Technical Papers
Estimation of Torque Used for Determining Pull-up Length of Keyless Propellers*
Song Yuzhong**, Hiroshi Iwakiri**
1. Introduction
Keyless propellers were first used on ships classed by our Society (NIPPON KAIJI KYOKAI) in the early 1970s. The percentage of keyless propellers in use has reached a high of as much as about 80% of large ships at the present time, because of their many advantages in terms of savings realized in machining costs of the key way and the avoidance of stress concentrations normally found in the key way.
However, three accidents involving propeller slippage, which is unique to keyless propellers, have been experienced by large bulk carriers. Hence, steps have been taken to determine the cause of these accidents and measures examined to prevent their recurrence. It was subsequently thought that over-torque resulting from the tendency towards a reduction in main engine power per displacement of the ship under crash astern condition was the most probable cause of these accidents.1
Requirements for the pull-up length of a keyless propeller consist of upper and lower limits. The lower limit of the pull-up length is determined from external loads such as transmitted torque, and thrust. The upper limit of the pull-up length is determined by the yielding point of the materials comprising the inner surface of the propeller boss not being exceeded. The equation describing the relationship between the lower limit of pull-up length and external loads was already established in the latter 1960s.2 The equation is still used in the Rules of the Society in a somewhat revised form.
The estimation of external loads such as transmitted torque has always been accompanied by problems of accuracy due to the extremely complex nature of the transitional behavior of shafting including the torsional vibration under crash astern condition, etc.
The torque transmitted between the propeller and shaft in transitional condition, including the crash astern period, is considered to be closely related to the major parameters of an individual ship such as its accelerative ability. However, examination of the transmitted torque while taking into account the major parameters of a ship has yet to be found in any literature thus far. As a result, the estimation of transmitted torque may not always be appropriate for any given ship.
With such a background in mind, the authors have tried in this paper to correlate the possible peak vibratory torque passing through the resonant range under crash astern condition with the major parameters of an individual ship. In addition to theoretical analysis, measured data are also used to find a way to estimate torque used for determining the optimum pull-up length of keyless propellers for various ships having different major parameters.
2. Basic Considerations
The transmitted torque in an intermediate shaft in crash astern condition measured over time is shown in Figure 1 together with the rotational speed of the shaft (in this case, it is the same as the rotational speed of the propeller because this example is taken from a directly-coupled transmission system). As shown in Figure 1, large torsional vibration torque can be observed at the resonant rotation speed of the propeller as it continues to rotate ahead. This occurs as both the propeller and ship decelerate and then as the propeller starts to rotate astern as it begins to accelerate in the reverse direction while in crash astern condition. The measured resonant rotation speed of the shaft shown in Figure 1 is 53rpm.
Fig. 1 The torque in an intermediate shaft during crash astern measured over time.
