Towing Winch
Towing winch design and rules: brake holding capacity at 80 percent of towline MBL, render and recover, emergency release, and IACS UR M79.
A towing winch holds, pays out and recovers the main towline, and its rated capacities are derived from the towline rather than directly from bollard pull. DNV sets the holding capacity at 80 percent of the towline minimum breaking force with the towline in the first layer. MSC/Circ.884 paragraph 12.3 sets it against the documented minimum breaking load of the largest towline to be used, calculated for the outermost layer at which towing will be performed, and paragraph 12.4 requires the winch and its supports to withstand the breaking load of the main towing wire without permanent deformation.
Emergency release is specified tightly. DNV requires the system to function within a maximum of three seconds after activation, with controlled pay out and enough resistance to rotation to avoid uncontrolled unwinding, released from the bridge and every other control stand, functional on power drive failure, with normal brake operation restored immediately and no automatic restart of the winch motor. IACS Unified Requirement M79, Towing winch emergency release systems, was issued in October 2018, with Rev.1 of February 2020 implemented from 1 July 2021.
The full article will cover drum and brake design, render and constant tension functions, the weak link at the drum end, tension measurement and display, ISO 7365:2012 for deep sea towing winches, and the difference between a towing winch and a mooring winch.