Page 224 - Introduction to Naval Architecture

9 Propulsion

The concept of effective power was introduced in Chapter 8. This is the
power needed to tow a naked ship at a given speed and it is the starting
point for discussing the propulsion of the ship. In this chapter means
of producing the driving force are discussed together with the
interaction between the propulsor and the flow around the hull. It is
convenient to study the propulsor performance in open water and then
the change in that performance when placed close behind a ship.
There are many different factors involved so it is useful to outline the
general principles before proceeding to the detail.

GENERAL, PRINCIPLES

When a propulsor is introduced behind the ship it modifies the flow
around the hull at the stern. This causes an augmentation of the
resistance experienced by the hull. It also modifies the wake at the stern
and therefore the average velocity of water through the propulsor. This
will not be the same as the ship speed through the water. These two
effects are taken together as a measure of hull efficiency. The other
effect of the combined hull and propulsor is that the flow through the
propulsor is not uniform and generally not along the propulsor axis.
The ratio of the propulsor efficiency in open water to that behind the
ship is termed the relative rotative efficiency. Finally there will be losses
in the transmission of power between the main machinery and the
propulsor. These various effects can be illustrated by the different
powers applying to each stage.

Extension of effective power concept
The concept of effective power (P E) can be extended to cover the power
needed to be installed in a ship in order to obtain a given speed. If the
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