1. History

The first true electric outboard was invented in 1973 by Morton Ray, founder of Ray Electric Outboards.^[1] This unit consists of a brushed DC motor in the powerhead, much like a conventional petrol engine. Production of these motors began in 1974, and approximately 99% of all motors sold are still in service.^[2] Ray Electric Outboards currently produces electric outboard motors that range from 135 lbs thrust to 270 lbs thrust. They also manufacture boats to accommodate their units. As the desire for clean technology increased and as battery technology developed, new manufacturers emerged. In 2005, German manufacturer Torqeedo started selling portable outboard units in Europe. In 2007 they started selling in the United States. They currently have outboards ranging from 1 hp to 80 hp. Their 80 hp outboard has the motor contained in the powerhead, much like the Ray Electric outboard. With this setup, the electronics and motor are not immersed like with trolling motors that use an immersed motor directly coupled to the propeller.[1] Another advantage of this setup is that gear reductions allow the motor to operate at higher specific speeds increasing energy density while the propeller can operate at lower speeds for efficiency. Motivated by the pollution of petrol engines, Joe Grez set out to build an electric outboard in 2008.^[3] He then founded PropEle Electric motors, Inc., currently the producers of the lightest electric outboards available at 14.4bs. This unit does not have the motor immersed in water, similar to the Torqueedo Deep Blue and Ray Electric outboard.

2. Power Supply

The advantages of electric boat propulsion systems are the low maintenance costs, the limited noise and emission-free operation. The disadvantage is the limited range due to the weight and size of the batteries. There are several battery types in use today with various power-to-weight ratios:

• Lead-acid batteries have low specific energy of 33 to 42 Wh/kg and when quickly discharged, loses 40% of capacity when discharged fully within 1 hour, but they are very cheap.
• Nickel-cadmium batteries have specific energy of 40 to 60 Wh/kg, but are rarely used today due to environmental concerns and strong memory effect.
• Nickel-metal hydride batteries have specific energy of 60 to 120 Wh/kg, contain no cadmium or mercury, have relatively low self-discharge and almost no memory effect. They provide compromise between cost and performances.
• Lithium polymer, Lithium-ion and other newer battery technologies have specific energy of 100 to 265 (or even more) Wh/kg, but at much higher cost per stored Wh.

Charging the batteries with a photovoltaic solar system, wind turbines and towed generators (when traveling under sails or anchored in strong current) can make the system independent of any external energy source. For longer trips, the system can have a range-extending gasoline or diesel generator which recharges the batteries.

3. Costs

Electric outboard motors cost two to three times the cost of a four-stroke petrol engine, but operation costs are lower. A 1-kilowatt electric outboard motor, including a lithium battery, can be purchased from US$1800.00 (2016) which is already competitive to a 3 HP petrol engine. Electric motors are usually much more efficient in power-to-weight ratios. Generally 17.0 pounds of thrust equals 1 HP. However, one requires at least twice as much power to compensate for added weight.^[4]

4. Models

In the following section an overview and comparison of electric outboard motors.

To compare with petrol engines: 1 HP = 0.745 kW and 1 kW = 1.341 HP.

4.1. Ray Electric Outboard^[5]

4.2. Electric Paddle^[6]

4.3. Torqeedo^[7]

4.4. ePropulsion^[8]

4.5. AquaWatt^[9]

4.6. Elco^[10]

4.7. Pure Watercraft^[11]

4.8. KARVIN ^[12]