Great increases in the cost of fuel and the advent of very large tankers and bulk carriers have focused the attention, during the last decades, on means to enhance the efficiency of ship propulsion. An obvious way of obtaining an efficiency increase is to use propellers of large diameter driven by engines at low revolutions, as can be deduced from the developments in Chapter 9. Such a solution is, however, in many cases not practically possible. This has then given impetus to the study and adoption of unconventional propulsion arrangements, consisting, in general, of static or moving surfaces in the vicinity of propellers.

A distinct indication of the serious and extensive activity in the development and use of unconventional propulsors may be seen in the report of the Propulsor Committee of the 19th ITTC (1990b) which lists seven devices including large diameter, slower turning propellers. Here we summarize the hydrodynamic characteristics of six of these devices omitting larger-diameter propellers. The six devices are: Coaxial contrarotating propellers, propeller with vane wheel, with pre-swirl stators, with postswirl stators, ducted propellers and propellers operating behind flowsmoothing devices.

Emphasis is given in the following to a variational procedure which, in a unified fashion enables nearly optimum design of several of these configurations.

Propulsive Efficiency

Propulsive efficiency is conventionally thought of as the product of the open-water efficiency of the propulsor, the hull efficiency and a factor termed the relative-rotative efficiency.