# Study on the Propeller Rope Cutter Concerning Transient Torsional Vibration Due to Cutting Action
## 5 Experiment in the Shipboard Test on an Actual Ship  
A rope cutter system has been installed on a passenger ferry. The main technical data of ship and propulsion plant are summarized in Table 4. The experiment procedure is as follows:  
- The engine stops, the ship stops. - A diver hooks a rope into the cutter system. - Start the engine, run at a speed of 600 rpm (ready to engage). - Engage the clutch to rotate the propeller shaft, and then the rope is cut immediately. Test No. 1 was carried out with a \(\emptyset\)50 rope for cutting and test No. 2 used a fishing net, as shown in Figure 12. Both are made of polypropylene. The measured vibration results are compared to the normal engagement. As with the laboratory tests, a strain gauge was installed at the intermediate shaft to measure the transient torsional vibration due to the cutting actions. The roller encoder measured the rotating angular velocity. The accelerometers were mounted on the engine body for the structure vibration measurement. The measurement diagram is described in Figure 13. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Ship** & & **Main Engine** & \\ \hline Name & SEOMSARANG NO. 13 & Manufacturer & CATERPILLAR \\ Gross Tonnage & 155 & Type & C32 DITA (ACERT) \\ Date of built & 2016 & Bore \(\times\) stroke & 145 \(\times\) 162 mm \\ Owner & Haekwang Shipping & Power & 634 kW (850 BHP) \\  & & Revolution & 1800 rpm \\ \hline
**Reduction Gear** & & Number of cylinders & 12 \\ \hline Manufacturer & NICO & Number of sets & 2 \\ Type & MNG90 BL & **Shaft Diameter** & \\ \cline{2-4} Gear ratio & 3.48: 1 & \
\begin{tabular}{l} Intermediate shaft \\ Propeller shaft \\ \end{tabular} &
\begin{tabular}{l} 120 mm \\ 145 mm \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specifications of the ship and propulsion system in the shipboard test. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Parameter** & **Unit** & **Rope \#1** & **Rope \#2** & **Rope \#3** \\ \hline Rope diameter (\(\varnothing\)) & [mm] & 6 & 9 & 12 \\ Cross-sectional area of rope & [mm\({}^{2}\)] & 28.27 & 63.62 & 113.10 \\ Stress measured at inter. shaft & [N/mm\({}^{2}\)] & 1.1 & 2.5 & 4.5 \\ Cutting torque (\(T\)) & [N-mm] & 1727.88 & 3926.99 & 7068.58 \\ Cutting force (\(F_{2}=F_{3}\)) & [N] & 52.36 & 119.00 & 214.20 \\ Shear stress in rope due to cutting action & [N/mm\({}^{2}\)] & 1.85 & 1.87 & 1.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Shear stress amplitude in rope due to cutting action. When the gear clutch engaged with the cut, the torsional vibration stresses were greater in the comparison with the normal engagement as described in Figures 14 and 15. Those oscillations were due to the cutting actions. In case of test No. 1, after the first one, the cutting occurred 2 more times because the rope continued to move into the cutter with the flow of water. As calculated in Section 2, the permissible transient torsional stress is determined by the lower value as 64.5 N/mm\({}^{2}\) for 520 reversals. The transient torsional vibration amplitudes due to cutting action were less than 5 N/mm\({}^{2}\), which is significantly smaller than the clutch engaging torsional fluctuation, 34 N/mm\({}^{2}\), which occurs many times daily. The maximum diameter of rope that can be safely cut is calculated as 180 mm. The cutting capacity of this cutter is determined. After clutch engaging, the angular velocity of the propeller shaft increased quickly, as shown in Figures 16 and 17. The cutting action affected the angular velocity slightly; it was lower but not remarkable in both test cases. The difference in engine structure vibration was very insignificant. There was almost no effect of rope cutting to the engine body. Figure 12: The overview of passenger ferry and cutting test materials (rope, net). Figure 13: Schematic diagram of the vibration measurement system in the shipboard test. Figure 16: Comparison of rotating angular velocity at the intermediate shaft during engaging test No. 1.