Auto-Position Escort Winch System
The safety and effectiveness of escort tugs operating in the indirect towing mode is a matter of concern to many owners and designers. When operating in this way the tug’ s towline is made secure on the stern of the ship being escorted and uses the dynamic properties of the tug’ s hull to generate steering or braking forces considerably higher than its static bollard pull.
In the simplest terms, the tug becomes a very large powered rudder. The position of the tug’s winch, fairlead and towline are critical and have a dramatic effect on the stability of the tug and its performance when towing in this manner. Winch manufacturer JonRie InterTech has collaborated with Castleman Marine of the USA to develop a winch system that can enhance the performance of an escort tug by introducing a two-plane mechanical advantage.
Known as the ‘Auto Position Escort Winch System’ (patent pending), the concept introduces a new form of winch mounting that enables the winch and its integral fairlead to rotate about its vertical axis and follow the line of force of the towline.
In most shiphandling tugs, whether azimuthing stern drive (ASD) or tractor, the fairlead is mounted on the deck and acts as a fixed ‘point of tow’, always on or very close to the centreline of the vessel. With the new concept, as the winch rotates, following the towline outboard to port or starboard, the position of the fairlead moves the ‘point of tow’ outboard and imposes a ‘righting effect’, improving stability and enabling the tug to exert a greater force.
The winch mounting is similar to that of a revolving crane and will allow the winch to rotate through 110 degrees during indirect towing operations but can be locked on the centreline for normal shiphandling operations.
An advantage of the system is that the whole winch and fairlead (tow point) rotates, on the bow of the tug in the case of an ASD vessel, and thus reduces the side loading on the winch and spooling gear (level wind). Based on conceptual calculations this system will increase tow line forces by 15% and reduce a tug’s heel angle by as much as 40%. This system works as one with the tug and enhances the vessel’s performance and reduces the possibility of girting, tripping or capsizing.
JonRie InterTech has commenced testing using a model of the new Auto Position Escort Winch on a typical 32m ASD tug design, supplied and operated by Burchett Marine Inc of Canada. Initial trials at different speeds in the direct and indirect modes of operation have produced promising results. The test figures will be compared to analytical data being developed by Glosten Associates of the USA. Formal tank testing will also be carried out using different ASD tug and Voith tractor hull forms.
Related Articles
TUGS & SALVAGE
EQUIPMENT
MARINE CONSTRUCTION