In theory, maintenance on a diesel-electric powerplant should also be less than on a direct drive. With an engine running at a constant speed and load regardless of sea conditions, it should closely mimic optimum dynamometer readings. Smith concurs that he's had few maintenance issues.
Electric motors also provide more maneuverability. Since the software in the monitoring bus can fine-tune the power at the props (you can literally dial in 116 instead of 117 rpm) and give them 100-percent torque, getting dockside doesn't require bumping the engine in and out of gear, waiting patiently to see the effect, and bumping again. You can finesse your boat exactly where you want it to go while maintaining vessel velocity.
He ran across another plus of a decoupled system after recently extending the April K by six feet. "I could have put the engine back there," he reflected, "and freed up even more room." Designers can put diesel engines anywhere they choose, so even placing one transversally is an option since there's no shaft linkage to the engine. His centerline engine setup also allowed Smith to install a pair of Wesmar stabilizers, which he believes would have been impossible to get to with the twin Cats in place.
But despite all of the benefits, there are also drawbacks. The foremost concern is weight, since adding extra components means there's more to move through the water. Smith got around this by going to one engine, which, due to the combined prop-to-engine efficiency loss of both powerplants, actually maintained his top speed.
April K's retrofit, cobbled together from different suppliers, was a way to get diesel-electric power on a smaller boat six years ago, but what about now? Are there any complete packages that can be installed into a boat? Or even better, that already come in a vessel?
Glacier Bay, based out of Union City, California, has been building and installing complete diesel-electric propulsion OSSA Powerlite drives (minus the props) to power small sailboats for the past few years. And now Custom Metal Yachts is installing the OSSA Powerlite system into a 73-foot passagemaker. The Gregory Marshall design will be the first Glacier Bay diesel-electric system to find its way into a pleasure yacht. The system enlists five 20-kW D.C. diesel gensets instead of conventional propulsion engines to provide all of the power and a predicted top speed of 20 knots.
"It was designed as a completely integrated propulsion and power platform, not just a composite of off-the-shelf components," says Glacier Bay's CEO Marc Hoffman, alluding to the plans that allow all the "hotel power" (any non- propulsion power) to be drawn from the gensets.
Another factor that increases the efficiency of the Glacier Bay system is the lack of mechanical gearboxes. In their stead, twin permanent-magnet D.C. electrical motors are splined directly onto the output shafts, reducing the roughly three- to five-percent loss attributed to the helical gears in conventional marine transmissions.
Hoffman claims that the Glacier Bay system is also quieter than conventional propulsion systems, but not just because all gensets are encased in soundshields made with Glacier Bay's Ultra-db insulation. The props are connected to the electric motors using a CV joint and thrust bearing arrangement. This minimizes the prop noise transmission into the boat. Finally, the electric motors themselves are ultra-quiet. The drive system gets rid of "the rumble," states Hoffman. He gets to the crux of it when he adds, "Think about it from a designer's standpoint—the feedback we get is 'Nirvana'."
When you improve space aboard, allow for redistribution of weight, and cut the amount of fuel needed for a trip, you can change some fundamental considerations of yacht design. One thing that doesn't look like it will change is the potential for growth in this sector. I wouldn't be surprised to see a few new drive diesel-electric drive systems on the horizon. Actually, some other full-packages are sneaking up as we speak, and soon this quiet propulsion may be making a lot of noise.
This article originally appeared in the January 2009 issue of Power & Motoryacht magazine.