In the January 06 issue of Yachting Monthly, Nigel Calder explains why he has opted to fit diesel-electric propulsion on his new boat.

Here he goes into greater detail about the development of diesel-electric power.

The January issue will be on sale from Thursday 8th December

OVERCOMING OBSTACLES

The efficiency gains on larger vessels are achieved by operating a series of AC generators in place of a single propulsion engine.

At low loads, only one generator is run, and then as the load increases more are brought on line.

In this way the generators are always operated at the optimum (most efficient) point on their power curves, as opposed to a conventional installation in which the engine has to be powerful enough for maximum power and then spends almost all its time running at an inefficient point on its power curve.

Obviously, on a boat of our size we are not going to have multiple propulsion generators.

If we were to use a conventional AC generator for the diesel-electric drive, it would need to be run at a constant speed (which is necessary to maintain the correct AC frequency) in which case we get back to the situation of the engine running at an inefficient point on its power curve for most of the time it is operating.

It has taken a fair amount of research over the past decade to develop alternative approaches that optimize engine efficiency in a single engine installation.

One approach (most notably pursued by Fischer Panda in Europe) is built around variable frequency (and therefore variable speed) AC generators feeding AC motors; the other (most notably pursued by Solomon Technologies and Glacier Bay in the USA) uses a DC generator (which, because it is not frequency sensitive, can be run at whatever speed is appropriate to the load) feeding DC motors.

In all cases, the propulsion motors are run at relatively high voltages (up to 800 volts AC or DC in larger installations).

What has made both approaches viable is the development of powerful, lightweight permanent magnet generators and motors (they use high density, ‘rare earth’ magnets) together with powerful electronic power conversion devices derived from the DC-to-AC inverter marketplace.

The DC motors, for example, are, in fact, AC motors that accept a DC input which is then converted to variable frequency AC by a ‘motor controller’ that employs inverter-based technology.

Fischer Panda’s AC motors, which are not directly driven by the AC generators, use similar technology between the generator and motor.

An outgrowth of this generator and motor development has been a range of high voltage motors suitable for such things as windlasses, electric winches, bow thrusters, and, in the case of the Glacier Bay product line, refrigeration and air conditioning compressors.

These are typically smaller, lighter and more efficient than the traditional motors they replace.

An added benefit is that any time motor voltage is raised, the supply cables can be reduced in size (a doubling of the voltage reduces the cable size to one quarter for the same rated output), which in many instances (notably windlasses and bow thrusters) gets a considerable amount of heavy cabling out of the boat.