Analysis of the Influence Factors on the Efficiency of Marine Tail Shaft Transmission System

The transmission efficiency of the marine tail shaft was calculated with the finite element method and experimentvalidatedthe rationality and feasibility of finite element models. The research conclusions as follows: power consumption increases with the shaft length, shaft diameter and speed increase. The results can provide a theoretical support for further research andit is also helpful for engineering applications.


Introduction
When the ship is sailing on the ocean, itsshaft between the stern bearing and propeller is working in water, which is normally called the shaft in water [1].There has always been scholars to question the power consumption of the shaft in water and then affect the ship's economy and efficiency.In particular, the shaft in water power consumption of the warship will further affect its operationalperformanceattract more attention from scholars and experts [2,3].However, it is limited to directly measurethe shaft in water's power consumption in real environmental conditions when theshaft in water runs in the water [4].So far there has been no relevant studies or reports about the shaft in water drive efficiency.
In this paper, firstly, finite element model of the shaft in water is built in GAMBIT and its friction torque in water is calculatedvia FLUENT, and then the power consumed by the resistance is obtained.Our project team designed the relevant design experiments to test the friction torque produced by the shafting in water and then calculate the power consumption.The theoretical results and experimental results are comparatively analyzed to find out the factors affecting the efficiency of the shaft transmission in water.

2.Friction torque calculation with FEM
The power consumption caused by friction can be calculated by torque, so the friction torque should be calculated by FLUENT firstly.The power consumptioncan be calculated with: x Where P(W) is the power consumption, S is the rotation speed of the shaft and T is the friction torque.The meaning is the same as in the following.For example, when the axial section of the shaft in water is 1200mm and the diameter of the shaft is 240 mm, the diameter of the rotating area is selected 360mm.Due to the large external flow field, the impact on the friction power consumption is small, the grid partition can be sparse, a total of 44350 units, the rotation of the basin and the axis of contact, the grid division of the higher density, a total of 43350 units.As shown in Figure 1.

Finite elementmodel
Thequality inspection of grid division meets the requirements and the finite element models of different axial lengths under different axial lengths are similar to that of example.
In in Table 1.Where, L is the length of the shaft in water, D is the diameter, DRis the diameter ofrotating basin, S is the rotation speed of the shaft and T is the friction torque.
Obviously, there is no effect on the friction torque calculation of the rotating basin under different coaxial section diameter.According to the calculation formula of the power consumption and the friction torque, the power consumption is not related to the size of the rotating basin, so any diameter can be used in the FEM calculation.

Calculation results
The length of the shaft is 1500 and 1200 mm

Experimental Results
Power consumption under different conditions is shown in Table 3, which is the interpolationof the tail shaft power consumption in the water and in the air, that is, power consumed by the shaft to overcome the resistance of water.According to the experimental results, it is concluded that when the length and diameter of the shaft are constant, the power consumption of the shaft increases with the increase of the rotating speed.

Conclusion
The following conclusions can be drawn from this research: the experimental results verify the feasibility of the finite element model, and computational results by FLUENT also demonstrate the rationality of experiment.
In finite element calculation, the author found that rotating area diameter size had no effect on the friction torque and power consumption, which can provide a theoretical reference for the other simulation.The power consumption of the shaft in water is increased with the increase of the length, diameter and rotational speed of the shaft.
Because of the gap between the experimental model and the actual ship, the experimental model is relatively small, the measured torque and power consumption are relatively small under different conditions.In the follow-up study the author will establish a larger experimental platform to get more experimental data to further explore the impact factor of the shaft in water drive efficiency and power consumption and describe them in a more specific numerical relationship.

Figure 3 .
Figure 3.Power consumption varies with diameter When the shaft length is 1500mm and the diameter is 390mm, the FEM calculation result is compared with the experimental results, as shown in Figure 4.Other comparison results are similar under different operating conditions, and here are no longer cited.

Figure 4 .
Figure 4. Comparison between experimentationand calculation Easy to see the various experimental conditions calculated with the resultant power variation and trends basically consistent with the lower section of coaxial rotational speed increases, the corresponding increase in the power shaft section of the water by the rate will slow and fast.

Table 1 .
Torquevalues under different rotating basin diameter