Reply
Thank you very much for your question, Mr. Takai. As you pointed out, the torque acting on propeller shafts is often considered to be greater than usual when a ship is in the racing condition. However, we do not have any measured data for this condition. Considering the fact that when a ship is going astern, the reverse thrust acts to pull the propeller outwards from the shaft, and further that all slip accidents have been experienced only while going astern, the authors believed that one possible maximum torque in the crash astern condition is suitable in estimating the torque when calculating the minimum pull-up length of a propeller in order to prevent propeller slippage.
Dr. Kazuyuki Ouchi, Nakashima Propeller Co., Ltd.
The increased ratio of torque is finally described by Equation 19. In this formula the pitch factor is apparently not taken into account. Do you think the pitch factor should be neglected in this case?
Reply
Dr. Ouchi, thank you very much for your comment. It is true that we did not take the pitch factor explicitly into account in Equation 19. However, in the process of deriving Equation 18 showing the relation between the rate of increase of torque and propeller diameter, several regressive relations, such as k over D shown in Figure 6, were used. If the sample data itself were used instead of a regressive relation, the |JT| over D shown in Figure 10 might be as shown in the following graph:
That is to say, even though the regressive relation would remain unchanged, the relation for an individual ship would deviate from the regressive relation. As mentioned above, if the various pitch factors are considered, the relationship between KQ and J would not be expressed by a single curve but as a number of curves as shown below:
As can be seen in the above figure, the relationship between λ and |JT| expressed by Equation 6 is also a regressive relationship. Since the reasoning leading to Equation 18 was based on regressive relationships, even if the pitch factor was taken into account in this reasoning, it is believed that the regressive relation would be unchanged but the variance would increase. To cover the effect of the deviation, we made the regressive relation shown in Figure 11 cover all the measured data.
