Electric Drives - Propulsion of the Future
by Richard Kastelein
In my opinion, electric powered boats have definite advantages. They are environmentally friendly, and very quiet; they run with only a whisper of sound. They are reef friendly, quiet in harbours, are cheaper to run and emit no pollutants.
So why are we still using fossil fuels and Marine diesel engines to provide propulsion for ocean going vessels?
Consider the typical internal-combustion engine. From the time a charge of fuel ignites in a cylinder, it has to push pistons, turn a crankshaft, turn a camshaft, open valves, pump water, pump oil, turn an alternator, and submit to reduction from a transmission to step the engine's thousands of revolutions down to something a propeller can use. By the time that's done, the engine's efficiency is somewhere below 25 percent. Also, diesel engines are rated at their maximum rpm - and on sailboats are rarely operated at that speed.
By contrast the the efficiency of Solomon Technologies
' electric motor is a percentage in the low 90s. Here's how it works: When the system is switched on, DC current from the batteries enters an electronic controller, which produces expanding and contracting magnetic fields in the motor's stator windings. These magnetic fields attract and repel the fields from three permanent magnets, made from neo-dymium iron boron, that are attached to the rotor.
The controller electronically modulates the pulse width to increase or decrease speed. At 13 inches wide, the motor provides ample contact with the shaft to produce high torque at low rpm, enough for the motor to turn particularly large propellers. Fixed three-bladed 18/18 (diameter/pitch, in inches) propellers are typical in many of STI's installations. From the flowing electrons to the turning prop, the shaft passes through only two bearings and a stern gland - and no transmission, all of which accounts for its high efficiency. Furthermore, with the electric motor, the relationship between rpm and torque is linear: You can use it to turn the boat's prop at 1 rpm or 10 rpm or 50 rpm or 100 rpm. An internal-combustion engine needs to cross an rpm threshold before its propeller is put in gear; otherwise, it would stall.
Probably the most amazing aspect of the STI's electric drive is its ability to produce electricity with a low-speed, high-output alternator driven by the prop shaft when a boat is under sails. In other words, the device is converting the prop's rotation into stored energy.
Other new developments in this area of Electric drives are being addressed with companies such as ASMO Marine
for utilising electric propulsion in production craft as well as Fast Electric Systems
and MW Line
in Switzerland for using straight electric drives in larger vessels.
For those that prefer a little more muscle in terms of power - and would prefer to have a dual propulsion system - the hybrid diesel-electric may be the answer. Whilst it's more environmentally friendly - straight electric is not practical for some commercial operators, powercats and motorsailors for a variety of reasons including availability of shorepower, horsepower issues, or lack of a backup system.
For those who prefer a dual system there's a couple of options.
One is using DC generator
input for electric drives. What's the difference in fueling up a DC generator with diesel to top up batteries in order to directly drive a propulsion system
... and just using a normal diesel engine? Lots.
There is a huge amount of savings in terms of fuel consumption as well as a much quieter generator as opposed to a chugging diesel. The emission differences are also significant. The downside is the size and weight of the battery banks needed and more money initially invested (which is eventually recouped through fuel savings). The DC Whispergen
is powered by a Stirling engine
that needs no oil, is almost completely silent... it operates unobtrusively with a noise level similar to a domestic air-conditioner.
Lightweight, compact and efficient, the WhisperGen converts over 90% of the fuel supplied into heat and electricity.
Then there's the Vetus option
- where electric propulsion is integrated into the diesel engine much like the hybrid cars one sees on the road today. The general idea is to use the diesel engine when you want the power and switch to electric propulsion when you want some peace and quiet. When motoring under diesel power, the electric motor, driven by the diesel engine, functions as a dynamo, charging the batteries for the next round of electric propulsion. It's a nifty system.
This technology is more than viable... the US military is planning to shift over a significant proportion
of their vehicles to hybrid technology in the future. The Humvee will be replaced by the more efficient Shadow RST-V - which is the US Marine Corps' first 4x4 hybrid-electric tactical vehicle. Richard Kastelein Avante Yachts LTDA