As a result of laboratory experiments and analyses of field data, four equations of bit performance were suggested as in the previous papers, i.e., euqations of performance line, penetration rate at the separation point, relative penetration rate and bit life. In this paper methods obtaining such factors as penetration rate, drilling time and length of a bit, and optimum combination of bit weight and rotary speed, are generally discussed by using of oremantioned equatious with the efiect of hydraulics included. An impact force developed at a bit nozzle is governed by quantity of drill collars to be used; penetration rate, rate of drilling operation, and drilling time and length of a bit may be affected by quantity of drill collars to be used. There is the optimum bit weight which gives the maximum penetration rate under a given rotary speed, and, in the most case, the rate of drilling operation also attains at the maximum value under the same conditions. As an example of application of it to actual driling practices, influences on the penetration rate of pump operations, i.e., single pump operation and parallel operation of two pumps, and also quantity of drill collars are examined on the basis of analyses of field date.
