Page 261 - Introduction to Naval Architecture

246 PROPULSION

speed. Power increases rapidly with speed. If enough readings are
available the humps and hollows due to the interaction of bow and
stern wave systems will be detectable. The figure shows a plot of
Admiralty coefficient This coefficient, or constant, is effectively the
inverse of circular C and is the given by:

A comparison of the power measured on trial and that estimated from
mode! tests, gives a ship-model correlation factor. This data can be used
for future similar ships.

Wake fraction from ship trials
If shaft torque is measured a torque coefficient can be calculated from
the shaft revolutions and propeller diameter. The advance coefficient
can be found from the ship speed and a plot made as in Figure 9.23.
From open water propeller tests the value of advance coefficient

Figure 9.23 Wake fraction

corresponding to any given torque coefficient can be found. This yields a
value of 14' The wake is the difference between the ship speed and V!,.
This is the mean wake through the propeller disc. In the absence of open
water model tests methodical series data can be used but with less
accuracy.

MAIN MACHINERY POWER

The objectives of the resistance and propulsion testing have been to
develop an efficient hull form and propulsor design and to establish
the main machinery power needed to drive the ship at the design