Theoretical and experimental analysis of a slack towline motion on tug-towed ship during turning

Concerning the safety of towing, a slack towline condition on tug-towed ship motions interaction during turning is necessary to be studied. This paper presents a fundamental investigation aimed at gaining a more comprehensive insight into the basic mechanism of the slack towline condition identified via several towing parameters affecting its occurrence. Through applying steady turning theory of the tug-towed ship presented by Fitriadhy and Yasukawa (2011a), a slack towline formula is derived, which is capable of defining a boundary limit of their turning ability before the towline falls into the slack condition. Several criteria of the slack towline are described and the reason behind its proposed formula explained. A comprehensive theoretical clarification is presented here using nonlinear analysis for a better prediction of their turning ability including capturing the slack towline condition, where effects of towline length and deflection-rudder angle are discussed. The results revealed that the tug was vulnerable to lose handling of towing with respect to the towed ship that led to the towline falling into the slack condition. For validation, a series of turning tests for the towed ship model were carried out at Towing-Tank in which the theoretical approaches agreed qualitatively with the experimental results