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ABSTRACT In difficult ice conditions it is sometimes necessary for the icebreaker to tow the merchant vessel which can be somewhat risky as high forces act in the towing line. However, relatively little is known about the forces acting during the notch towing operations. The purpose of this study is to investigate the notch towing operations by measuring the towing cable forces. Measurements were performed onboard Finnish icebreaker Kontio during winter 2016. The results indicate that in general the towing line forces are linearly dependent on the mass of the towed vessel. High force peaks are usually related to slack & rapid tightening of the towing cable. It is easier to generate slack with large vessels as a gap has to be left between the icebreaker and the towed vessel in order to maintain maneuverability of the icebreaker. In addition, skills of the helmsman of the towed vessel are crucial factor for towing line forces. If the towed vessel is not able to follow the icebreaker in straight path, the risk of breaking loose from the towing fork and high forces increase.
INTRODUCTION In difficult ice conditions, especially with ice compression, the merchant ships are occasionally unable to follow the icebreaker with their own propulsion power. Therefore it is necessary for the icebreaker to tow the merchant vessel. It is believed that notch-towing operations will increase in the future as the merchant vessels' engine powers tend to decrease due to emission regulations. At least the need for icebreaker assistance is foreseen to increase in the Baltic Sea (Westerberg, 2014).
Towing operations require skilled crews and can be somewhat risky as high forces act in the towing line and there is also a risk of collision. However, relatively little measured data is available about the forces acting during the notch towing operations. Previously Starshinov et. al. (1990) have measured the forces acting on the icebreakers stern during different type of towing operations. Their study gave information regarding the forces to the structures of the icebreaker stem due to contact between two vessels especially when the vessels are rapidly decelerating.
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