Page 225 - Introduction to Naval Architecture
P. 225
210 PROPULSION
Installed power is the shaft power (P$) then the overall propulsive efficiency
is determined by the propulsive coefficient, where:
The intermediate stages in moving from the effective to the shaft power
are usually taken as:
Effective power for a hull with appendages = P E
Thrust power developed by propulsors = P T
Power delivered by propulsors when propelling ship = P D
Power delivered by propulsors when in open water = P&
With this notation the overall propulsive efficiency can be written:
The term PE/PE is the inverse of the appendage coefficient. The other
terms in the expression are a series of efficiencies which are termed,
and defined, as follows:
PE/PT - hull efficiency = r/ n
PT/PD = propulsor efficiency in open water = TJ O
PD/PD = relative rotative efficiency = r) R
PD/PS ~ shaft transmission efficiency
This can be written:
The expression in brackets is termed the quasi-propulsive coefficient
(QPC) and is denoted by t] D. The QPC is obtained from model
experiments and to allow for errors in applying this to the full scale an
additional factor is needed. Some authorities use a QPC factor which is
the ratio of the propulsive coefficient determined from a ship trial to
1
the QPC obtained from the corresponding model. Others use a load
factor, where:
Transmission efficiency
load factor = (1 + x) =
QPC factor X appendage coefficient
In this expression the overload fraction, x, is meant to allow for hull
roughness, fouling and weather conditions on trial.