Why use electric propulsion?
Since no electric motor or generator is 100% efficient, it seems counter-intuitive that placing them between the diesel engine and propeller could improve fuel efficiency. Nevertheless, the fact is that, when a diesel-electric system is properly designed and configured, it can and does substantially improve fuel economy.
When in port, the battery bank can be recharged at the dock using shore power charging. Under sail, the battery bank is recharged as water rushes over the prop, spinning the blades and shaft, turning the electric motor into a generator that charges the battery bank – a process we call regeneration. Regeneration is the ability to produce electricity when a boat is under sail. When the boat is under sail, the propulsion works backwards, converting the prop's rotation into stored energy.
“For years the presumption has been that breaktroughs in fuel cell technology would be the seminal innovation that would transform the way energy and propulsion systems are set up on boats. The necessary breakthroughs have been discussed for at least ten years but do not seem to be a whole lot closer! Meanwhile, I believe that a series of incremental improvements and innovation in electric motor and DC generation technology have brought us to the point where the diesel/electric boat has moved from the realm of interesting experiment to potential widespread practical application.” – Nigel Calder. Boatowner’s Mechanical and Electrical Manual. Third Edition.
The result of a regeneration/battery bank electric system is phenomenal efficiency, fuel economy and vastly increased cruising range. For the pure electric power sailor, bound for distant ports, raising the sails to take advantage of nature’s energy – wind and sun, the cruising range can be unlimited.
For extended cruising needs, auxiliary power generation is normally accomplished through the addition of a diesel generator or solar array backup power system. Even when motoring in hybrid-electric/diesel mode, that is, with a generator running, far less fuel is burned than with a conventional fossil fuel engine. The reasons for greater fuel efficiency are simple. While motoring with the STI system, boat speed is altered by varying the motor’s rpm electronically, with no wasted energy. By contrast, the fuel-hungry operation of a combustion engine loses efficiency and wastes fuel at every level of operation from idling to accelerating. As the fossil fuel engine builds rpms in order to reach productive speeds, parasitic losses can reduce efficiency by as much as 50%. The auxiliary diesel generator used to recharge the battery bank is blueprinted to run at optimum performance, operating in a narrow rpm range where fuel efficiency is maximized.
Fuel economy also is improved through aggressive prop sizing. The high torque, low rpm electric motor with its large, high-pitch prop transfers force more efficiently to the water. The conventional fossil-fuel engine must turn at a relatively high rpm to produce the torque necessary to turn its prop, hence the need for reduction gears to keep rpm in the range required by the prop.
Benefits of the electric system:
- Dramatically reduces noise.
- Reduces or eliminates fuel smell and exhaust odor. No exhaust installations, which are a common source of problems.
- Uses renewable sources of energy to fuel itself. Reduces or eliminates air and water pollution completely.
- Better maneuverability.
- No vibration. Simpler mounting points.
- Instant power; no need to warm up.
- Achieves hull speed and stops faster than conventional power plants.
- Extremely long life, designed to last 250 thousand hours, which is longer than the expected lifetime of most sailboats!
- Significantly less routine maintenance, no filters, belts, valves, or pulleys.
- Greater house power usage potential.
- Ample fresh water onboard with lower tank capacity required for diesel.
- No winterizing required, extending the sailing season.
- About the same weight overall, but instead of a few very big and heavy diesel equipment, plus a heavy fuel tank when loaded, the electric system is made of many components that are smaller and the individual pieces are much lighter, making it easy to distribute them around the hull, an easier to carry in and out during installation and maintenance.
Recent advancements in electrical propulsion, energy recovery, composite materials and better insulation, designs capable of astounding performance, and advancements in navigation, have made possible just very recently, to build vessels that are dramatically more efficient and the relation between energy consumption and the energy produced in a modern boat finally crossed the break-even point.
Electric propulsion systems, in conjunction with renewable technologies such as photovoltaic (solar), wind, and hydroelectric to renew a significant portion of the energy required for that vessel's operation, are a new trend just about to become very popular. While shore-power and alternate forms of processed energy renewal can also be used, the marine operating environment is ideally suited for the use of sustainable, naturally renewable energy systems.
Modularity
Following the military requirements for which the system was designed, it is divided in a few modules
that can be easily replaced and maintained. The advantages are obvious: New modules may be added in the
future. Solutions that are not viable today may become affordable in the future and incorporated in the
system. If in the future fuel cells become practical, all one has to do is install them on-board and plug
them to the main distribution box. For the Vini-56 project the modules initially in place are these:
- Electric motors (2)
- Motor controller (2)
- Generators (2)
- Main distribution box
- Electronic commands
- Energy storage bank
- DC-DC converters
- DC-AC Inverter
- AC battery charger
- Wind generator
- Solar panels
Why diesel engines are so bad?
Electric motors are 94% efficient in warm weather, and better than that the colder it gets. They have only one (1) moving part and therefore suffer none of the multitude of parasitic losses of fossil fuel engines, including waste due to transmissions, fuel pumps, gears, and literally hundred of parts that create heat, vibration and the resulting major energy losses.
Even bigger fuel savings are realized in boats that are heavily equipped with on-board appliances. The generator efficiently produces electric power for all auxiliary systems as well as for propulsion. There is no need for a separate generator to run the air conditioner, and one never has to run a diesel propulsion engine as a generator to recharge batteries and sustain onboard electrical systems. Additionally, stored battery bank power allows you to use electricity on demand, and the generator only runs to recharge the battery bank, rather than running continuously while high voltage appliances are in use.
Electric motors can outperform higher horsepower diesels because they deliver more of their horsepower to
the propeller. Typical diesel engines have losses before power is delivered to the propeller.
The way a diesel engine functions is much more complex than an electric motor. 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 thousands of revolutions down to something a propeller can use. By the time that's done, the engine efficiency is somewhere below 25 percent. Also, diesel engines are rated at their maximum rpm – and on sailboats are rarely operated at that speed.
These losses are further exacerbated by the typical use of a diesel engine. Maintenance on diesel engines very seldom includes bringing the engine to operating temperatures at least once per week during the life of the engine. Typically, many weeks and in most cases, months will pass between engine start-ups.
When a diesel is not run for extended periods of time, the lubricating oil necessary to prevent damage to the cylinders will work its way to the oil sump at the bottom of the engine. When the engine is started after even a relatively short period (4 weeks), the oil pump must pump oil back to the top of the engine to provide lubrication. This may take 5 to 15 seconds depending on the engine, which is sufficient time to result in scoring of the cylinder walls.
This scoring, over time, will decrease the ability of the cylinders to maintain the high levels of compression necessary in a diesel. Accordingly, over a relatively short period of time, significant power losses occur. These changes are gradual and frequently not noticed until the problem has reached engine failure. This cannot happen with electric Motors. They will maintain their rated HP efficiencies for their entire lifespan.