Calculation of Propeller-Excited Whirling Critical Speeds

TLDR: In this article, a two-support model of the propeller and tailshaft was used to estimate the critical speed of a vessel shafting system, which is suitable for manual calculation or use with a small, desk-top calculator.

Abstract: This paper presents a calculation method, suitable for manual calculation or use with a small, desk-top calculator, for estimating the propeller-excited whirling critical speed of a shafting system The method is especially adaptable to use with a small programmable calculator Based on a two-support model of the propeller and tailshaft similar to that used in earlier whirling calculations, this method "brackets" the natural frequency by first assuming the forward end of the tailshaft to be simply supported, then repeating the calculation, assuming the forward end to be fixed This gives an upper and lower limit on the natural frequency The actual natural frequency is estimated by interpolation between these two values The proposed calculation method includes propeller gyroscopic and inertia effects, as well as shaft mass effects Entrained water may also be taken into account Comparisons between the results of the proposed hand calculation and the results of other well-known methods of whirling analysis are presented for a typical vessel shafting system Extensions of the proposed calculation to include the effects of the line shafting and the sterntube bearing stiffness are also presented