Flexible Power Solutions*

On many vessel types the required propulsion power varies greatly during a single voyage. Remarkable differences in load, trim or cruising speed are the cause.

Many shipowners have noted that conventional single-engine solutions do not provide the flexibility required for such wide fluctuations.

The effects are even greater for ships where the original trial speed was far above the actual requirement.

Trial speeds and thus the installed main engine outputs are purposely selected to be too high.

This is done to make the ship more interesting on the second-hand market, while the actual traffic where the vessel is used moves at much lower speeds.

Multi-engine solutions are often chosen for many ship types. This permits generation of propulsion power required in any operating mode at engine loadings close to the optimum fuel consumption loadings.

The most spectacular example is the passenger ferries, where the usual layout is two propeller shafts and four main engines. There are also examples where as many as four engines are served by a single gearbox.

For a typical passenger ferry with a four-engine, twin-screw installation, the installed outputs can exceed 45,000 bhp. The reasons why passenger ship operators select multi- engine configurations are obvious: availability; flexibility; and compactness.

Another way to achieve flexibility is to install a diesel-electric solution, where the propulsion power comes from a common "power station." The power station normally consists of from two to even nine engines and since this power station feeds both the electrical equipment as well as the propulsion motors, it is easy to install an electronic performance control that starts and stops engines based on the actual power requirement.

In this way, the engine loadings are always on or close to the optimum fuel consumption point.

A twin-engine installation would be normal for a single-screw vessel.

In some cases the optimum is that both engines are the same size. In other cases, it is better to use a "father- and-son" installation, which is a machinery arrangement where the main propulsion is handled with a multi-engine arrangement consisting of a larger engine (father) and a smaller engine ( son). The engines are connected to a common gearbox and drive a single controllable pitch propeller.

The father-and-son solution is usually very efficient for ships with a varying speed or loading profile.

By selecting the sizes of main engines wisely and by installing suitable shaft generator arrangements, remarkable savings in both capital expenditure and operating costs can be achieved.

Further savings can be made when onboard electricity is generated with the main engines. Due to the CPP installation, the configuration does not require expensive constant speed gears for a shaft generator.

The cost and complexity of such devices are normally much more than what would be anticipated in the initial stage.

In conclusion, the use of multiengine installations has proven a useful alternative to conventional slow-speed engine installations. The flexibility needed to run at any speeds and loading conditions is highly appreciated by owners already using the system.

The availability of the system is increased, since the ship is maneuverable at all times, even if one of the main engines is being overhauled.

The economy of this system results not only from lower operating costs, but also from low capital expenditure and space savings.

For free literature on diesel engines from Wartsila, Circle 62 on Reader Service Card *Editor's Note—Excerpted from a paper by Kenneth J. Jofs, president, Wartsila Diesel do Brasil Ltda.

Flexible Power Solutions*

Other stories from April 1991 issue

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Maritime Reporter