the wind forward in a gust and luffing the sails is what keeps a multihull from capsizing, as they do not heel. So, I would usually disengage the vane gear so she wouldn't go back on course until the wind abated or I eased the sheets.
As most cruisers try not to beat a lot, any wind direction from 60 to 70 degrees off of the bow should keep the vane gear working well, and anything from the beam to DDW shouldn't be a problem. Certainly, for a downwind ocean crossing I can see no reason a well designed vane gear shouldn't work.

 

If i have my sails adjusted properly my autopilot uses around 3 amps and i sail with it on a setting of 2-3 out of 10 to reduce steering. My boat balances well and 5 degrees of rudder is a lot normally.

 

For most catamaran designs current today, a vane on one hull would not even see the wind until it was aft of 90 degrees from the far side. It is almost impossible to mount a vane centerline on a catamaran, or at least it would be a tradeoff with davits and dinghy stowage I would never make. I wouldn't want a vane that needed to be disconnected to prevent capsize because of its steering characteristics.

We operate differently than you. We stay on our feet by reducing sail to be appropriate for the gusts, then sailing it optimally to the prevailing winds. I don't have much experience with trimarans, though.

Personally, I think vanes are anachronisms today for most boats. They make almost no sense on a catamaran, even if they could work well, because you give up an entire hull access for attachment, put weight on the most sensitive area of the boat (the hydrovane weighs the same as carrying another dinghy back there), carry the big ugly thing around most of the time for no reason, and one can easily fit enough permanent solar on a catamaran to run a small town so power isn't an issue. With two rudders, dual electric autopilots provide redundancy should one quit or one rudder be compromised.

Mark

 

Can someone explain the physics to me: Why does the AP use more power if the helm is unbalanced? I wouldn't think the AP motor would need to move any more than in a balanced boat. Is it because it takes more power to move, when it does?

 

If the helm is unbalanced the rudder takes more force to turn, which translates into more instantaneous power needed. In addition, an unbalanced boat will head up (or fall off) as soon as the AP stops making its adjustment, which increases the duty cycle of the AP - more average power needed.

Mark

 

My cat uses a lot of power if I dont have the windward side centre board partially down,
My boat is like a cork and with out a centre board partially down to keep it in a straight line,
My boat wanders all over the place from side to side and the Autopilot is grinding away heavily,
Drop the centre board and the Autopilot is almost unheard while its working, It dont work as hard to keep it straight,

Cheers, Brian,

 

If my AP is engaged... if the helm is moving a lot ie the AP is constantly making corrections the power draw increases. I can turn the yaw feature down to get the AP to make fewer corrections. With small waves the helm is / can almost be rock steady. Down wind the waves tend to make the AP work harder. I am thinking the draw is in the 2 amp range but I will check this next week.
Don't use a 12v refer so that is not a continuous draw. As I do run the engine at least 1 hr / 12 hrs the batts get topped up and the hot water replenished... and of course I am motor sailing for those 2 hrs.

 

There are two types of power usage with an autopilot - the instantaneous current needed to move the rudder, and the average current over time it uses to do its work. You've described how the latter changes with seastate, and that is my experience also. However, Telesail is describing an instantaneous 30A draw to move his rudder, and that seems excessive. It is near the upper limit off what many larger recreational AP's can supply.

Mark

 

30A is the actuation current - the spike at the start - which can be reduced by investing in a $600 gizmo that L&S sell as an aftermarket item but I have not done so yet. The draw during a rudder move averages 10A and in sporty conditions, it is activating every couple of seconds.

It is not tough to turn the helm, especially when you are in flattish conditions but the boat wants to go where the wave is pushing it in quartering seas and that is where the effort comes. The AP does not predict the next wave (unless you spend Vendee bucks) so expends more effort than a human helmsman would. Perhaps it should not be a surprise but less power used when beating than broad reaching with the exception of Gulf Stream crossings where all bets are off!

Maybe part of the explanation is that she only weighs 11 tons at 54 ft so has less inertia to plow on in a straight line than "proper bluewater boats" that don't let you feel every bump in the road.?

 

Waves and sea pushing the boat around demand more attention to the helm obviously. So a lighter boat will be more subject to forces at sea that a heavy one and that makes sense.
MOST rudder corrections should be small unless the course and sea conditions make for more demand on the helm.
So on broad reaches the AP may cause the boat to S because it cannot apply corrections quickly and with enough rudder... such as hard over. My AP cannot put the rudder hard over.... and so for example, the AP cannot tack on a dime so to speak as a helmsmen might. For cruising this hardly matters. What you want is for an AP to hold a straight course when averaged over time... easier in calmer conditions and harder in rougher conditions. We all understand that tacking to a destination is more distance sailed and takes longer than sailing DIRECT to the destination. Same with AP... we want a responsive AP which holds a straight course and doesn't "zig zag" using energy and essentially travelling further and taking longer. Smart APs are computing optimal heading for a designated course/destination using environmental data.

My boat and my experience is that the draws are not what OP is seeing for most conditions. But... when the conditions are challenging a helmsmen out performs an AP.

 

There is a big difference between the below deck unit on a 50 footer and a wheel pilot on a mid sized cruiser or a tiller pilot on a smaller boat. The older Raymarine wheel pilot on my Cal 33 has a motor that is the size of small flashlight. I think max current draw is maybe 3 to 5 Amps. Duty cycle is less than 10% if the boat is well balanced. Pretty insignificant.

 

This is true as far as the motor rating, but not necessarily for operation in practice. For example, the below deck unit on our 46' boat is rated to draw 30A. In actual operation, it draws 3-5A. I think the difference is that 30A could be brought into play in conditions or situations where a wheel pilot would simply stop working. Otherwise, they operate with similar power because they are doing the same amount of work.

Mark

 

A few generalizations:
Autopilots steer to a course.
Wind vanes steer relative to wind direction. When the wind shifts, so does your course.
Autopilots usually draw a lot of power. Often more than other "big loads" like refrigeration.
The current draw of an autopilot varies with the size of the boat, the type of actuator, the sea conditions & the trim of the sails.
Some boats don't track well going down wind with an autopilot. Some work well at all points of sail.
The performance of a vane can vary with the quality of it's installation & the choice of it's location. It can vary a lot.

If I was planning a crossing, I would want a vane on the boat for sure. An autopilot would also be nice. My reason for saying that, is that after 10 days under sail, the vane will not have killed the batteries and in the event of any electrical failure, the vane would still be an option.

 

A lot of boats track down hill much better than their owners think, because most really haven't spent the money on a tradewind rig. Getting rid of the main and having twin headsails on poles, is the sercret to a good, controllable down wind rig. If the sails are pulling the boat along, the seas will have much less effect on the boat and she will ride and track much better.

 

Running the Pelagic system on a Tartan 34C. During the last 7500nm, I only had a 200ah house bank, no solar, only the Yanmar 3GM30F. All lights on board are LED. Underway, I only have the Garmin Chartplotter, VHF and occasionally the IridiumGo running. SOP was to run the engine for 1 hour after sunrise and 1 hour at sunset. (Best compromise at the time). Always ensured sails were balanced (gave it 100% attention at change of watch). I never saw below 12.5v on my house bank. See video for the work the Pelagic was having to do on occasion. You can hear how hard it was working at the time. Based on my energy use and rough calculations, a 170w Sunpower panel will keep up with the AP and underway electronics in the tropics.