# Study on the Propeller Rope Cutter Concerning Transient Torsional Vibration Due to Cutting Action
## 1 Introduction  
Korea is a country with many islands as well as very busy fishing ships. Accompanied by that bustle, there is an increase in the amount of waste in the sea posing a danger to ships [1, 2, 3, 4]. At least in the seas of Korea, the annual inflow of marine debris was estimated to be 91,195 tons. Of this, the land-based sources account for 36% (32,825 tons), whereas the sea-based sources account for 64% (58,370 tons) [5]. A rope or a piece of fishing net can wrap around the propeller shaft and then prevent the propeller from rotating. Accidents due to these suspended objects are frequently reported at least once a week. The accident rate is expected to increase further in the future due to the increased operating time and intensification of ships in the fishing area, as well as competitive fishing activities. Mostaccidents delay the sailing schedule by at least one hour or more, resulting in increased financial costs [6; 7; 8]. Sending a diver underwater to solve this problem is potentially risky, not just in rough weather. A vessel not under command (NUC) at sea may become an obstacle and then endanger other ships moving nearby. In a more serious case, a rope can immediately stop the rotating propeller, damage the transmission gears, shut the engine off, or even drop the engine off the platform. If a rope remains entangled in the propeller shaft and continues to rotate around the seal, it will eventually cut the seal and cause the propeller shaft seals to leak. [PERSON] also mentioned incidents where fishing line wrapped into the stern seal [9]. During strong waves and winds at sea, many cases of ship overturning accidents have been reported. The ship named Dolgorae capsized, causing the death of 15 people and 3 missing people [10; 11; 12]. The MV Seohae ferry sank on 10 October 1993 in the Yellow Sea due to a 10 mm thick nylon rope wrapped around the two propeller shafts, killing 292 of the 362 passengers and crew on board [13; 14; 15]. To avoid that kind of accident in advance, many efforts have been made to remove the harmful substances by increasing the activities of collecting the floating materials at sea such as abandoned fishing nets and ropes [16]. This work is helpful but it is very laborious and not efficient enough since it is impossible to remove all the harmful substances at sea. In addition, although the propulsion systems without propellers, such as water jet propulsion [17; 18], have been developed and applied, there are practical limitations such as high capital expenditure for installation on a small ship. Another solution is to install a rope guard, which acts as a barrier to protect the propeller shaft from line entanglement [19; 20]. However, there is a problem that the downside of this device increases the weight and water drag, forcing the engine to generate more power for the same ship speed. Besides, after a while, it will be fouled and contaminated at times, making it difficult for inspections or repairs. In this study, the development of a rope cutter is concerned. This cutting system is an assembly of two main parts. The static part includes a non-rotary blade held by a holding block, which is fixed on the propeller shaft strut housing by bolts. The rotating part including two blades is mounted on the propeller shaft and rotates together. It was designed to cut anything that is wrapped around the propeller shaft, such as a rope or a piece of fishing net. The rope cutter had posed a great advantage in protecting the propeller shaft from suspended objects. However, the cutting action produces the transient torsional vibration, similar to an ice-propeller interaction [21; 22; 23; 24; 25]. After many of these actions, the lifetime of the propulsion system is shortened due to cumulative fatigue. Furthermore, if the tangled object is too hard for the cutting capability, it can stop the shaft immediately or significantly damage the shaft in terms of fatigue strength. That object may not be long enough to wrap around the shaft, such as a piece of wood.