Page 387 - Introduction to Marine Engineering

Appendix 357

thrust on the thrust block and the propulsion of the ship at some
particular speed. The propeller efficiency is a measure of effectiveness
of the power conversion by the propeller.
« Cr • thrust force X ship speed
propeller efficiency = El—E-
shaft power
The power conversion achieved by the propeller is a result of its
rotating action and the geometry of the blades. The principal
geometrical feature is the pitch. This is the distance that a blade would
move forward in one revolution if it did not slip with respect to the
water. The pitch will vary at various points along the blade out to its tip
but an average value is used in calculations. The slip of the propeller is
measured as a ratio or percentage as follows:
theoretical speed or distance moved
„ ,. — actual speed or distance moved
propeller slip = : —*- :
theoretical speed or distance moved
The theoretical speed is a product of pitch and the number of
revolutions turned in a unit time. The actual speed is the ship speed. It is
possible to have a negative value of slip if, for example, a strong current
or wind were assisting the ship's forward motion.

Example
A ship on a voyage between two ports travels 2400 nautical miles in eight
days. On the voyage the engine has made 820000 revolutions. The
propeller pitch is 6 m. Calculate the percentage propeller slip.
u • u- 820000x6,. - , -, 1Qca x
theoretical distance = (1 nautical mile = 1852 metres)
1852
2656.59 n.miles
theoretical distance — actual distance x 100
percentage slip =
theoretical distance

2656.59 - 2400 x 100
2656.59

= 9.66%

Power estimation

The power developed by a ship's machinery is used to overcome the
ship's resistance and propel it at some speed. The power required to