Two core decisions we must make when designing a cruising boat’s electrical system for living aboard full-time and making offshore voyages are:
- the size of the battery bank, and
- which charging sources we will need: generator, solar, main engine, wind, etc.
But the first thing we need to do, before getting into all that fun stuff, is think about electrical consumption and how to keep it reasonable.
That’s what I’m going to cover in this chapter, and then in the next we will move on to battery bank specification.
It’s Worth It
Whatever your situation, do make the effort to go through this process, particularly if you have not yet got out there cruising and/or done a multi-day passage, since unrealistic assumptions about power usage, leading to undersize battery banks and charging sources, is one of the most common cruise and/or voyage ruiners.
And even for me, after all my years of cruising successfully on the same boat, this exercise has proved both interesting and useful.
The Old Way
Typically, when designing an electrical system, we are supposed to fill out a form listing every single load on the boat in order to calculate our expected consumption.
But there are several problems with that approach:
- It pushes us into thinking about the details ahead of the big picture—pretty much always a bad idea.
- It’s difficult to estimate electrical usage for a given piece of kit. Sure we can look up the specification, but that doesn’t help much unless we can also make an accurate assessment of how much of the time the machine in question will be running, and how its consumption may vary over time; not always, or even often, easy.
- On a modern cruising boat, with scores of machines that use electricity, this kind of bottom-up analysis will take a huge amount of time.
A Better Way
Given that, I’m going to come at this from the top down, by looking at the loads that really matter, along with rules of thumb for estimating them based on my some-25 years of living on boats.
Will this approach be dead-nuts accurate? No, but it will let us get as close as we need to. And, anyway, the inaccuracies in the bottom-up approach are probably as bad or worse—just because we write down a lot of numbers doesn’t make ’em right.
Do We Even Need To Fix It?
The other advantage of this approach is that we can reverse the process by using the same rules of thumb to understand what level of electrical consumption the system we already have will support (more on how in Part 2). Increasing battery bank size and charging sources is a huge project, at least when done right, that can seriously screw with our cruising budget both money- and time-wise.
Assumptions
When thinking about at-sea loads, I will be primarily focusing on sailboats, since a motorboat, at least one with proper alternators, does not need to worry about loads while underway. But I will also cover loads at anchor, which will be of interest to both sail and power owners.
- I will be expressing usage, load, and capacity in amp hours (Ah) for a 12-volt system. If you have a 24-volt system just divide my numbers by two.
- When I write “day” I’m referring to 24 hours, unless otherwise stated.
Power Suckers
Let’s start by looking at the four horsemen of the flat battery:
John, I’m glad you’ve emphasized the importance of being realistic and honest in estimating electrical consumption.
It’s really easy to add everything up, get something like 6 kWh/day ( = 250 Ah/day @ 24 V, 500 Ah/day @ 12 V), and think “My gosh, that’s like a thousand pounds of batteries, that can’t be right!” So you go back and say “well, the autopilot won’t really take that much, and the radar won’t be on that long” until you get the number you want.
But, if you’re honest about your consuption up front, you end up with a much better balanced system than if you try to cheat the input data. You decided up front that you wanted to run the radar all night on passage, and that you wanted to run under autopilot 24/7 offshore, and that you wanted to go 3-4 days at anchor before smelling diesel fumes. Since those desires formed the basis of the design, your system can handle them without a fuss. It’s a much more relaxing position to be in than watching the breeze come up and wondering “do I really have enough juice in those two 8Ds for this autopilot to last the night?”
Hi Matt,
Good point. It is so easy in this project, and many things in life, to start tweaking the numbers until we get the answer we want, rather being realistic.
Toasters? Pah! European boats often have a grill under the stove burners: great for toast, a chop, fish – the list goes on. I don’t know why we never see them on North American boats, but they’re great. Failing that, a large coffee can with a wire grid over the top and holes punched in the bottom, set over a stove burner, makes great toast fast. It’s not elegant, but it’s efficient.
And as for microwaves, they’re only any use if you have a freezer or take a trip to the supermarket every day: it’s wonderful how removing one system can remove the need for another.
Mind you, having lived for years without, I have a secret hankering for refrigeration…
A great article (again) though, John. Your remarks about autopilots are particularly germane: it’s all too easy to get into a situation where autopilot use drains the batteries and you get exhausted steering. Frequent attention to sail trim really pays off.
Ironically, I use more power in the winter, when I have less solar. I run the lights more, I tend to watch movies, and the heater draws power. The propane solenoid is also a sneaky one.
Thanks for another excellent article John, but I have to comment on methods to arrive at an average daily consumption. There are long spreadsheets online to help calculate the Ah usage of every piece of electrical equipment, but any calculation on the current consumption and the time used, as you say, will be mainly guesswork. In my opinion the only accurate way is to measure the Ah discharged using a shunt based Battery Monitor. All cruising boats should have one of these but it must be correctly installed to measure the current taken by all equipment on the boat.
So by turning off all loads and all charging sources there should be zero current draw, but add any piece of equipment like radar or the fridge and the current draw will be shown immediately. Turning off wind and solar should be easy but disabling the engine alternator for 24 hours is the hard part. This can be done by removing the 12v supply that powers any regulator via the ignition switch. On some engines the regulator can be disabled by simply turning off the ignition switch, but this will disable any electrical instruments or fuel pump used by the engine. Some regulators once exited can’t be disabled this way.
Depending on how you use your boat accurate measurements can now be made of the real Ah used by all equipment with all charging sources disabled to determine the worst case scenario.
Sailing over 24 hrs
Anchoring 24 hrs
Anchoring overnight
Sailing overnight only
Sailing daytime only
These need to be done several times to obtain an average. I have to admit that I have only done anchoring overnight – but this depends on how much the computer is used, especially watching movies!
Hi Matt,
Sure, you are absolutely right, by far the best way. But the problem is that most people going through this either don’t have a boat yet and are trying to figure if the boats they are looking at have adequate systems (most don’t) or are upgrading an existing inadequate system where batteries are only a part of the whole plan, so we are faced with the classic chicken and egg problem.
In both these caes I think my way will get them close enough to be practical.
This is beyond the scope of this excellent article (looking forward to Part II), and sorry for the long comment. Perhaps there is another article that covers it, which I haven’t found, or perhaps a prompt to share your experience…
Anyway. Fridges/Freezes and their power consumption. I have spent about 10 years fighting this problem on a range of cruising boats. Because I have a degree in Electrical Engineering, that one always landed on my desk, whichever boat I was on. Oliver, we can’t seem to keep the fridge/freezer cold and/or the batteries charged. Can you fix it? Sigh. The many other electrical/electronic problems on boats, which I was asked to help with were usually pleasantly easy to solve in comparison.
I know you don’t like engine driven compressors and I understand why and mostly agree. However, there is a really key feature, which, if we can replicate it without the belt driven monster, can revolutionize a cooling system.
That feature is: Very high cooling output power “for free”. Dumped into holding plates, as you suggest above. My experiences are quite dated (over 20yrs old). But not THAT much has changed I think. This essentially allows us to use the holding plates as a “better kind of battery” just for that purpose. A “battery” which can accept charge at a very high rate, until it’s “full”. Big compressor, no problem => max power right in there. Power to drive that big compressor is “free” when the engine is running (or the genset, see below). So the question is how can we achieve these benefits without the engine driven compressor nightmare.
I haven’t quite been able to glean exactly what your setup is on MC. But what I understand, is that you have holding plates in your fridge and freezer and a single big 12VDC compressor (judging by the size of it on your photos, maybe 1/2 hp or even 3/4 Hp??, ie 375 – 600W or 30-50A at 12V). So that’s good, and has some of the features. When your genset is running you can cool down those plates, rapidly and “it’s for free” because the genset has spare capacity and is running anyway. Extra benefit: it loads the genset nicely. Ideally the compressor would be even bigger to take even more power when it’s “freely” available. Ideally it would also have an automatic “always run when the genset is on” wire to make sure you take full advantage (needs thermal protection for the plates and compressor!).
First shortcoming: Because your compressor is 12VDC, and your genset is 120VAC you have to install extra “battery charger” capacity to take full advantage of this. The 3x 40A battery chargers I remember reading about elsewhere, don’t quite seem to do that. Are you not compromising your max bulk battery charging rate for the batteries because the fridge/freezer compressor is running? Need more battery chargers? (ie make more amps available at 12VDC when the genset is running). The same applies to the engine alternator. We to make it big, really big. Or have 2. – Again, the belt driven refrigeration compressor monster is suddenly appealing, but we know why not.
The second problem: Lead acid batteries suck. Badly. As you have eloquently written about elsewhere. Basically any power that you need to put there in order to take it out again costs you. Big time, for a range of reasons, which I won’t repeat here. So when your genset is not running, or, as on Colin’s Ovni, there is no genset but more low power alternative energy sources (MC has some of those too, but less). The best way to reduce the lead-acid nightmare is to not put it in there in the first place. ie try to consume the power when it’s available. In terms of refrigeration compressors this means that a 30-50A fridge compressor is FAR from optimal, because you are very unlikely to produce that much “spare” from low power alternative energy source. So what you want here is a 120W (10A@12VDC) compressor at most, less on a smaller boat with smaller fridge/freezer.
So the conundrum of avoiding the black hole which is the lead-acid bank, is to be able to cool the fridge/freezer in 2 modes: VERY High power when the engine or genset is running, and only low to moderate power when it’s not – using up the amps available from solar, wind and hydrogens.
Do agree with the above analysis? Do you know a good solution to this problem?
I know that these days there are “multivoltage” compressors available (ie 120/240VAC or 12VC). I am not sure that solves it, because in 12VDC mode you would still be sucking far too much power. The only idea I have is: Have 2 compressors. one high power connected to the plates. And one low power on an evaporator. The low power one will be 12VDC. The high power one could be 120/240VAC if you have a genset (although then it doesn’t work on the engine – except through an inverter). Or the high power one could be 12VDC (like yours) but then you need extra battery chargers for the genset.
We can also think of the problem in “Joules or Watt hours of Energy” (Watts multiplied by time running – similar as Ah, but not dependent on voltage). The 600W compressor running for 1hour while the genset is on, could be complimented by the 100W 12VDC low power compressor running for 6 hours while solar/wind/hydro power is available. In each case you have 600Whr of electrical energy available for refrigeration. With NO lead-acid black hole involved.
Other ideas? Look forward to your input.
PS: Have you ever seen a 120/240VAC generator driven by the main engine? Is that more realistic than 2 160A alternators?
Hi Oliver,
In my experience your analysis regarding the potential advantages of dual compressor modes is correct. Our boat came equipped with a SeaFrost system that cooled both a fridge and a separate freezer with dual-circuit holdover plates. By dual-circuit I mean there are two separate cooling coils inside each holdover plate that are not connected to each other in any way – four copper tube ends exit each plate. On each holdover plate one of the two circuits was cooled by a high-capacity engine driven compressor, similar to an automotive AC compressor, and the second of the two circuits was cooled by a moderately power hungry 120 Volt AC compressor. We essentially never used the 120 Volt AC system as we do not have a generator and our boat is rarely tied to a dock. We could have powered the AC system with the inverter but the Amp draw would have been in a punishing 30 to 40 Amps range. As we began taking longer cruises it didn’t take long for us to arrive at your analysis: we wanted to be able to rapidly transfer surplus energy from the engine into the refrigeration system when the engine was running. And when the engine was not running we wanted a 12 Volt DC system that would operate at an efficient low Amp draw such that it would be completely powered by our 300 Watt solar system during sunlight hours and not incur the inefficiencies involved in charging and discharging lead-acid batteries during that time.
After evaluating several different paths and consulting with Cleave and Nate from SeaFrost we elected to retain the engine driven compressor system and to replace the 120 Volt AC compressor system with two completely separate 12 Volt DC compressors, one for the fridge and one for the freezer.
We are pleased with the efficiency and versatility of the upgraded system. Less obvious, there are a series of redundancies inherent to the upgraded system. We can shut down the freezer and run just the fridge, saving considerable energy. We can utilize the freezer as a back-up fridge and shut down the fridge should a component of the fridge system fail. We can operate on 12 Volts DC alone should the engine or a component on the engine driven side fail. We can operate on the engine driven system alone should we have a partial or complete electrical side failure. The DC compressors are efficient enough that they do not add significant load to the battery charger during the infrequent times when we are plugged into shore power, so they provide everything the AC compressor offered plus much more. Lastly, the DC compressors will cool to either sea water or air, a redundancy we appreciated when an unexpected maintenance haul-out was required and we lived on the hard for a while.
The real impact is that we don’t think much about the refrigeration system any more. If we’re motoring we try to remember to run the engine driven system for the last 45 minutes before we shut the engine off. Then, after half a day or so, depending on the climate, when the holdover plates have mostly thawed, the DC system takes over and keeps the box(s) cool going forward. In the Bahamas, with our dark hull, 300 Watts of solar was enough to cool the fridge alone indefinitely without the engine. If we ran both the fridge and the freezer we would slowly loose ground and our 450 Amp/hour house bank would need charging every two days or so. Our next upgrade will be some additional solar capacity but we still like having that engine driven compressor under the sole.
Hi Grinnel
That does sound like a nice setup.
I like the redundancy, and the multiple choices for tapping into power to get things chilled at the rate at which the power is currently available. I have seen dual circuit holding plates before, just not had the privilege of cruising on a boat which had them. And yes those engine mounted compressors are basically automotive AC units (and can be quite reasonably priced).
John has good points too. Engine mounted anything reduces access, increases complexity and puts vibration where you don’t want. Even the obligatory alternator, is slightly problematic in this respect. John is also right, that his system is certainly simpler with less components, piping, joints etc than yours.
But then again, I have seen plenty of boats which make the engine mounted thing work and work reliably, and you have found that too. Those Automotive AC units do bounce around under bonnets of cars for their whole life normally, and have similar issues re flexible connections under vibration etc.
12VDC enclosed compressors have become somewhat more efficient over the last 20 years, so perhaps the engine mounted monster is less required? But great if you have it because it gives you options, and as you said “cooling ceases to be an issue at all”, without any lead-acid impact to boot – which we can then use to run those myriad of computers and other similar low to medium power devices that present a more suitable load for lead-acid.
I would always try to get a windvane to work with any boat that I cruise in, and have good experience with those up to 60ft. So on passage, it should be possible to achieve a power surplus due to the hydrogen options available. That surplus can be used for the low power 12VDC refrigeration system. No engine or genset required, ideally for the whole passage.
At Anchor high power topup would be required (as you have found too). On a genset boat that would be maybe a 1-1.5HP 120/240VAC compressor for me – I spent my youth maintaining high power low voltage DC motors and I dislike them. Without a genset, need some high power way to tap into engine power. Lots of 12VDC alternator juice or 120/240VAC engine driven generator or the engine mounted compressor. Running the engine for an hour every 1-2 days seems reasonable to me. And often naturally happens by motoring on/off anchor from one bay to the next.
Your point about 12VDC working nicely on shore power in the boatyard if you ensure you have an air cooled condenser as one of your options, it a really good one. I didn’t know you could plumb in something which cooled the condenser with “either water or air”. How does that work? Is that with a “double 3 way valve” of some sort?
I know there is lots of content on this site about high latitude cruising. That’s great, but obviously refrigeration is much easier in colder climes. I think my next trip will be in the heat, lots of it, not only because my cruising partner likes it that way. So powerful refrigeration is essential. If you’re going to have refrigeration at all, then you need to make it work properly and not just have a luke warm coolbox.
Thanks for sharing you experiences. It’s a complex subject.
Oliver
Hi Mal19,
Sounds like a good system. And I too am a huge fan of holding plate systems. That said, it has worked better for me to move the compressor off the engine and drive it with a 1/2 hp 12 volt motor. (I have had both types) Less clutter on the engine, no problems with vibration, and the same motor can run from the batteries. Of course that does require a larger alternator.
Hi Oliver,
Yes, you have the analysis right. That’s exactly what we are doing: using the holding plates to store energy instead of the battery. Or to put it another way, we pretty much never run the fridge and freezer on electricity from the battery.
And yes you are right that this requires bigger chargers and a bigger alternator (I don’t like dual alternators): https://www.morganscloud.com/2013/11/06/10-tips-to-buy-and-install-a-liveaboards-alternator/
You are also right that engine driven would be more efficient (less power conversions).
Having said all that, in my experience, and I have owned both, a powerful compressor being run via a 1/2 hp electric motor is still better than engine driven because the maintenance is so much lower and the reliability so much higher.
As to an AC generator off the main, that’s possible and in fact there used to be a freezer you could buy configured this way, but the clutter on the main engine and the maintenance problems were horrible (have a friend that had one).
So, in summary, this is one of those situations where reliability and maintainability outweigh best efficiency, particularly since with the main or generator running we have plenty of spare power in comparison to the inefficiencies.
Bottom line we kept keeping 8 cubic feet of food (~100lb) hard frozen for months at a time for many years with just 500 Ah in the main bank (since upgraded) with this set up.
The other advantage is that with our new bigger battery bank (800 Ah) we can, if it makes sense, run this 1/2HP motor from them, not an option with an engine drive. For example, if the freezer needs running first thing in the morning and I know we will be motoring for a few hours later in the day, I might run the freezer without starting the generator knowing that the batteries will get charged later. (A full run of the freezer is about 60 Ah and takes around an hour.)
And of course an engine drive can’t be run from the generator or shore power, so with engine drive most boats are fitted with a Danfos type compressor as well, more complication, clutter and maintenance problems. (I have owned this setup too).
Hi Oliver,
This is a response to your response. The site’s comments engine does not appear to allow comments indented more than 4 layers deep, so I reply here and hope you see it.
To answer your questions:
The two 12 Volt DC compressors are SeaFrost Tradewinds XP Air-Water Condensing Units. They feature both a small refrigerant to air heat exchanger (looks just like a small heater core) as well as a compact refrigerant to water heat exchanger. The boat was already equipped with the through hulls and strainer for the old water cooled 120 Volt AC condenser unit. It was therefore an easy decision to specify the water cooling option on the 12 Volt Tradewinds units and utilize the legacy through-hulls to achieve greater cooling efficiency.
The engine driven compressor is not mounted directly to the engine but rather to a bracket adjacent to the engine. Therefore, it is not subject to engine vibrations. However, it is driven by a rather long belt from a pulley on the engine and that belt is subject to considerable vibration.
Hi Grinnell and Oliver,
Having the compressor mounted off engine may work for Grinnell, but it’s generally bad practice unless the engine is mounted on non-rubber hard mounts (rare on a yacht). The reason is that the compressor belt tension will affect engine alignment and even if we get it dead nuts with the compressor tensioned, the mounts are being subjected to sideways forces they were not designed to take. And worse, compressor load changes over the holding plate freeze cycle so the engine alignment is being changes too.
I’m not trying to be argumentative here, but just to make sure others reading this understand the drawbacks.
Oliver,
With a similar background to yourself I’m equally unimpressed with Lead Acids myself and I’m committed to find better solutions myself.
In terms of scheduling refrigeration properly I completely agree with you. This Australian company seems to have thought all this through very nicely:
https://www.ozefridge.com
Cheers
Hi Philip,
The Ozfridge looks like a nicely engineered system. That said we need to be aware that the compressor is nowhere near big enough to freeze the plates in a short period so as to remove the need for larger battery banks. So, to me, it misses delivering the biggest benefit of holding plate systems that I discuss in the above post. Also even their largest unit will only run a freezer of three cubic feet.
And I will be discussing the drawbacks of lead acid in the next, but one chapter. In the mean time, I took a look at lithium here: https://www.morganscloud.com/2018/05/05/battery-options-part-1-lithium/
Hi John
Of the four evil energy users onboard cruising boats the electric autopilot stands at the head of the list.
Except that by observation many (most?) cruising sailboats rarely go anywhere, and if they do they usually motor instead of bothering to put up the sails. So that $5,000 autopilot connected directly to the quadrant is the ideal tool for the job and its power thirst is unimportant.
For the subset of cruisers who are actually sailors, the sophistication of electronics and controls of the modern autopilot make it a very convenient device— one that is reliable most of the time until Murphy pays you a visit in the middle of the night far from land. It is not rocket science to design an electric autopilot that requires magnitudes less electrical power buy using the energy of the vessel moving through the water to control a trim tab or servo paddle. But neither wind vane manufacturers or electronic autopilot manufacturers specifically design for this use. Is there no demand or are we caught in the common human trait of conventional thinking?
I think you will find that most windvane suppliers do support solutions of using low power tiller pilots with the servo pendulum generating the power. This should not be the primary mode of operation, but works as a backup or in very light wind.
You are right that there are many “motorsailing cruisers” out there. For these people, autopilots and also refrigeration are much less of an issue.
For those of us that like the sailing bit – and I include myself here – electrical power is always a problem that requires careful design and planning, hence Johns article here. For the keen sailors who understand how to trim their boat and enjoy that, it is my view that a windvane should be one of the self-steering systems on board, and if well chosen and installed it should be the primary one – meaning it steers more than 85% of the time on passage.
Gi Richard and Oliver,
I too am a vane gear fan:
https://www.morganscloud.com/2007/05/19/windvane-or-autopilot/
Hi John
4 level indent limitation here. Replying to your comment above about dangers/problems with engine compressor.
I completely agree, as I hope I made clear, that attaching anything to front of main engine is a headache.
People, even the suppliers of such kit, seem to talk about mounting compressor on stringers in boat, not on engine frame. That seems mad to me. Those reference points move by 2 inches relative to each other sometimes (at certain low rpm resonance frequencies). I would be really not keen to do that, although Grinnel seems to have had a positive experience.
Can we instead focus on what the problems are – and am still conscious of article scope creep here – with mounting an engine driven refrigeration compressor or indeed a 120/240VAC generator (actually those don’t really exists, it’s a 240VDC alternator with an inverter to make the right (50/60Hz) frequency) suspended on a frame which is attached to the same reference frame as the engine crankshaft, above the flexible mounts.
This is no different to mounting a high output 200A alternator to the engine, is it? I know it needs flexible pressurized refrigerant hoses. But other than that? What is the concern? (I personally have some, but I am trying to not pre-bias the discussion).
Hi Oliver,
At least for me, we are over thinking this. Let’s take a step back. My view is that we must have an alternator, so it’s best just to make that a big one and run everything else off it. Why do I say “best”? Because it is the simplest solution, with the least amount of kit, and one thing I have found over some 40 years of maintaining offshore boats and going to sea in them is that the simplest solution pretty much always causes the least problems.
Also, do keep in mind that for me this is not just theory. I have had two engine driven systems in the past, both also had a secondary system with separate coils in the holding plates. And when I bought my present boat it had two alternators and a compressor on the front of the engine. The whole thing was an unnecessarily complicated mess. Is that just one experience and not relevant? Maybe, but probably not. Simplest is pretty much always best on a cruising boat.
For example, you will note in that in the above article I make very clear that all holding plate systems, including mine, have drawbacks and that for most people an evaporator system is a better bet. Why? Because it’s simpler.
More on that thought: https://www.morganscloud.com/2011/01/17/the-beauty-of-simplicity/