One notable implementation of diesel-electric propulsion is the Azipod, developed by the German firm ABB. The Azipod system utilizes an electric motor mounted externally in a pod beneath the hull. The motor drives a fixed-pitch propeller mounted at one end of the pod, and the entire pod is free to turn 360 degrees. By locating the motors outside of the hull, problems like alignment, noise, and vibration that are associated with a conventional shaft, strut, and bearing drive train are eliminated. Moreover, because the pods can turn, they offer extraordinary maneuverability compared to a normal shaft and rudder system on a boat.
One of the early adaptors of Azipod propulsion in the megayacht arena is the German builder Lrssen. It had a client who demanded that his yacht (originally named Air but now named Ice) be as clean and environmentally friendly as possible, with low emissions, minimum vibration, and sound levels like that of a cat on plush carpet. (The yacht's emissions standards are so stringent, the yard proudly touts her as its response to the Kyoto agreement.)
To meet these criteria the yacht was fitted with eight diesel gensets, each rated at 1,000 kVA (800 kW) and mounted in pairs on elastically supported foundations located in four separate rooms. The yacht's power-management system is designed to maintain the gensets operating as close as possible to 95 percent of rated capacity, their peak efficiency. Moreover, the diesel in each genset is fitted with a "soot" cleaning system (not feasible for large main-propulsion engines) that continually cleans the exhaust gas.
Because the Azipod has no struts or shafts, it operates in largely undisturbed water, so it creates much smaller pressure pulses on the hull as the propeller turns. That, coupled with the absence of bearings and gearboxes, makes for much less noise and vibration in the hull. After running the yacht for more than 18 months, the captain and the chief engineer have found that the system is easy to maintain, and they are quite satisfied with its performance.
The system on Benetti 's recently delivered Ambrosia III is similar, but with only two main gensets rated at 1,200 kW each powering a pair of 1,070-kW Azipod propulsion units. Complementing the main gensets is a pair of smaller (300-kW) units located in a soundproof compartment; the smaller units are used for station keeping and for leisurely cruising at speeds of up to about 9 knots.
Taking the pod propulsion concept to the next step, Feadship has designed a hybrid system, which it unveiled at the Monaco Yacht Show last September and refers to as X-Stream. The concept is based on contra-rotating propellers, using a pair of Azipod units (with forward-facing propellers) in proximity to a pair of conventional shaft-driven propellers. At slow speeds the shaft-driven props will free wheel, while the Azipod units provide propulsive power. At higher speeds the Azipods and the conventionally driven shafts will be driven in opposite rotation, achieving an additional boost in propulsive efficiency.
As it's presently conceived, Feadship's X-Stream concept uses direct-connected diesels to power the main propeller shaft. But I'd be willing to bet that when it finally becomes a reality, the main shafts will also be diesel-electric drive. It just could be a match made in heaven.
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This article originally appeared in the January 2007 issue of Power & Motoryacht magazine.