There's a dilemma that prevents diesel-electric propulsion from being widely adopted in boats and yachts, and it involves one of the basic laws of thermodynamics: There's a loss whenever you convert energy from one form to another—such as when you convert rotational engine power into electricity then back into rotational power.
But there's another issue that needs to be considered. Ignoring for the moment the issue of hull efficiency, a propulsion engine is most efficient when the load on the propeller matches an engine's peak torque output. In general, if a vessel's props are both the correct diameter and pitch then optimal loading only occurs when the engines are at WOT. At any other time there's a loss of power.
Engineers can take advantage of this difference by increasing the torque applied to the props at low rpm; one way to do this is to use variable-speed electric motors. A diesel-electric system can make up for the loss of power it incurs (up to a certain RPM) while converting rotational energy to electricity and back again if it maximizes the load on the diesel.
There are three major ways to incorporate an electric motor into a diesel-electric system. (To see examples of two very different kinds of diesel-electric hybrid boats, see "Silent Revolutionary" and "From Austria With Love.") The first keeps the shaft coupled to the drivetrain and enlists the electric motor to boost prop torque at low speeds. By contrast, some systems remove the engine-to-shaft coupling entirely, instead converting all the power to electricity first before sending it to the electric motors. The third way enlists both the aforementioned models, with a low-horsepower decoupled setup running one prop for slow-speed operation and a bigger engine with an intact shaft taking care of high-speed performance.
Without entering into a cost-benefit analysis of each setup, it's safe to say that each attempt to cut out the disparity between the engine output and the torque applied to the prop, if done properly, will have the potential to increase overall fuel efficiency. The preliminary questions then become the reliability and cost of these more complex systems.
One person who can vouch for the reliability of diesel-electric drives is Paul Smith. Back in 2002, he retrofitted his 1993 42-foot Ocean Alexander 423 named April K with a decoupled diesel-electric setup (see "The Next Big Thing," January 2005).
"The system that I have was modified…[but it] comes out of a bus," he enthuses. A 210-hp Cummins B200 with a flanged-on variable-speed generator/electric motor replaced the twin 210-hp 3208 Caterpillar inboards. The rest of the drivetrain was also rehashed, with the stock ZF marine gear and both the output and input shafts removed to make way for twin aluminum-encased electric motors and custom-made FAST marine gear with 4:1 reduction. So, what replaced the input shaft?
"Wires," said Smith. The wiring connects the initial electric motor to a converter that rectifies the incoming voltage. Software in a monitoring bus then allocates power to the two outboard electric motors, which spin the gearboxes and output shafts.
Smith's reason for installing the system was not fuel economy, (still, he claims a 22-percent savings over his old setup, partially aided by increasing wheel diameter two inches and removing an engine) but space. "I couldn't get outboard on the Cats," he explained, adding, "If you can get to things, you can fix things."
But Smith hasn't had to fix much. "Zero. That's how much maintenance I've done on the electric motors." Indeed he says the electric drives have quit just four times, each due to excessive voltage and each solved with a system reset, a fix that took a matter of seconds.
This article originally appeared in the January 2009 issue of Power & Motoryacht magazine.