Speed of electric boats

Using electric engines does not limit boat speed. Service speed is only limited by the amount of onboard energy (see Range)


Range of zero emission electric ship depends on the capacity of onboard batteries (see Solar energy), which is limited by the weight of batteries. A 100 kWh LiFePo4 battery, which is currently a safe and cost effective technology, weights about one ton whereas only twenty litres of gasoil are required to provide the same amount of energy.
For a boat, hull drag, and so propeller thrust, increases very quickly with speed. It is so necessary to reduce speed to be able to run all day long without intermediate refill.
In order to limit energy consumption due to battery carrying, we limit battery weight within 15% of ship weight (passenger weight represents 30% of total weight), thus limiting onboard energy to around 100 kWh using NiCd technology and 200 kWh using LiFePo4 technology.
In these conditions, and thanks to energy savings due to lightweight construction, hull and propeller efficiency, our ships can reach an operating range of 200 km (100 Nm) at a 12 km/h (6 knots) boat speed. This allows standard use in river, on lake or coastal water.
Operating speedRange without intermediate battery charge
9 km/h / 4.5 knots 400 km / 200 nm with LiFePo4
12 km/h / 6 knots 200 km / 100 nm with LiFePo4
15 km/h / 7.5 knots 100 km / 50 nm with LiFePo4

Range as a function of operating speed (15m plate-form)

Battery cost and life span

Battery cost represents a large amount of the initial budget of a zero emission ship (from 10% to 20% depending on technology and range). Moreover, batteries have to be replaced periodically.
This battery cost must be seen as an energy cost and can be compared to the fuel cost of a ship fitted with heat engine.
Life span of a battery depends mainly on the amount of energy delivered by the battery all along its life. So we can estimate the cost of energy that it supplies by summing power grid supply cost, battery life span and initial cost, electrical engine efficiency and other less significant aspects. Then, this cost can be compared with the one of a heat engine taking into account fuel cost and engine efficiency. Facts are:
  • Costs of battery energy and fossil fuels are currently very close to each other
  • Cheapest batteries do not lead to an optimized overall cost
That’s the reason why we currently propose LiFePo4 batteries, whose life span is at least five years for a daily use.
It is also obvious that “battery energy” will show an increasing advantage in term of cost compared to fossil energy due to battery cost reduction and due to the unavoidable increase of fuel price.

Lithium batteries

Lithium batteries obviously offer an advantage in term of weight, size and maintenance. A wide range of products, depending on cost and technology, is currently available. Based on a fifteen more year experience, we know that charge incident or deep discharge may happen and that it is absolutely necessary to prevent any risk of fire or explosion.
To match a demanding service and to guaranty passenger security, battery (having a capacity of 100 kWh or more) and its operating management (charge, temperature monitoring,…) must present a great level of quality.
Lithium Iron Phosphate technology (LiFePo4) has been massively adopted by transportation industry because of its advantages in terms of security. We now propose LiFePo4 batteries manufactured by top level companies that can guaranty security and longevity of their products.

Composite materials

The use of FRP (fibreglass reinforced polyester) composite materials in zero emission ship building offers many advantages compared to the use of aluminium and steel.
Very high density of steel increases so much consumption that an acceptable range can only be reached with an unacceptable amount of batteries.
Aluminium is a possible choice but FRP allow a better weight reduction if they are used using vacuum bagging or infusion, to reduce maintenance costs as they are not subject to corrosion and can be used without paint,
FRP have proven their strength in the field of naval applications and are now widely used for rescue and pilot ships.
The mistrust that FRP suffer from some professional sailors is mainly based on the damage that can results from frequent docking. Indeed, FRP build ships, as well as aluminium ones, may suffer more than a steel boat during uncontrolled docking or if hull and peer lack protection. Nevertheless, no damage happens if hull and peer are properly designed ; and FRP can be repaired in a few hours.
Carbon footprint (see Carbon footprint and recycling) arising from hull and structure building using FRP has only a minor impact compared to the one stemming from propulsion emission. The advantage shown in this field by steel ships will be largely balanced by the increase of energy consumption due to the extra weight of steel ships. Carbon footprint difference between aluminium and FRP is so small that it has no real influence on the ship overall carbon footprint.

Carbon footprint and recycling

Analysis of life cycle (including recycling) of the main GHG sources for our 75 pax ship are summarized in the following table :

 BuildingBattery (5 years span life)Electric grid charging
Carbon weight equivalent 25 tons CE 20 to 35 tons CE 3 kg CE / day

GHG emission for a conventional ship using heat engine, based on similar number of passengers and service, is about 120 kg CE per day.

GHG emission per year :

 AltEn Electric zéro emission 75 PAXConventional heat engine
  Building (life span > 20 years) Battery (life span > 5 years) Electric grid charging Fossil fuel
Carbon weight equivalent 1.2 tons CE 2 to 4 tons CE 1 ton CE 40 tons CE

Zero emission electric boat can save dozens of carbon equivalent tons per year of use.

The second most important source of emission is battery but its emission has decreased by the use of lithium technology. Note that industrial batteries are recycled from 80% to 100% whatever is their technology.

Building has a small impact on overall emission.

Solar energy

Photovoltaic cells can provide between 200 Wh/m2 and 1000 Wh/m2 per day depending on their technology (mono or poly crystalline), geographic location and sunshine. For a 75 passenger ferry, the maximum available area is 40 m2, leading to an amount of solar energy production between 8 kWh and 40 kWh, whereas daily consumption for a 80 km service at a 12 km/h boat speed is about 70 kWh.
This shows that, even using the best available technology, area and sunshine, photovoltaic panels could provide more than half of the required energy, but actually they will only provide a small part of daily ship energy consumption. Moreover, battery has to be designed to fit the worst situation.
Note that in the case of a reduced service speed, below 8 km/h, solar energy may cover a large amount of the ship needs, as it is the case for Marseille Ferryboat.

Maintenance and reliability

Maintenance of electrical propulsion is largely reduced compared to the one of heat propulsion. Moreover, propulsion and energy parts have a long life span :

MotorMotor driveBattery chargerBattery
7 years (possible refit at 50% of initial cost) 15 years 14 years 6 years

Life span for a daily 12 hours use.

Reliability of parts fitted on our ships plus the automatic and remote monitoring system allows a very demanding professionnal service and lead to a reduced failure rate :

 CharacteristicsOperating speedh/year/shipd/year/shipAvailability
Passeurs La Rochelle (2) 10 m, 35 PAX, 8t 3-4 nds 4050 h 365 d 99%
Bus de Mer La Rochelle (2) 15 m, 75 PAX, 19t 6-7 nds 1000 h 190 d 96%
Navettes Paris ICADE (3) 15 m, 75 PAX, 19 t 6-7 nds 3500 h 312 d 99%

Service availability rate excluding scheduled maintenance operations

  Hybrid propulsion

Mixing various source of energy is a way to expand the range of application of electric boat. Using high efficiency propulsion systems combined with low drag hulls allows a dramatic reduction of energy consumption and so of associated emission, even in the case where energy is provided by a generator.
We offer three hybrid solutions depending on operations:
  • Range extender: A medium range generator is used in addition with battery in case of specific energy need: flood, strong tidal stream, strong wind, extended day service,… The generator is plugged on battery charger. This system avoids oversized battery pack in case of exceptional use.
  • Serial-hybrid: Electric Propulsion Motors are directly powered by a generator, alternately with battery. The generator power allows sailing at higher speed in roads whereas battery is used to sail without emission at a moderated speed close to harbour and urban zones.
  • Parallel-hybrid: The propeller shaft is moved either by a electric motor, either by a Internal Combustion Engine. The ship can sail at low speed using the electric motor, for example in protected areas.


Selected technology for our electric boats is more elaborated and less spread than the one of conventional ICE ship, the cost (excluding battery) is higher. However, maintenance cost and replacing used part is less than that for a conventional boat.

As already shown (see Battery cost and life span), battery cost all along its life span is close to fuel cost of a conventional boat. Many studies predict that fuel price would increase by 7% per year for the next years. On the opposite, with the spread of land-borne electric vehicles, the price of lithium batteries is steadily decreasing.

Taking into account all those facts shows that the cost of an electric boat is already lower than the one of a conventional ICE boat