Let's Talk Electric Propulsion Part 1: Why Electric?

So, I’ve promised a discussion on the electrical installation for a while now, and I figured with this week of wet, rainy, misty weather that I’d take a bit of time off from the outdoor work I can’t get done. In between indoor projects I’ll be writing a mini-series of articles covering the following topics on my recent electric repower:

1. Why electric?
2. Installation tour
3. Recharging and power management
4. Dealing with “range anxiety”

If you’d like to see something specific covered that you don’t think I’ll get to in one of those discussions or if you have any further questions on a particular topic, please leave them in the comments and I will write a 5th section covering your questions in detail.

I’m going to try to commit to one of these articles every other day for the next week or so, though I might whip out two in one day and nothing for the next four - I’m juggling quite a few projects right now so sometimes I get time in a day and sometimes I just have to punch through and finish something while I have the opportunity (weather, temperature, parts, etc) to get it done right then. So please bear with me but I will try to get these out on a sort of reasonable schedule here for those of you who are really interested in the topic.

That said, here’s the first part!

****

Why Electric?

****

The subject of electric propulsion for boats has been under intense discussion for nearly a century now, believe it or not. Ever since the first electric motors were assembled and the first reasonable-capacity batteries were available, the idea of powering a propeller via electric drive was on the menu. All non-nuclear submarines to this day use electricity as their primary propulsion power. Obviously there are significant reasons why this system of propulsion isn’t widespread on the surface of the water today, or, well, it would be. However, though the times may be ‘a-changin’ slowly, ‘a-changin’ they are indeed¹, as modern technology steadily moves the usefulness, range, and cost/value ratio ever more in favor of the electric drive. Nevertheless, it is still very much a loaded decision that requires acute awareness of the tradeoffs required to reap the considerable benefits.

While the main purpose of this article is to discuss why *I* chose an electric drive for my boat, I hope that by sharing my thought process and key information it might help provide some useful fodder for discussion, perhaps even encouraging you to consider the switch for your own boat.

Before I begin, for those of you who want to dip your toe in the technical “how can an electric motor replace a diesel” conversation, which I won’t really get into here (but might expand in future episodes), see footnote 2 at the end of this article.

For most of you, a good place to start is to introduce what a typical electric setup looks like. I find it helps to relate it to your typical diesel engine for those of you who like comparisons. A typical diesel has a fuel tank to store energy that will later be burned to power the prop. It has a throttle to control speed by varying the amount of fuel injected (or by controlling the airflow, but this is more common on gasoline engines). It has a transmission to switch between neutral, forward, and reverse gears, and it has exhaust, cooling, and lubrication systems to help keep everything running smoothly. It also tends to drip those fluids all over your bilge and spatter them across the inside of your engine bay.

An electric system, on the other hand, has a battery bank to store energy which will later be converted via magnetic fields to power the prop. It has a throttle which, with the assistance of a semi-intelligent power controller, adjusts the speed at which the prop spins by varying the force of the magnetic field inside the motor. Some of them have a gear or belt drive to change the ratio of motor speed to prop speed, but none that I am aware of have an actual transmission, since the motor can spin both forward and backwards (and doesn’t care which way is forward or reverse, its all the same to the motor) and does not need to keep spinning even when its power is not being used. Electric motors are much more thermally efficient than a diesel or gasoline engine (meaning they produce less wasted energy in the form of heat) and therefore can typically be cooled with nothing more than the air around them - although certain high performance electric motors also use a water cooling system - though not usually one containing antifreeze. Lubrication is only needed in the bearings and since the motor does not get very hot, these are typically sealed and need no maintenance or oil changes. Finally, the exhaust system is gone because the electric motor does not produce any gasses that need to be vented.

So far, in a typical sailboat electric drive compared to the typical diesel, we’ve eliminated three major systems: cooling, oil, and exhaust. We’ve removed or simplified the transmission and reduced the heat generated by the motor (making it vastly more efficient - more on that shortly). The motors are also MUCH smaller and weigh MUCH less than an equivalent diesel, since there are no explosions taking place inside of them that need to be contained by large blocks of heavy steel or aluminum. On the flip side, we have a large and heavy battery bank in place of the (generally equally large and heavy - at least when full) fuel tank.

So let’s tally up the scorecard here:

Diesel Motors: Heavy for a given power output. Requires Oil, Coolant, and Exhaust systems and associated maintenance. Noisy. High vibration. Low thermal efficiency translates to high temperatures in engine room and wasted fuel.

Electric Motors: Lightweight for a given power output. No regular maintenance other than a quick cleaning and very occasional belt replacement, if equipped. Whisper quiet. Low vibration. Very high thermal efficiency translates to much cooler temperatures in engine room and very little wasted energy.

Weight and storage space of typical battery banks and fuel tanks roughly cancel.

If that were all there was to it, we’d all have electric motors in our boats (and cars, and maybe even planes) today! But there’s more to the story.

Gasoline and diesel fuel are some of the most efficient forms of storing energy we know of, from a weight and volume perspective. Nuclear fuel might be more efficient but that’s a bit out of the question on our scale here. Fossil fuels are, relatively speaking, safe to handle and easy to control. They are fast to refill, and available around the world, for the right price. But they have a serious problem: they are very finite and impossible to produce without tremendous effort and extremely complicated equipment.

Electric energy, on the other hand, has nearly the exact opposite situation. We store it - even to this day - in a nearly archaic and highly inefficient form of lead and sulphuric acid that dates to over 150 years ago (yes, lithium technology continues to tease us with its benefits, but it is still only just break-even from a cost perspective across a given lifespan if all goes perfectly well, and lithium is a horrifically environmentally devastating metal to retrieve and process). All batteries are comparably inefficient from a weight and volume perspective (and lithium is still volumetrically inefficient even if it is much lighter weight). Fortunately, electrical power is safer in many ways than liquid fuels, but it is not very fast to refill (ultracapacitors and other exotic technology notwithstanding). It is available nearly everywhere, for pretty darn close to free, and can be generated as long as the sun shines and the wind blows with very simple and reliable equipment that can be carried and repaired aboard even the most humble of sailing craft. You might think of electric power versus diesel as very analogous to the philosophical differences between sailors and powerboaters. In fact, that’s precisely the reasoning that makes me believe that eventually, all sailors will come to have electric auxiliary propulsion, and the rest will go buy powerboats. A guy can dream, can’t he?

But I’m getting ahead of myself. One of the major results of that combination of “slower refill” and “volume inefficiency” is this: a battery bank of equal size and weight to a tank of fuel will result in MUCH LESS RANGE. Particularly speaking, about an order of magnitude less, give or take. That means that for a given weight and space of fuel storage in your boat, where you could go 100 miles on diesel you can go 10 on electric power.³

… <crickets> …

Now that those of you who really should own powerboats have left (I should note that I am definitely not disparaging responsible and competent powerboating - let’s not go there), I can talk frankly and openly with the rest of us. You sail because you enjoy the peacefulness, because you appreciate the skill it takes, because there’s something special about the time you turn off the engine and feel the pull of the sails, am I right? For some of you, you never turned off the engine because you don’t even have one. Even moreso to you, my friend. Whatever the reason you sail, the fact that its NOT accompanied by a go-fast, get-there-now lifestyle and a noisy, roaring engine is a major factor. In fact, if we could draw an axis of two extremes, on one hand we would have the Cigarette boats with their high speeds and engines that require earplugs even at idle. On the other hand you’d have a sailboat with a well made, efficient, and seaworthy hull, a perfectly tuned rig, and no engine whatsoever. Most sailors with auxiliary diesels fall squarely in the middle on that line - a large, heavy, overly powerful engine on a moderately efficient hull with a sort-of-tuned rig. I’m sorry if I offended you. Actually, no, I’m not. I’m just speaking the truth, and sometimes its not nice. But I’m not sorry, its the truth. The point is, we sail TO SAIL. And we know all too well that the diesel is the antithesis of that.

Now, I’m not going all apeshit on the idea of auxiliary propulsion. No, no no. Sailors have for centuries, maybe even millennia, understood the benefit (and more than occasionally, the necessity) of an oar or two. Those of you who are still reading, who are true sailors at heart, and who are genuinely bummed about really needing and using the diesel in your boat - I feel your pain more acutely than you know, having been in your ranks not even two months ago. There is a very true utility to auxiliary propulsion, even to the most stalwart of sailing aficionados. The wind often dies JUST as you enter that narrow inlet with the outbound tide. Marinas pack hundreds of boats along channels so narrow that a full pivot is often only possible with the aid of an empty slip - and woe to the full keelers who find themselves facing a dead end in one of these with a foul wind and a rapidly shoaling bottom. No longer is it common to lay to anchor in a harbor and find the assist of scheduled transport to the shore a mile or more distant. I have heard rumor that even those who are well known in the cruising community for sailing without engines find themselves to require a tow into or out of harbor more frequently than even they would like to admit. So indeed, there is much utility in an auxiliary form of motive power; the question is: how to ensure that the auxiliary propulsion does not distract from the fundamental point of sailing? Yulohs aside, it is my firm belief that the electric motor has finally come of age as the practical answer to this question. But it took until the recent invention of certain efficient drive electronics and highly reliable and efficient all-weather industrial motors to get here.

Without going into excruciating detail, the gist of the modern story is this: electric motors have finally reached the level of reliability, ease of control, and intelligence of recharge that makes them a PERFECT fit for the ethos of the modern sailor, whether cruiser or daysailer. And perhaps most relevantly to why the time has finally come about, there are a variety of vendors whose sole specialty is the fitment of these systems inclusive of all necessities, and the subsequent support of such systems, such little as is needed from time to time. But the point remains: if range under power alone is your end-all, be-all metric for choosing between these systems, may I suggest a little too loudly that you sell your sailboat and buy a motor cruiser.

Let’s Bust Some Myths

This brings me to the crux of the discussion here. In order to really appreciate the enormous benefits of an electric drive, you must fully embrace the benefits of sailing, along with the pertinent requirements. This leads me to The List of Truths about sailing with electric propulsion, to counteract The Fear that so many people express in the various forms of the phrase: “I’d really like to switch but…” :

The List of Truths about Electric Propulsion

1. Range is overrated. Speed is overrated. As long as you have “enough”, you are fine. Enough on a sailboat is much, much different from enough on a powerboat. You’ve got to wrap your head around this if you are used to cranking up the iron genoa every time the wind dies or you want to “get there sooner”. However, a well designed electric system with wind or solar recharge CAN add a knot or two to your speed for hours or even days without needing to be topped off with a genset or a dock recharge. And the range on even the smallest electric setups is PLENTY to get in and out of a typical harbor, on and off a mooring or two, and even to hold your position against a storm wind off a lee shore long enough to set proper sail and/or reposition to lay to a storm anchor or drogue.


2. That lee shore is not going to be fought by a diesel any better than it is by your electric motor. If you’re in a survival situation where you think, from the safety of your armchair, that your diesel is the savior, you need to think again. Your sails will, time and again, get you better out of that situation than your engine which, in all likelihood has seized up from a clogged fuel filter and/or air getting in the fuel lines from the violent sloshing. An electric motor is vastly more reliable in these circumstances, but most importantly you sail without the expectation of the engine saving you, which makes you a better sailor - and in this type of situation, that’s what’s actually going to save your bacon (or keep it out of the frying pan in the first place).


3. Running out of power is not the stressful event it is with a fossil fuel engine. Instead of NEEDING to get to a place to refill, you have FAR more options. You can
   • Wait until solar or wind power regenerates enough to move along.
   • Start up a small gasoline generator to quickly provide some electric power (this is called a “series hybrid” and basically turns your sailboat into the equivalent of a Prius).
   • Raise your sails or pitch out an anchor, whichever makes the most sense for your situation, like a Real Sailor^TM would.
   Which of those required a call to Tow Boat US, hm?


4. Fixing problems is pretty dead simple. Instead of “drain the fluids, pull this part, check it, replace it, nope didn’t fix the problem try something else”, if it can’t be fixed handily with a multimeter (and if you’re the non-techy type, a quick phone call to the vendor), you KNOW exactly which part gets replaced and it fixes the problem nearly 100% of the time first time, the end. Bonus: you just freed up all the space in your spares locker and your oil/coolant locker for more storage purposes. Double bonus: your engine toolkit is now just a couple wrenches, two screwdrivers, and a multimeter. Oh, I have your attention again?


5. And finally, electric propulsion offers all of the needed benefits of auxiliary power while enforcing the requirement for a common level of seamanship as ought to rightfully be expected of any sailor. It eliminates the deplorable category of “sailors” who never raise a sail, motoring about all the while the covers rot in place on the boom. Yet it provides all the services much beloved to those of us stuck in dreadful marinas and backwater windless bayous who must thread a certain needle before being able to hoist glorious canvas and thrill to the wind in the rigging.


As you can hopefully see, I’m pretty gung ho about electric auxiliaries. They offer all the truly useful conveniences of prop-based auxiliary propulsion: enough range and speed to truly enjoy the sailing capabilities of your vessel and keep her out of trouble in all the manners in which an auxiliary engine can reasonably be expected to do so. They make it convenient to enter and exit even very challenging harbors as well as any diesel could do, and better since they aren’t likely to clog a filter or somesuch. They remain true to the heritage and experience of sailing: propulsion that is utterly, blissfully quiet, can recharge from the very forces of nature than propel the craft herself, and are lightweight and small so as not to encroach unreasonably on the sailing performance and interior accommodations of the vessel. They require only the same level of forethought and planning any responsible sailor would perform normally, and offer an extraordinary host of benefits from a practical standpoint aboardship such as space, weight, heat, tools and parts storage, repair skill, and field serviceability. It is my opinion that all of the perceivable downsides of an electric motor are directly attributable to a mentality of careless and thoughtless boating, not that of a competent yachtsman. While they are not a suitable powertrain for all motorsailors or powerboats just yet, I have seen some very lovely powerboats that are 100% electric - and they require the same level of competent handling and seamanship as a similarly equipped sailing vessel, perhaps moreso as they don’t have sails to provide primary propulsion.

A Brief Discussion on Cost

Now that we’ve covered the benefits and tradeoffs, such as they are, of electric propulsion, I would like to add some final thoughts. First of all, the actual numbers for your vessel are going to be different from mine, but the general principles are those of physics and therefore remain true for all vessels. This means that to have more range, you get more batteries that hold more energy. To power a bigger boat, you need a bigger motor which requires more power to run it, and so forth. But just as diesels come in a variety of sizes and abilities, so do motors. Though I have not covered specifics, the cost of repowering with an electric motor setup compares to a diesel roughly as follows: The motor itself will cost about half what a bare diesel engine appropriately sized for your boat will cost, give or take. The battery bank will cost somewhere between the same as the motor on up to two, four, or even more times that depending on what type of batteries and how many you want, which is mostly a factor of range and reserve power. For a setup I would describe as “enough” for me, the ratio is about 1:1/2, in other words, the battery bank cost roughly half as much as the motor, and the sum of those two is roughly about 3/4 as much as a diesel repower would be, perhaps even less in my case. Most people will want about twice the battery capacity I have, but even so, when you factor in the fact that the electric setup will never need another fillup at the fuel dock, another oil or coolant change, and can in fact generate its own power while under sail, its not only paid for itself in the even swap of initial costs, but repaid for itself again over its life perhaps some twice or more. Again, while the exact numbers for my situation won’t match yours, the general principle I have found to be true by comparing many different setups on many different boat types. If anything, the reduction in global oil production due to peak oil and the resulting escalating oil prices combined with ever better electrical system and sailboat design efficiency means that this cost ratio will only continue to be increasingly in favor of electric propulsion.

*** It’s All About YOU, Baby. ***

Can I say with certainty that you should drop your diesel and go electric? Almost, but not quite. It really comes down to you. Are you able to accept the responsibility that comes with owning a sailboat whose primary purpose is to sail, rather than to motor? Are you interested in sailing for the sake of sailing and not solely as cheap transport, however slow, to some foreign dream of yours? Are you willing to learn the skills and put in the time on the water to develop the ability to trust your sails more than your motor? And what is it worth to be able to just turn a throttle, back out of your slip, go head to wind and hoist sail, all without the racket, maintenance, and mess of a diesel? If you’re in it just for daysailing, what are you waiting for, you’re never out of electric range of home anyway. If you’re primarily a racer, well, that’s up to your class rules, I guess. If you’re cruising, you’ll have to take a harder look inside yourself to find out what sort of sailor you really are to determine if electric is right for you yet. And for those of you with the rackety old repurposed farm tractor engine (that’s all of you with diesels, by the way) still in your boat but you know it shouldn’t be, hopefully this is the encouragement you needed to put that old engine out to the pasture it belongs a bit sooner and reclaim your rightful bliss. Because while you’re changing the oil, fixing that coolant leak, or waiting in the hot sun in line at the fuel dock, I’ll be swinging at anchor next to you laying in the hammock with a warm friend and a cold beer that extra hour or two.

Stay tuned. The next article in this series will be a guided tour of my installation and a discussion on the differences from when I had the diesel.


_________ /) ____________

¹ Listening to Bob Dylan. It seems appropriate.

²: Basically, boat speed through the water happens because of energy transferred through the propeller. The propeller must spin at a certain RPM to put this energy into the water, and that RPM is provided through the application of power to the propeller by the engine or motor. A lot of people use the term “power” and “energy” equivalently, but they are slightly different: power is how fast you are using energy. The unit of power is universally the Watt (you’ve heard of a 100 W lightbulb no doubt), and when engines and occasionally motors are concerned you hear the term “horsepower” (HP) a lot. A horsepower is simply very close to 746 Watts, so they are measuring the same thing and using two different but related numbers to do so. Electric motors are most commonly rated in kW, which is 1000 Watts. (For the math junkies, if I have a 7.46 kW motor that would be almost exactly a 10 HP motor). For those of you confused about power versus energy, Wikipedia has a nice explanation.

The reason I bring that point up is because very commonly you hear electric motors rated in kW instead of HP whereas, in the US at least, almost all internal combustion engines are rated in HP. Elsewhere in the world they are both rated in kW and those of you who live there are yawning and wondering when I’ll start making sense. The major point is that for YOUR boat, it will require a certain amount of power to push it through the water. It doesn’t matter where this power comes from, it just matters that it is produced and delivered to the prop. So an electric motor that produces the same useful POWER to the prop as a diesel, regardless of what they are ‘rated’ at, will move your boat just as fast, and as you will see, will probably consume less ENERGY doing so due to not wasting so much of their input energy in the form of heat and mechanical losses.

I won’t get into “electric motors have more torque” etc. etc. because that’s pointless. All that matters is a) the motor delivers enough power to drive your hull at a given speed and b) your prop is sized properly to match your hull’s required power to the motor’s deliverable RPM at peak efficiency. You can adjust the motor’s RPM through gear ratios and you can adjust the prop with pitch and diameter. There are multiple combinations that yield practically equivalently efficient setups, its not worth fussing too much over once you get a good combination unless you’re a yacht designer and/or just really picky about minutiae. Its best to get a torque / power curve for your motor, some sort of hull speed / power data from your designer (who hopefully has specified a good prop for the hull as a starting point), and a good prop guy who can take those data points and recommend a gear ratio that puts the engine at its peak efficiency where the prop is at its peak efficiency, and size the prop to put your hull at an optimum cruising speed where those efficiencies meet up. This is true regardless of the propulsion method. The reason you see so many people happily replacing 40 hp diesels with 10 hp electric motors is not because of torque or more efficient props or any of that bullshit, its because the diesel was both horribly inefficient (and thus wasted a lot of its ‘rated’ power) and ridiculously oversized for the boat. Bigger engines on sailboats are so overrated as to be ridiculous. John Vigor recently wrote a more amusing diatribe on the same subject.

³ Note that this has nothing to do with “MPG”, and only a bit to do with “range”. This is for “a given weight and volume of energy storage”. One of the benefits of electric power is that the batteries are smaller and more modular than a fuel tank, and can be stowed in places where their weight can contribute to the balance of the boat, often displacing additional lead ballast (and thus canceling its negative weight impact) such as in the case of the POP 25 sailboat. Plus you can add more as long as you have space and weight capacity for them. So absolute range is a factor of battery capacity on board, as well as a factor of any recharging (wind generation, solar, etc) that is also taking place to help reduce the load on the batteries.

Since electricity does not directly use fuel, there is no way to relate efficiencies in terms of MPG, but its very easy to compare efficiencies in terms of ENERGY. One gallon of diesel fuel provides approximately 135.73 MJ of energy. Using my boat as an example, my diesel engine burned about 1/2 gallon an hour to move the boat at about 3.5 kts, give or take (obviously we are talking long range cruising speed here, not peak, which is very inefficient). This is approximately 67.86 MJ / hour, or 18.85 kW of consumed energy. The electric motor, to move the boat at the same speed, theoretically consumes (need to do more in-practice verification before I am comfortable saying DOES consume) just shy of 3 kW to do approximately the same thing. This is more than 6 times more efficient, or in “MPG-equivalent” it means my electric motor gets ~40 NM/gallon. This sounds great, but in the same exact parlance, my particularly chosen battery bank is the equivalent of a third of a gallon of fuel. That means my maximum theoretical range is not more than 15 NM, and most likely it’s going to be a bit less in the real world. This is where the increased efficiency of the electric motor by at least half an order of magnitude meets the decreased volumetric efficiency of batteries by at least a full order of magnitude. Fortunately, its not the whole story as you’ll see.


_________ /) ____________