Electric Boat is sometimes used as a colloquial abbreviation for the US Electric Boat Corporation. This article is about electric boats themselves, rather than this corporation.
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 boat engines by electricity too.
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 silent, non-polluting and potentially renewable energy source has been increasing steadily again. With the present state of technology so developed, many believe that the time is right for electric boats to become popular again.
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.
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. The use of this may call into question claims that the electric boat is 'non-polluting' and uses 'renewable energy', but only among the more pedantic purists. The boat is not directly polluting the water in which it is sitting, as are most diesel- and gasoline-powered boats. The power stations that generate the electricity are subject to strict environmental controls, and an increasing number of these in the near future will be using renewable and non-polluting energy sources themselves, see Kyoto Protocol.
- 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-crystaline 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 - we are not trying to create a perpetual motion machine! The extra drag from the generator will 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.
- If the boat is to have an internal combustion engine anyway, then its alternator will of course provide significant charge when it is running. This does rather defeat the original purpose, however. The weight saving that we would expect by not having this engine and all its associated tanks, pipework and other fittings would help to add to the efficiency that electric propulsion needs.
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.
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 ther 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.
- 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 (2005) however they are very 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.
To make the boat usable and manoeverable, 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 technology is also complex and changing. Permanent magnet, brushless motors are considered very suitable by some specialists.
Depending on the size of the boat and the choice of electric motor, a standard propeller shaft, bearings and propellor may be available. In some cases some reduction gearing may be required, but from the point of view of efficiency, wear-and-tear and routine maintenance this should be avoided if at all possible, perhaps by choosing a different propeller.
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.
- River and lake boats. Electric boats, with their historically limited range and poor performance against a strong headwind or current, have tended to be used on inland waterways. In this environment, their complete lack of local pollution is also a significant advantage.
- 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 by being a one-piece item apart from that, the manufacturers have been able to optimise the combination of the motor, the speed controller and the drive chain including the all-important propeller. Electric outboards have for some years provided an ideal drive systrem for inland waterway fishermen, being quiet and pollution-free so as not to scare away or harm the fish and other wildlife. As technologies improve, they should bring these benefits to many other dinghy users too, such as for yacht tenders and other inshore pleasure boats.
- Cruising yachts usually have an auxillary enine, 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 manoevered into a tight berth in a crowded and confined marina or harbour. Electric propulsion is ideally suited to the second case. The first case provides many - especially beginning yachtsmen and women - with a cause for anxiety, as a powerful diesel auxillary has often been known to get a boat into harbour when sail power alone, or sail with limited help from an electric system, would miss the tide, the hours of daylight, or a rising gale. In fact with good seamanship, good passage-planning, sailing skills, the ability and willingness to keep to the deep sea or to anchor off in unfavourable conditions, none of this need be a problem, except perhaps for those in a hurry to get home, maybe for work in the morning or to feed the cats.
Diesel-electric. There is a third potential use for the trusty diesel auxillary 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 manoevering 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.