Electric boat

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 has to be obtained for the battery bank from some source like the sun.

• A mains charger allows the boat to be charged from shore-side power when available. 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 operate electric boats using sustainable or renewable energy. For large vessels, an onshore battery may be necessary to provide more short-term power than the grid can supply.
• Solar panels can be built into the boat in reasonable areas in the deck, cabin roof or as awnings. Some solar panels, or photovoltaic arrays, can be flexible enough to fit to slightly curved surfaces and can be ordered in unusual shapes and sizes. Nonetheless, 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 has sails as well, and will be used in deep water (deeper than about 15 m or 50 ft), then a towed generator can 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 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 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.
• In hybrid electric boats, if a boat has 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 are both coupled to the drive (parallel hybrid), or the combustion engine drives a generator only for charging the storage batteries (series hybrid).

In all cases, a charge regulator is needed. This ensures that the batteries are charged at their maximum safe rate when power is available, without overheating or internal damage, and that they are not overcharged when nearing full charge.

An alternative to charging is changing batteries while in port. It offers the benefit of removing the need to wait for the recharging to complete before sailing. This approach has the potential to allow ships and ferries with tight schedules to be electrified as charging can be done in port with no time limitations.

Battery bank

There have been significant technical advances in battery technology in recent years, and more are to be expected in the future.

• Lead–acid batteries were still the most viable option until the advent of larger, lithium-ion batteries mass-produced for electric cars from approximately 2012 onwards. Deep-cycle, 'traction' batteries are the obvious choice. They are heavy and bulky, but not much more so than the diesel engine, tanks and fittings that they may replace. 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 the strong acid being spilled in the event of a capsize as this could be very dangerous. Venting of explosive hydrogen and oxygen gases is also necessary. Typical lead–acid batteries must be kept topped-up with distilled water.
• Valve-regulated lead–acid batteries (VRLA), 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, lithium-ion and other battery types 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 different charge controllers to those that suit lead–acid types.
• Lithium-ion in this case usually means lithium iron phosphate batteries, which although are heavier than other lithium-ion, is safer for marine application. They are expensive but in applications which need reliability and ruggedness like ferries which run most of the day (10–12 hours/day) this is the best option. It has a much longer life – 5 to 7 years life-cycle.
• Fuel cells or flow batteries may provide significant advantages in years to come. Today (2017) however they are still expensive and require specialist equipment and knowledge.

Among the various battery chemistry the choice between a fast charging (LTO, NMC etc.) versus slow charging (LFP) is decided by economic analysis considering Capital expenditure (CAPEX), Operating expense (OPEX), Total cost of Ownership (TCO). It is observed that for higher energy need because of high speed or large weight with intermitted charging is an area where fast charging batteries become more economical.

The size of the battery bank determines the range of the boat under electric power. The speed at which the boat is motored also affects range – a lower speed can make a big difference to the energy required to move a hull. Other factors that affect range include sea-state, currents, windage and any charge that can be reclaimed while under way, for example by solar panels in full sun. A wind turbine in a good wind will help, and motor-sailing in any wind could do so even more.

Speed controller

To make the boat usable and manoeuvrable, 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. One of the most common types of speed controllers uses pulse-width modulation (PWM). PWM controllers send high frequency pulses of power to the motor(s). As more power is needed the pulses become longer in duration.

Electric motor

A wide variety of electric motor technologies are in use. Traditional 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 through 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. 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 eliminate it by using slow high-torque motors. The electric motor can be encapsulated into 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, such as the Mary Gordon Electric Boat, exist and are often important projects for those involved.
• Range anxiety is a common concern for those considering electric propulsion on a boat. In 2018, the crew of Rigging Doctor on board Wisdom crossed the Atlantic Ocean with an electric motor.
• 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 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. 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, birds and other wildlife. Combined with modern waterproof battery packs, electric outboards are also ideal for yacht tenders and other inshore pleasure boats.
• Electric personal watercraft have been invented but yet to be fully commercialized. These watercraft use a high powered internal battery systems that are waterproofed and propel the vessel at high speeds using a jet drive propulsion. The most first successful project of electric personal watercraft is by United States Company ELAQUA Marine.
• Cruising yachts usually have an auxiliary engine, and there are two main uses for it: 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 not suitable for prolonged cruising at full power although the power required to motor slowly in light airs and calm seas is small. Regarding the second case, electric drives are ideally suited as they can be finely controlled and can provide substantial power for short periods of time.
• Commercial ferries:
  • Norway's first battery–electric ferry is MV Ampere, with capacity for 120 cars and 12 trucks. As of November 2016, it has operated for 106,000 km. Its battery holds 1 MWh of energy, but the 9-minute charge time is sometimes not enough, and more battery capacity is to be installed. Norway has scheduled several other electric ferry projects. Based on operational data, Siemens concludes in a life cycle analysis that 61 of Norway's 112 diesel ferry routes could be replaced by electric ferries with a payback time of 5 years. The analysis includes auxiliary costs such as chargers, grid, and so on.
  • In Finland Föri, the historic Turku city ferry across the Aura River to Abo, was converted to all-electric propulsion in April 2017. The vessel was introduced as a wood-burning steam ferry in 1904, converted to diesel operation in 1955 and now provides a continuous daily service from 0615 to late evening for foot and cycle passengers on battery power. Charging takes place at night.
  • Other projects are considered in Canada, Sweden and Denmark.
  • India's First Solar Ferry, a 75-passenger boat, that is powered by sun and grid charging with lithium batteries, began service in 2017. Based on the predictions of consumption the payback time is 3 years.
  • Some ferries can charge their onboard batteries while docked by using a pantograph.
  • On the other hand, ferries can include, sometimes free, charging points for the passengers' transported electric bicycle, electric motorcycles and electric cars.
• Diesel–electric hybrid: There is a third potential use for a 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 may sit at anchor running large diesels for hours every day, these are not too big an issue, compared to the savings that can be made at other times. An example is the fishing boat Selfa El-Max 1099, with 135 kWh battery and 80 kW diesel generator. An LNG-powered supply vessel started operation in 2016 with a 653 kWh/1600 kW battery acting as spinning reserve during dynamic positioning, saving 15-30% fuel.
• Solar powered: A boat propelled by direct solar energy is a marine solar vehicle. The available sunlight is 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. 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. (see also List of solar-powered boats)

Wired electric boats

Trolley boats are a special category of electric boats are the vessels receiving their electrical power by wire. This may involve overhead wires, where one or two wires are fixed over the water and the boat can make contact with them to draw electric current, or a waterproof tether cable may be used to connect the boat to shore. In case of a single overhead wire the electrical circuit has to be closed by the water itself, giving rise to a larger resistance and corrosion of the electrodes. In case of two wires no electric current has to be sent through the water, but the twin wires, which cause a short-circuit whenever they come into contact with each other, complicate the construction.

Naturally the boat has to stay close to the wire, or its tether point, and therefore it is limited in its manoeuvrability. For ferries and on narrow canals this is no problem. The Straussee Ferry in Strausberg, Germany is an example. It crosses a lake along a 370 m trajectory and is powered by 170 V from a single overhead wire. The Kastellet ferry crosses a 200 metres (660 ft) wide shipping channel in Sweden, using a submergible tethered supply cable which is lowered to the sea-bed when the ferry is docked at the opposite terminal to its tethering point.

In the Mauvages tunnel on the Marne-Rhine Canal a bipolar overhead line provides 600 V DC to an electrical tug, pulling itself and several ships through the 4877 m tunnel along a submerged chain. This prevents the buildup of diesel exhaust fumes in the tunnel. Another example was the experimental electrical tug Teltow on the Kleinmachnower See, 17 km south-west of Berlin. It was used from 1903 until 1910 and had current collection poles based on those used by trolley buses.

Pollution and embodied energy

A 2016 life-cycle study in Norway states that electric ferries and hybrid offshore supply ships compensate for the environmental effects of producing lithium-ion batteries in less than 2 months.

Solar ships

The first passenger solar vessels started to appear in Switzerland in 1995 with the Solifleur (pictured above), which was also the first solar vessel to feed more energy into the electricity grid than it consumed, on a yearly average, via a grid connection when docked.

In 2010, the Tûranor PlanetSolar, a 35-metre long, 26-metre wide catamaran yacht powered by 537 square metres of solar panels, was unveiled. On 4 May 2012 it completed a 60,023 kilometres (37,297 mi) circumnavigation of the Earth in Monaco after 585 days and visiting 28 different countries, without using any fossil fuel. It is so far the largest solar-powered boat ever built.

India's first solar ferry – the Aditya – a 75-passenger boat fully powered by sun, is under construction. It is expected to be completed by the middle of 2016.

Japan's biggest shipping line Nippon Yusen and Nippon Oil Corporation said solar panels capable of generating 40 kilowatts of electricity would be placed on top of a 60,000 tonne car carrier ship to be used by Toyota.

The Monaco yacht company Wally has announced a "gigayacht" designed for billionaires torn between buying a mansion and a superyacht. The Why 58 x 38 is designed to have an autonomous cruising range of 12,000 miles at 12 knots by means of 900m² of solar panels which generate 150 kW to assist the diesel–electric motors and optional Skysails.

List of battery–electric ships

Solar electric catamaran vessel to carry at least 50 passengers.