This article is about electrically powered boats in general. For the company called "Electric Boat", see General Dynamics Electric Boat.
While most boats on the water today are powered by diesel engines, and sail power and gasoline engines are also popular, it is perfectly feasible to power boats by electricity too.
Passenger solar boat Solifleur, Switzerland 1995
Electric boats were very popular from the 1880s until the 1920s, when the internal combustion engine took dominance. Since the energy crises of the 1970s, interest in this quiet and potentially renewable marine energy source has been increasing steadily again, especially as solar cells became available, for the first time making possible motorboats with an infinite range like sailboats. The first practical solar boat was probably constructed in 1975 in England (see Electrical Review Vol 201 No 7 12 August 1977). With the present state of technology, many[citation needed] believe that the time is right for the increased usage of electric or solar boats.
Components
The main components of the drive system of any electrically powered boat are similar in all cases, and similar to the options available for any electric vehicle.
Charger
Electric energy will have to be obtained for the battery bank from some source.
- Mains charger allows the boat to be charged from a shore-side power point when one is available. This calls into question claims that the boat is \'non-polluting\' and uses \'renewable energy\', but at least it does not directly pollute the water in which it sits as would the use of any petroleum-based motor. Shore-based power stations are subject to much stricter environmental controls than the average marine diesel or outboard motor. By purchasing green electricity it is possible to indirectly operate electric boats with any source of sustainable energy or renewable energy.
- Solar panels can be built into the boat in reasonable areas in the deck, cabin roof or as awnings. Modern solar panels, or photovoltaic arrays, can be flexible enough to fit to slightly curved surfaces and can be ordered in unusual shapes and sizes. It is still true that the heavier, rigid mono-crystalline types are more efficient in terms of energy output per square meter. The efficiency of solar panels rapidly decreases when they are not pointed directly at the sun, so some way of tilting the arrays while under way is very advantageous.
- Towed generators are common on long-distance cruising yachts and can generate a lot of power when travelling under sail. If an electric boat is to have sails as well, and will be used in deep water (deeper than about 15 m or 50 ft), then a towed generator will help build up battery charge while sailing (there is no point in trailing such a generator while under electric propulsion as the extra drag from the generator would waste more electricity than it generates). Some electric power systems use the free-wheeling drive propeller to generate charge through the drive motor when sailing, but this system, including the design of the propeller and any gearing, cannot be optimised for both functions. It may be better locked off or feathered while the towed generator\'s more efficient turbine gathers energy.
- Wind turbines are also common on cruising yachts and can be very well suited to electric boats. There are safety considerations regarding the spinning blades, especially in a strong wind. It is important that the boat is big enough that the turbine can be mounted out of the way of all passengers and crew under all circumstances, including when alongside and when coming alongside a dock, a bank or a pier. It is also important that the boat is big enough and stable enough that the top hamper created by the turbine on its pole or mast does not compromise its stability in a strong wind or gale. Large enough wind generators could produce a completely wind-powered electric boat. No such boats are yet known although a few mechanical wind turbine powered boats exist.
- If the boat is to have an internal combustion engine anyway, then its alternator will provide significant charge when it is running. Two schemes are in use: the combustion engine and the electric motor coupled to the drive in parallel, or a dedicated generator with the combustion engine charging the storage batteries.
In all cases, a charge regulator will be needed. This is to ensure that the batteries are charged at the maximum rate that they safely can stand when the power is available. It must also ensure that they are not overcharged when nearing full charge and not overheated when they are discharged and a great deal of charge current becomes available.
Battery bank
There have been significant technical advantages in battery technology in recent years, and more is to be expected in the future.
- Lead-acid batteries are the most viable option at the moment. Deep-cycle, \'traction\' batteries are the obvious choice. There is no denying that they are heavy and bulky, but not much more so than the diesel engine, tanks and fittings that they may be replacing. They need to be securely mounted, low down and centrally situated in the boat. It is essential that they cannot move around under any circumstances. Care must be taken that there is no risk of spilled, strong acid in the event of a capsize as this could be dangerous or even fatal. At the same time, venting of explosive hydrogen and oxygen gases is also necessary. Finally, typical lead-acid batteries must be kept topped-up with distilled water.
- Valve-regulated lead-acid (VRLA) batteries, usually known as sealed lead-acid, Gel, or AGM batteries, minimize the risk of spillage, and gases are only vented when the batteries are overcharged. These batteries require minimal maintenance, as they cannot and usually do not need to be refilled with water.
- Nickel metal hydride and other hi-tech, solid-state batteries are becoming available, but are still expensive. These are the kind of batteries currently common in rechargeable hand tools like drills and screwdrivers, but they are relatively new to this environment. They require specialised charge controllers.
- Fuel cells are going to provide significant advantages in the years to come, and one day heavy lead-acid batteries will seem \'pre-historic\' by comparison. Today (2007) however they are still expensive and require specialist equipment and knowledge, making them all but impractical for any but their dedicated enthusiasts.
The size of the battery bank will determine the range of the boat under electric power alone. The speed that the boat is motored at will also affect this - a lower speed can make a big difference to the energy required to move a hull. Other factors that affect range include sea-state, windage and any charge that can be reclaimed while under way, for example by solar panels in full sun. A wind tubine in a good following wind will help, and motor-sailing in any wind could do so even more.
Speed controller
To make the boat usable and maneuverable, a simple-to-operate forward/stop/backwards speed controller is needed. This must be efficient - i.e. it must not get hot and waste energy at any speed - and it must be able to stand the full current that could conceivably flow under any full-load condition.
Electric motor
A wide variety of electric motor technology is in use. Traditionally field-wound DC motors were and still are used. Today many boats use lightweight permanent magnet DC motors. The advantage of both types is that while the speed can be controlled electronically, this is not a requirement. Some boats use AC motors or permanent magnet brushless motors. The advantages of these are the lack of commutators which can wear out or fail and the often lower currents allowing thinner cables; the disadvantages are the total reliance on the required electronic controllers and the usually high voltages which require a high standard of insulation.
Drive train
Traditional boats use an inboard motor powering a propeller though a propeller shaft complete with bearings and seals. Often a gear reduction is incorporated in order to be able to use a larger more efficient propeller and/or a smaller motor. This can be a traditional gear box, coaxial planetary gears or a transmission with belts or chains. Because of the inevitable loss associated with gearing, many drives do without and use especially slow high-torque motors. The motors are often encapsulated in a pod with the propeller and fixed outside the hull (saildrive) or on an outboard fixture (outboard motor).
Types
There are as many types of electric boat as there are boats with any other method of propulsion, but some types are significant for various reasons.
- Historical and restored electric boats exist and are often important projects for those involved. See the Mary Gordon Electric Boat for example.
- Canal, river and lake boats. Electric boats, with their limited range and performance, have tended to be used mostly on inland waterways, where their complete lack of local pollution is a significant advantage. Electric drives are also available as auxiliary propulsion for sailing yachts on inland waters.
- Electric outboards and trolling motors have been available for some years at prices from about $100 (US) up to several thousand. These still require external batteries in the bottom of the boat, but are otherwise practical one-piece items. Most available electric outboards are not as efficient as custom drives, but are optimised for their intended use, e.g. as an ideal drive system for inland waterway fishermen. They are quiet and they do not pollute the water or the air, so they do not scare away or harm fish and birds. Combined with modern waterproof battery packs, electric outboards are also ideal for yacht tenders and other inshore pleasure boats.
- Cruising yachts usually have an auxiliary engine, and there are two main uses for this engine. One is to power ahead or motor-sail at sea when the wind is light or from the wrong direction. The other is to provide the last 10 minutes or so of propulsion when the boat is in port and needs to be manoeuvred into a tight berth in a crowded and confined marina or harbour. Electric propulsion is ideally suited to both cases, but not for prolonged cruising at full power. Regarding the first case, the power required to motor slowly is so small that many hours of operation are possible even with a small battery capacity. Regarding the second case, electric drives can be so finely controlled that it is easier to perform difficult manoeuvers than with combustion engine drives. Additionally electric motors can be used at several times their rated power for short periods, so that even quite small motors can provide adequate thrust when required.
- Diesel-electric. There is a third potential use for the trusty diesel auxiliary and that is to charge the batteries, when they suddenly start to wane far from shore in the middle of the night, or at anchor after some days of living aboard. In this case, where this kind of use is to be expected, perhaps on a larger cruising yacht, then a combined diesel-electric solution may be designed from the start. The diesel engine is installed with the prime purpose of charging the battery banks, and the electric motor with that of propulsion. There is some reduction in efficiency if motoring for long distances as the diesel\'s power is converted first to electricity and then to motion, but there is a balancing saving every time the wind-, sail- and solar-charged batteries are used for manoeuvring and for short journeys without starting the diesel. There is the flexibility of being able to start the diesel as a pure generator whenever required. The main losses are in weight and installation cost, but on the bigger cruising boats that sit at anchor running large diesels for hours every day, these may not be too big an issue, compared to the savings that can be made at other times.
- Solar powered. A boat propelled by direct solar energy, almost always converted to electricity by solar cells, temporarily stored in accumulator batteries, and used to drive a propeller through an electric motor. Power levels are usually on the order of a few hundred watts to a few kilowatts. Solar powered boats started to become known around 1985 and in 1995 the first commercial solar passenger boats appeared.[1] Solar powered boats have been used successfully at sea. The first crossing of the Atlantic Ocean was achieved in the winter of 2006/2007 by the solar catamaran sun21.[2]
Of course, all the component parts of any boat have to be manufactured and will eventually have to be disposed of. Some pollution and use of other energy sources are inevitable during these stages of the boat\'s life and electric boats are no exception. The benefits to the global environment that are achieved by the use of electric propulsion are manifested during the working life of the boat, which can be many years. These benefits are also most directly felt in the sensitive and very beautiful environments in which such a boat is used, by a careful and caring owner and crew.
Example of a modern production electric boat
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See also
External links
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia
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