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Are electric motors ready to replace diesels?

Jan 22, 2013

A boat owner’s perspective on the state of electric propulsion

Electric propulsion is a hot topic these days, for both cars and boats, but it presents particularly challenging questions for the cruising sailor. Perhaps the most important is whether being good to the environment comes at a cost in reliability and performance that could increase the risks faced at sea. Having sailed on production diesel auxiliary sailboats from the Chesapeake to Labrador, I have been comforted on many occasions with the friendly sound, and the concomitant relief, that comes when the diesel engine fires up when it is needed most.

But sometimes the incessant noise gets on the nerves, or the following breeze blows a sickening smell into the cockpit. And a few times, the unexpected silence of a broken-down engine can take your breath away. Now, as my wife and I begin our fourth season with our 31-foot wooden sloop, Amanda, powered by an electric motor system, we have answers to some of the most often asked questions.

First, here are the easy questions:
Are electric motors powerful enough? Absolutely. Our motor is a 7-kW unit, equivalent to 10 hp. But electric motors deliver high torque and more power to the propeller shaft at low rpm. Power of 3.5 kW (or 70 amps at 48 V) drives our boat at five knots. In calm conditions, 1 kW drives the boat at three knots. The motor is not absolutely silent, but you won’t hear it if there is a little breeze. One of the things we love to do is motor with 500 watts of power to point up higher in a light breeze. You can hardly hear the motor, and it feels more like sailing than motoring.

Are they reliable? Electric motors are mature technology and are much simpler than diesel engines. The number of things that can go wrong is much smaller. Generally they are more reliable. But between the batteries and the motor is a control box, usually black, with all that implies in terms of how easy it is to repair. You won’t find a replacement unit in your local chandlery or be able to order it from West Marine. Your marina cannot fix it. It is not difficult to replace, so carrying a spare, although not cheap, would be the prudent approach if you are doing extensive coastal cruising in a remote area, for example.

Are they maintenance free? Well, they don’t need to be winterized. They don’t need their oil changed. They may need new electrical brushes every few years depending on use and on design of the motor (some are brushless). If you include AGM batteries you won’t have the problems of maintaining flooded cell batteries. The maintenance advantages are significant, but below we will talk about the importance of back-up charging from an onboard generator. And although they are smaller and more lightly used, they are internal combustion engines and they need standard maintenance.

Does an electrical propulsion system decrease weight on board? Our motor and its control systems weigh less than 75 pounds. That’s a real advantage if it needs to be removed or replaced. It is certainly a lot less than a diesel engine. But then there are the batteries. We have two banks of four batteries each to supply 48 volts. Each battery is a group 31 AGM weighing 75 pounds, therefore 600 pounds total. These can be placed with some flexibility (you need heavy gauge cables, so not too far away!) and used for the house systems as well. So it might be argued that not all the weight should be assigned to the propulsion system — some is needed anyway, some is good ballast. But it is more than a diesel weighs. We discuss adding a generator to the system below, which can be another few hundred pounds.

How much does it cost? For the motor and the batteries installed, probably around $10-12,000, which is considerably less than a comparable diesel installation. If you add a generator, you are probably close to the cost of a diesel motor. But there is the advantage of flexibility with different operational modes, and there will be better fuel economy.

How do you optimize the propeller? The appropriate parameters for propeller diameter, pitch and gear ratio are chosen to allow the motor to reach maximum rpm at the maximum boat speed, which is essentially the hull speed. One can use the same approach that is described in Dave Gerr’s Propeller Handbook for diesel engines. Optimizing for electric propulsion is actually more forgiving than with a diesel engine because it is not as important for an electric motor to operate at a certain rpm for optimal efficiency. For example, if you set the pitch higher, or the gear ratio lower, then the motor will run at a lower rpm for the same boat speed. But the power drawn from the batteries will be similar thanks to the chopper power supply which adjusts the power to the motor to maintain an rpm set by the throttle.

In our case we chose a 16-inch max-prop, which feathers to reduce drag under sail. The high torque of an electric motor allows it to easily drive a big prop. Fred Hutchinson, of PYI Inc., which supplies max-props, advises that a pitch of about 20 degrees optimizes the efficiency of the propeller for propulsion. This pitch works well, when combined with a gear ratio of 2.5. The motor’s top rpm is 2,250 at 48 V, leading to a maximum shaft rpm of 900. An electrical current of 70 amps drives the boat at five knots at a motor rpm of about 2,200. Unlike a diesel, it is fine to run an electric motor at or near its maximum rpm for long periods. In fact, our motor (Lynch Motor Company model 200-D135) can operate up to 4,000 rpm if driven by a higher voltage, so 2,250 rpm is far from its maximum mechanical limit.

How far can you go? We have a battery range of up to 20 nm at three knots, and 10 nm at five knots under modest sea and wind conditions if there is not significant power drawn by house loads. This is a simple matter of battery capacity and depth of discharge. However, simple motoring is seldom the preferred way to go. Motor sailing in light winds seems surprisingly effective at relatively low currents, say 10 amps. We spent a few years sailing in Nantucket Sound where there can be currents of two to three knots. These can be extremely useful to extend the range. And, although it is probably obvious, fighting strong head winds, currents, and large seas is going to limit range and drain battery capacity fairly quickly.

How long does it take to recharge? We have a 1-kW battery charger on each bank which can supply a little over 15 amps to discharged batteries. About four to five hours restores their full charge using a 20-amp marina circuit.

Now the more difficult questions:
What about regeneration under sail? Regeneration of power under sail, by allowing the prop to turn the motor, which in turn acts as an electrical generator, is the Holy Grail of electrical propulsion. I was strongly attracted by the advertising of ASMO Marine with respect to regeneration. The words in their product brochures were strongly encouraging, but they were suspiciously qualitative. However, their U.S. representative (NGC Marine of Wisconsin, currently no longer in business) provided very optimistic curves of expected regeneration performance. That was four years ago, and I have learned a great deal since then. The NGC graphs, showing the potential to generate 10’s of amps at modest boat speeds, do not represent realistic situations. In fact, such current levels are not physically possible. I also know that many customers have purchased systems from ASMO Marine and no one has reported any significant regeneration. ASMO is currently represented in the U.S. by Annapolis Hybrid Marine, but they are not able to back up any of the ASMO claims about regeneration with real data. So the bottom line on regeneration is “buyer beware.”

Let me add a more optimistic note, from my background as a physics professor. It is fundamentally possible to generate power by dragging a propeller through the water. The company Ampair sells a device with a 12-inch prop that can generate up to 100 watts. A 16-inch prop should be capable of generating 1.77 times that power due to its larger area. A similar power level can be estimated from propeller tests that were done at MIT in 1994 by Beth Lurie and Todd Taylor for Practical Sailor. They show that the difference in drag power from the freewheeling (nearly zero torque) situation to the fixed propeller (maximum torque) situation could be about 350 watts for a three-blade 16-inch diameter propeller at five knots. It is this difference that is available for conversion to electricity.

Detailed tests would be required to determine what fraction of this power could be converted to electricity, but my intuition is about 50 percent, or 175 watts. This could be a useful power level for maintaining batteries supplying the house needs on a modest 30-foot sailboat. I want to caution, however, that it is difficult to use the same motor for propulsion and regeneration. Since there is a big difference in the power range necessary for propulsion, and that available for regeneration, the same motor performs both functions effectively. Trevor Lees from the Lynch Motor Company told me they have used their motors in both motoring and generating configurations, but never at the same time. They work very well as generators aboard Hydroptere, the sailing hydrofoil that recently broke the 50-knot speed barrier for a full nautical mile. But if your boat won’t reach those speeds, the power obtained by regeneration will be significantly less! Solomon Marine Propulsion (Ikanos Technologies Inc.) also claims to offer systems which regenerate power, but their website also contains no quantitative performance data. But the site does have a good explanation of how a feathering max-prop can be used for regeneration, which is a bit beyond the scope of this short article.

Should the system include a generator? Yes, unless you are always berthed at a dock with electrical power and day sailing for distances that are not outside your battery range, or, if you are completely comfortable with an engineless boat. Even if regeneration works, it will generate only modest power. My advice would still be to take what you get from the wind as a bonus — it will reduce the generator hours used — but have a generator for back up power if and when nature conspires against you. Our Honda 2000i is inexpensive and reasonably effective, although it will not support simultaneous operation of the chargers for both battery banks when they are each working at full charging current of 15 amps. We plan installation of a permanent marine generator of at least 3 kW.

A little weekend outing makes the case for a generator. We keep Amanda in a very protected marina, but the price we pay is that it is 2.5 nm up a very narrow channel, with a strong tidal current. No sailing is possible in this channel. With a full battery charge, we left the dock recently to go out for a short sail and overnight stay. There was a good wind. We motored down river and used 20 amps for an hour. The weather was improving so we sailed for a couple hours to a nearby anchorage and we decided to stay overnight. Another 10 amp-hours were used for motoring into the anchorage and finding a good location. We also use an average of about 1.5 amps for our electronics, refrigerator, and other minor needs like our LPG valve, water pressure, etc. Overall in this respect, our boat systems are very efficient. However, over a 24-hour period the house use amounted to 36 amp-hours. By the next day we had used around 70 amp-hours, which is about the useful capacity of one battery bank. We expected to sail back to the river entrance and motor back to the marina. However, two things made the situation difficult. There was an absolute flat calm all morning. And the tide was going to be turning against us going up the river in the afternoon. It was Memorial Day and we needed to be back in Boston that evening. We could not sail in the calm, and motoring all the way (about six nm) would not have left enough margin to be sure we could make it up the river, especially against the tide. I have a Honda 2000i generator that we carry for such circumstances and we “motored” home with the generator producing 15 amps to partially offset the motor use of 25 amps at 3.5 knots. A little wind even came up briefly.

Some conclusions. A well-integrated system, including a marine generator and having an electric motor with regeneration of a few hundred watts under sail, could be similar in cost to a conventional diesel engine installation. But such a system would 1) provide better fuel economy and 2) offer significant operational flexibility. The better fuel economy is due to the low cost per kW hour of electricity when charged from shore, and the more efficient operation of the onboard generator, since it can operate at an optimal rpm whenever it is charging the batteries.

The flexibility comes in different ways. For short day sails, you might not need to turn on the generator. Or for long passages, the regeneration current could be enough to continuously supply house needs. If you are careful to keep the batteries well charged, then failure of the generator does not present the crisis situation that failure of a traditional diesel might present, because you would still have battery power for a number of hours of motoring.

Aboard Amanda we now believe it is not possible for our particular motor to function as a generator, but we still believe that systems with modest regeneration are possible — we know it doesn’t violate the laws of physics. For us a permanent installation of a marine generator capable of supplying both battery chargers will be the most practical approach, and hopefully completed this summer.

Bottom line. So far, electric propulsion remains a small niche compared to diesel engines. The companies are small, and many are start-ups with little capital. There are not well-established dealer networks, and repair knowledge is not assimilated into the marinas and boatyards as it is for diesel engines. There are fundamental questions about the practicality of regeneration, and about the amount of power that regeneration can deliver. In summary, I would say the following: if you have the adventurous spirit for which sailors are famous, and that spirit extends to adventures on the cusp of technology, then electric propulsion challenge can be great fun. If not, then I would wait and watch. There will be a lot of progress in the next few years. Particularly attractive are hybrid systems that combine electric and internal combustion engines.

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David E. Moncton teaches physics at MIT and lives in Boston. He keeps his boat Amanda in Yarmouth, Maine.

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