Electric outboard motor
Electric outboard motors are an option for boat propulsion. Most electric outboard motors have 0.5 to 4 kW direct current (DC) electric motors, operated at 12 to 60 volts DC. These systems have the propeller fixed directly to the motor, which is mounted in the lower unit under water – see trolling motor. This setup limits the power output. Recently developed outboard motors are powered with an alternating current (AC) or DC electric motor in the power head like a conventional petrol engine. With this setup, a motor can produce 10 kW output or more and is able to replace a petrol engine of 15 HP or more. The advantage of the induction or asynchronous motor is the power transfer to the rotor by means of electromagnetic induction. As these engines do not use permanent magnets, they require less maintenance and develop more torque at lower RPM.
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.
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.
- "Calculator for electric boat motors". Retrieved 2010-11-11.