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Catamaran Electric Propulsion

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There has been growing interest in electric propulsion for sailboats. The lure of clean, silent electric motors capable of replacing the diesel auxiliary inboard(s) seems to be especially strong for multihull sailors, but then they have always been able to think outside the box and are, in general, more open to new concepts than their single-hull counterparts. Despite their forward thinking and willingness to embrace new technologies, however, they ultimately want equipment and systems that work well. And they are, in general, quite willing to spend some extra money on elegant solutions on board, but they also have an uncanny ability to sense when the cost is inappropriate. Electric propulsion systems available today can work well and their cost can be within the realm of reason, but they are not for everyone. Determining if they are right for you and your boat requires a little investigation.

The idea of electric propulsion for boats is not new. There have been electric boat companies in business for decades and plenty of enterprising sailors have managed to create their own home-brewed systems, with, as you might guess, varying degrees of success. I can remember meeting a young electric propulsion visionary in a harbor in Florida back in 1983. He had replaced his aging diesel engine with a large 24VDC electric motor. His system was fairly crude by today's standards, and his motoring time was seriously limited due to his reliance on solar panels as the only method of recharging the batteries. Yet it's important to note that he was doing it; we all watched in wonder as he motored silently in and out of the harbor. More importantly, he completely understood not only the benefits but also the limitations of his particular system, and he was willing to adjust his sailing and motoring habits to accommodate them.

This is an important first step in achieving a successful electric propulsion system, getting into the right frame of mind and establishing a realistic frame of reference. You must first fully understanding what the benefits of an electric propulsion system are, for in pursuing the design, purchase, and installation of this system you will undoubtedly expend more time, effort and money than with a conventional diesel inboard set-up, and that will typically be true whether you are purchasing motors for a new boat or replacing the diesel engines in a previously owned one.

Eventually (and the day is not far away) electric propulsion systems will be as 'conventional' as diesel engines are now, and it will be every bit as easy for consumers to purchase and have these systems installed. Why do I say that the day is not far away? As I write this article a leading manufacturer of production catamarans is considering offering Solomon Technologies electric propulsion systems as a standard factory option. This is a quantum leap in the industry's frame of mind, and it will most certainly change our frame of reference before too long. This simultaneous leap in faith and technology is in many ways similar to what occurred in the automobile industry when hybrid gas/electric vehicles were introduced.

The analogy with hybrid vehicles is appropriate for electric propulsion systems for boats, since most multihull sailors who embrace this technology will end up with some type of a hybrid diesel/electric system, employing a gen-set of some size to provide house electric loads and adequate power to run the engines directly when needed. To help understand why this is so, let's look at two ideal scenarios for electric propulsion systems. We can then fine tune our analysis in part two of this article by looking beyond the ideal to help individual sailors determine if electric propulsion is a good match for their situation.

In my mind the two ideal scenarios for electric propulsion are at opposite ends of the spectrum in terms of engine use.

In the first scenario we have a modern multihull owner (not necessarily the owner of a modern multihull) who is a true sailor who wants to minimize dependence on an engine, and who is concerned about the environment and wants to address this concern as much as possible when outfitting their boat. Renewable power and clean, quiet operation of the boat play heavily in this sailor's thoughts. This sailor, like the sailor I met in Florida in 1983, is willing to adjust his lifestyle (at least slightly) to make electric propulsion work.

The sailor described above can get by with an electric propulsion system of modest size and complexity. A gen-set is not a necessity, as long as house loads don't require it and the electric motor use is balanced with the ability to recharge the batteries, either through 'regeneration' (using the electric motor(s) to create charging power when the boat is under sail), solar power (individual solar panels must be wired in series to create the necessary voltage of the propulsion system), or a battery charger used when dockside.

In the second scenario we have a modern multihull owner who might imagine sailing as their primary method of propulsion, but who in reality is quite dependent on their auxiliary engines to get them from A to B. In addition, their house loads are of sufficient scope and sophistication to make a gen-set an essential item on board.

For this sailor an electric propulsion system works well, not because of their ability to limit their needs but because their needs require the use of a gen-set. Once a gen-set is part of the program, a truly hybrid power system can be designed, one that takes care of all propulsion and house loads when needed. Of course the gen-set must be of a sufficient size to power the electric motor(s) directly , and the electric motors must be of sufficient size to provide comfortable and safe operation in all conditions. But it is a reality that a properly designed and installed electric propulsion system incorporating a gen-set can be both efficient and reliable, and these systems are available today.

In part two of this article we'll look beyond these two extreme scenarios and take a closer look at the components and peculiarities of an electric propulsion system.


 

Part II - The Major Components

In Part I of this article we reviewed electric propulsion and briefly outlined two systems at opposite ends of the spectrum in terms of complexity and how the boats were used. In this article we'll review the various components needed for a successful multihull installation. For illustrative purposes, we'll use the Solomon Technologies system.

Unlike the primarily mechanical components of a diesel inboard propulsion system, the components in an electric propulsion system are almost exclusively electrical devices for making, distributing, storing and controlling electrical power. In addition, with electric propulsion the electrical drive system and the 'house' electrical power system are typically integrated to a large degree, since operationally the drive motors are treated as large electrical loads.

The main component in an electrical propulsion system is the electric drive motor itself. For trimarans only one motor is required, whereas catamarans typically have two motors (one each hull) for best maneuverability in port. On smaller cats a singe motor aft midships with a catamaran drive leg is also an option. Drive motors in an electric propulsion system are much smaller and lighter in weight than a diesel inboard engine, but they get mounted in much the same way. The motors are located fore and aft for convenience and best weight distribution, and positioned on a standard set of engine mounts down low in or near the bilge and at an appropriate angle from horizontal to most easily accommodate the prop shaft. The dramatic difference with electric drive motors over diesel engines is how clean, quiet, and free of fumes the 'engine compartment' is, especially when the motors are located under an aft cabin berth.

The shaft and stuffing box or prop seal are all as per standard inboard diesel installations. The props on an electric propulsion system are similar to models used for diesel inboards, but the prop diameter may be larger and the blade pitch altered to suit the motor and application, in both a drive mode and regenerative mode. In the Solomon Technologies system the motors can serve as electrical generators when the boat is under sail. Even when motor-sailing the motors can discern between the need to drive the boat forward and periodic surges forward due to sailing momentum. This phenomenon is similar to the regenerative braking used on high-tech land vehicles, where energy developed from slowing down or stopping the vehicle is converted into drive power for normal operation. Some folding prop models can be used successfully in electric propulsion systems, but the increased drive efficiency of a fixed prop combined with the use of the rotating prop for electrical power regeneration under sail make the expense of a folding prop questionable on a multihull.

Batteries to store electrical power are next on the component list. Battery bank capacity and configuration depend on the type of system used, the total weight carrying capacity of the boat, and the motoring time desired strictly from battery power. The Solomon system operates at 144VDC, which requires a bank of 12 batteries wired in series. The individual batteries in the bank can be Group 27, Group 31 or 4D case size. While the battery bank for an electric propulsion system represents a substantial amount of weight that must be wisely positioned on the boat, the total system weight is usually equivalent to or even less than the total weight of diesel engines and drives, start batteries, and fuel and fuel tanks. On a multihull the battery bank can be split into two and mounted one each hull, as long as the cables connecting the two banks are sized to carry the full system amperage draw.

The next major component to consider is a genset. This component is actually an option, although one that most sailors choose to incorporate into their electric propulsion systems. First let's review how a genset is used in the system, then we can look at sizing considerations. With a genset onboard an electric propulsion system behaves more like a hybrid electric-diesel propulsion system. The genset provides electrical power to run certain high-amp loads directly and power to be stored in the battery bank for later use. The genset can be located anywhere that is convenient and allows for proper weight distribution. On many larger cats the genset is mounted midships in a forward locker. In a Solomon Technologies system the genset is designed for 144VDC output, which increases the efficiency of power going to the batteries or to the motors directly. House loads are supplied from the battery bank through a 144VDC-to-12(or 24)VDC converter (for all DC loads) and a 144VDC-to-110(or 220)VAC inverter (for all AC loads).

A small genset can help off-set the power draw of the electric drives when motoring, but if it is too small it won't be able to run the motor(s) directly. This scenario is fine for limited use of the motor(s), but is unsuitable for long-term motoring since the electrical draw will outstrip the battery bank's ability to supply power. It's important to understand that by choosing a genset that is too small to run the motor(s) directly, or by choosing not to install a genset at all, you are limiting your motor run time. This can still be a practical compromise for some applications, however, with substantial savings in cost and weight.

In part three of this article we'll look at the balance of system components of an electric propulsion system, including system distribution, control and monitoring equipment.


 

Part III - Balance of System Components

In Part 2 of this article we reviewed the major components of an electric propulsion system: the drive motors, the shaft and prop, the batteries, and a gen-set (optional but usually recommended). In this article we'll review the balance of system components, including the
'house' power system and the distribution, control and monitoring equipment. For illustrative purposes, we'll again use the Solomon Technologies system.

Motor Control, Throttle, Key Switch

For each electric drive motor installed on a multihull there is a motor controller, in essence a black box that allows the appropriate levels of power to flow to and from the drive motor. Electricity used to power the motor passes in one direction, and regenerative power created from the motor when sailing (or motor sailing) passes in the other direction on its way to the battery bank. There is also a throttle and key switch for each drive motor, similar to those used for diesel engine installation.

Power Distribution Center

Every modern electrical power system should have an enclosed power distribution center that contains the heavy duty circuit components and connections. Solomon Technologies currently has a separate center (they call it a SPMD: Safety Power Management Device) for each drive motor. These centers provide a distribution point for wiring to and from the drive motors, battery bank, throttles, key switches, and meters. A Master Breaker Enclosure houses the circuit protection for the system, and integrates the propulsion system with the house DC and AC electrical power systems.

Inverters & Chargers

With 144VDC as the operating system voltage for propulsion, various inverters and chargers are needed to provide electricity suitable for house loads and to recharge the propulsion battery bank and the house battery bank. Of primary importance are devices for recharging the propulsion battery bank from shore power (remember that in most Solomon systems a 144VDC gen-set provides a high level of charge to the propulsion batteries when away from a source of shore power), and for providing AC power from the propulsion battery bank. These can be stand alone devices in the form of a stand alone charger (110/220VAC to 144VDC) and a stand alone inverter (144VDC to 110/220VAC), or as a combi unit that performs both functions.

For DC house loads a 'Cross Charger' is used. This device converts 144VDC (from the main battery bank, shore power, or from the gen-set directly) to either 12 or 24 VDC to power DC house loads using a small intermediary house battery bank.

Renewable Power Equipment

If you do it properly, solar, wind and water chargers can be integrated into an electric propulsion system. They help cut down on gen-set run time and allow you to conserve the power in the main battery bank for propulsion. Essentially you must maintain a separate 12 or 24 VDC system that is supplied by renewable chargers. At one time it seemed like a good idea to set up a bank of solar panels (10-12 modest size panels) in series to charge the propulsion bank directly, but there are drawbacks: installation is more costly and shading can greatly affect the output voltage from the solar array. I realized that it's much better to use solar, wind and water power to charge a house bank, which in turn supplies house loads at 12 or 24 volts. The size of this bank and how much the cross charger is utilized depends on both the size of the DC house load and how much renewable power is available; the more renewable power produced, the more house battery capacity is required and the less need there will be for the cross charger. Some sailors choose to eliminate the cross charger altogether and rely solely on renewable chargers and shore power for DC house loads.

Monitoring Equipment

Finally, single function meters and multi-function system monitors are used to make electricity visible, both for the propulsion system and the house power system. Monitoring the status of the propulsion bank and being able to track the current consumed or supplied by the drive motors is essential, as is monitoring the status of the house bank and tracking the charging and load current. Up until now E-Meters have been used for both purposes; the meter for the propulsion system uses a prescaler to make it appropriate for monitoring 144VDC. In future specially designed monitors will provide information about all aspects of the propulsion and house power systems.

I hope this three-part article has given you food for thought and some criteria for determining if this type of system is right for you and your multihull.

Independent Energy GuideKevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor's Multihull Guide. Sailors Multihull Guide

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