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Tow Forces in Waves for Canadian Navy Vessels

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Technical Report

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Defence R and D Canada - Atlantic Dartmouth, NS Canada

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Towing operations are often conducted by the Canadian Navy. Evaluation of associated forces is required to determine whether towing can safely occur without breakage of rope. This report examines forces that can influence towing operations in a seaway. Ship hydrodynamic resistance and air resistance are evaluated using drag coefficients. For a vessel being towed, additional resistance arises from propeller parasitic drag, which will vary greatly depending on whether a propeller is freewheeling or locked. The presence of a seaway introduces added resistance, and also dynamic forces due to surge excitation acting on the two vessels in a towing operation. A frequency domain method has been developed for predicting dynamic forces during towing. Towing force predictions in sea state 5 show that the presence of a seaway can contribute greatly to total towing forces. It is recommended that propellers on a towed vessel be allowed to freewheel during towing operations, and that towing speeds be reduced in higher sea states.

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