Page 264 - Practical Ship Design

222 Chapter 7

7.5.5 Propeller design

The author wanted to include in this book approximate formula that would enable a
naval architect deprived of any other data to make a complete initial design, but has
decided to admit defeat with propeller open water efficiency. The subject appears
to be too complex for any simplified treatment and readers are referred to the
standard text books on this subject.
Fortunately, there is rarely any need to consider open water efficiency at the
initial design stage as Emerson’s QPC formula seems to fill the immediate need in
power estimation with reasonable accuracy, enabling open water efficiency to be
dealt with by propeller designers at a later stage.

7.5.6 EfJiciency of a controllable pitch propeller

The efficiency of a controllable pitch propeller is generally I-2% less than that of a
fixed pitch propeller for the same design condition - this being mainly due to the
much larger boss diameter.
If, however, there is a requirement for extensive operation at either a speed or a
displacement significantly different from the design condition a controllable pitch
propeller can offer substantial advantages in fuel economy and/or speed at the
second operating condition.

7.6 HULL EFFICIENCY

If it is impracticable to include in this book a satisfactory way of estimating open
water efficiency, there is no point in giving formulae for hull efficiency as it is only
the product of these - the QPC - which is of real interest. It is, however, worth
discussing the components of hull efficiency to try to identify whether these can be
influenced in the design process in a way that will improve powering efficiency.
It will be recalled that:

where
I-t T-R v -v,
qh =- and t=- and w, =-
1-w, T V
To optimise qd, the thrust deduction t should be minimised, the Taylor wake
fraction w, should be maximised as should the relative rotative efficiency q,.