As noted, the goal is to meet all of a boat’s electrical generating needs when the vessel is underway. We control the electric generating load to optimize engine efficiency. At idle speeds, there are substantial gains in propulsion efficiency, but as engine speed and propeller load increase, these efficiency gains diminish until full power is approached, at which point we shut down the generating device in order to avoid overloading the engine. With this regime, so long as the batteries can accept the charging current, the engine runs at close to peak efficiency almost all the time.
The propulsion load is met more efficiently than with a conventional installation, and electrical energy is supplied to the boat at, or very close to, peak engine efficiency. Fuel consumption per kilowatt-hour of electrical energy generated is far below the average fuel consumption of a stand-alone generator (as much as five to 10 times lower, given that most AC generators spend most of their time operating lightly loaded, which results in incredibly inefficient electrical energy creation). Engine run hours and maintenance are substantially reduced. If amortization, maintenance, fuel and other costs are factored in, the kilowatt-hour cost of electrical energy is many times lower than with a stand-alone generator.