Page 242 - Introduction to Naval Architecture

PROPULSION 227

Figure 9.13 Thrust, torque and efficiency curves

torque coefficients decrease with increasing advance coefficient
whereas efficiency rises to a maximum and then falls off steeply.
This format is good for presenting the data for a given propeller but
not very useful for design purposes. In design the problem is usually to
find the diameter and pitch of a propeller to provide the desired power
at set revolutions and speed. The thrust power, P T, is the product of
thrust and speed.

5
2
That is P T(N /pV a ) = G(/), where G is a new function of/.
Taylor used U to denote thrust power and using seawater as the fluid,
dropped p and took the square root of the left hand side of the above
equation to give a coefficient J3rj. He used a corresponding coefficient,
Bp, for shaft power which he designated P, That is:

For a series of propellers in which the only parameter varied was pitch
ratio, Taylor plotted By or Bp against pitch ratio in the form of contours
for constant 6 values, <5 being the reciprocal of the advance coefficient.
A typical plot is shown in Figure 9.14.
To use the plot the designer decides upon a value of revolutions for
a given power and advance coefficient. This gives B\j or Bp. Erecting an
ordinate at this value gives a choice of values of <5 from which the
diameter is obtained. Associated with each diameter is a value of pitch
ratio. For a given Bp the maximum efficiency that can be obtained is