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.
   220   221   222   223   224   225   226   227   228   229   230